Novel perspectives on fermented milks and cardiometabolic health with a focus on type 2 diabetes

Abstract

This review will explore the observational and mechanistic evidence supporting the hypothesis that fermented milk consumption has beneficial effects on metabolism. Live cultures in fermented dairy are thought to contribute to gut microbial balance, which is likely an instrumental mechanism that protects the host against gut dysbiosis and systemic inflammation associated with cardiometabolic diseases. Lactic acid bacteria (LAB) release bioactive metabolites, such as exopolysaccharides and peptides, that have the potential to exert a wide range of metabolic and regulatory functions. In particular, peptides derived from fermented dairy products are likely to exert greater cardiometabolic and anti-inflammatory effects than nonfermented dairy. It is hypothesized that LAB-derived bioactive peptides have the potential to protect the host against cardiometabolic diseases through antimicrobial actions and to effect changes in gene expression of glucose regulatory and anti-inflammatory signaling pathways. The peptides released through fermentation may explain some of the health effects of fermented dairy products on cardiometabolic disease risk observed in epidemiological studies, particularly type 2 diabetes; however, mechanisms have yet to be explored in detail.

INTRODUCTION

Aside from promising cardiometabolic preventive properties, plain yogurt is a nutrient-rich food¹ that corresponds with food-based dietary guidelines worldwide and established healthy diet recommendations (Mediterranean Diet and Dietary Approaches to Stop Hypertension [DASH]). Milk and dairy products, including yogurt, are highly acceptable to consumers, widely available, and have low cost/nutrient ratios,¹ making them ideal foods to recommend to the general population for cardiometabolic disease prevention.² Despite equivocal observational data (see below) on the benefits of fermented dairy or yogurt consumption on specific disease outcomes, such as obesity, cardiovascular diseases, hypertension, and metabolic syndrome (MetS), the evidence to date on type 2 diabetes (T2D) has been strong and consistent.^3–6 A standard portion of yogurt is equivalent to 1 serving from the dairy food group in the US (227 g, 8 oz or 1 cup). Although the observational evidence suggests fermented milk is part of a healthy dietary pattern, there is a lack of experimental evidence to support clinical recommendations for the specific use of yogurt or other fermented milks in T2D prevention.⁷ Although yogurt consumption is generally associated with healthier lifestyle factors among consumers,⁸ there are likely other factors unique to fermented milk that explain the consistent beneficial effects of yogurt intake on T2D risk found in epidemiological studies. The aim of this review is 2-fold: first to summarize the state of evidence and knowledge gaps in fermented milk and cardiometabolic health research, and second to suggest targets for future study and validation. Although many mechanisms are applicable to all fermented dairy, as a group, cheese demonstrates greater heterogeneity than other fermented milks in terms of nutrient content (eg, sodium, calcium, and milk fat); therefore, this review will be limited to liquid and semisolid fermented milks, with yogurt as the main focus, because it has been the most frequently reported fermented milk to have purported cardiometabolic health properties.

CURRENT EVIDENCE

It is difficult to separate the isolated effects of consuming a single food from other lifestyle factors. This is because study designs using an intention-to-treat model with strict adherence guidelines are inappropriate in the context of food consumption and dietary patterns that have variable levels of adherence in real-world settings.⁹ Fermented dairy products represent a group of foods that have similar nutrient profiles, contain live bacteria that are generally shelf-stable, and are backed by epidemiological evidence supporting regular consumption for inclusion in healthy dietary patterns and protection against cardiometabolic diseases.²

Despite the overwhelming number of long-term prospective cohort studies that have identified beneficial associations between fermented dairy consumption, notably yogurt, and cardiometabolic diseases such as type 2 diabetes,^3–6 the identification of a causal relationship has remained elusive. A number of mechanisms have been proposed but have yet to be validated. For several years the scientific community has been calling for greater clinical and more basic preclinical research to confirm a direct role and clarify the key mechanisms by which fermented dairy can promote cardiometabolic health. The dairy matrix,¹⁰ bioactive metabolites (eg, peptides and exopolysaccharides) released during dairy fermentation and gastrointestinal digestion,¹¹ and actions of live ferments on the gut microbiota¹² are the most plausible mechanisms responsible for the health benefits of fermented dairy (Figure 1). To date, there is a paucity of evidence on the topic, and the majority of clinical or animal studies on fermented dairy have investigated combinations of specific probiotic strains in relation to short-term cardiometabolic outcomes (fasting blood glucose, insulin, hemoglobin A1c, homeostatic model assessement of insulin resistance (HOMA-IR), total serum cholesterol, low-density lipoprotein cholesterol, triglycerides, and blood pressure)^13–15 using yogurt as a vehicle for delivery.¹⁶ Many of the proposed mechanisms are speculative in nature and have been extrapolated from a mix of epidemiological data and plausible physiological mechanisms.⁷ Here, an overview of the epidemiological evidence for a relationship between fermented milk consumption and cardiometabolic diseases is presented (Table 1).

OBESITY

Few studies have investigated the relationship between fermented dairy foods and weight status, but evidence of the benefits of yogurt consumption on obesity appears promising.¹⁷ However, there have been inconsistent conclusions among studies, which may be the result of a lack of statistical power, different outcomes measured (eg, body mass index [BMI], weight change, waist circumference, or weight status), age group, sex and gender differences, type of yogurt (eg, low-fat vs high-fat), adjustment for different covariates such as baseline weight, and vitamin D or calcium status.^{19,20,38–45} One meta-analysis evaluated the topic and found an inverse association between yearly changes in body weight and yogurt intake. Each incremental serving of yogurt was associated with 40.99 g (95% CI, 33.8–48.09 g) less weight gain per year.¹⁸ These results were pooled from a single, well-reputed study that examined 3 large American prospective cohorts: the Nurse’s Health Study I, the Nurse’s Health Study II, and the Health Professionals Follow-up Study.¹⁹ In a different American cohort from the Women’s Health Study (n = 18 438), it was found that ≥1 serving of yogurt was associated with a 16% (95% confidence interval [CI], 3%–31%) increased risk of becoming overweight or obese compared with no yogurt, after a mean follow-up of 11.2 years.²⁰ A small prospective cohort from the Quebec Family Study compared yogurt consumers (n = 269) to nonconsumers (n = 570) and found significantly lower percentage body fat in male yogurt consumers at baseline (P = 0.005), which was accentuated at the 6-year follow-up, adjusting for age and nutrient-rich food index (P = 0.03). These findings were not observed in women.²¹ Earlier results from the same cohort (n = 248) found no association between intake of low-fat yogurt only and weight change, but a positive association with a small, yet significant (P = 0.02), increase in waist circumference from baseline to the end of the study, adjusted for initial age, baseline body weight or adiposity indicators, and change in daily physical activity level.²²

The apparent discrepancy between studies may be due to the fact that only data for low-fat yogurt were reported in the earlier study and different covariates were adjusted. A prospective cohort study examining dairy and calcium intake among Portuguese adolescents at age 13 years and BMI at age 21 years found no association between yogurt intake and BMI.²³ Despite the large, prominent studies^44,46 that have found favorable associations between yogurt intake and adiposity measures, studies that have found negative²⁰ or neutral associations²³ indicate that observational data, although promising, are not sufficiently consistent to draw definite conclusions.

TYPE 2 DIABETES

Currently, the observational evidence specifically linking yogurt consumption to cardiometabolic diseases is only consistent for T2D, as it is supported by numerous high-quality systematic reviews of large, prospective, cohort studies.^4–6,47,48 In 1 meta-analysis, based on 7 cohort studies, there was a 14% lower risk of T2D associated with 200 g of yogurt intake per day.⁶ A lower risk was also associated with fermented food intake in this meta-analysis, but this was based on only 2 studies.⁶ Subsequent meta-analyses have reported similar findings. An analysis based on 9 prospective cohorts found that consuming a 244 g serving of yogurt per day was associated with an 18% reduced risk of T2D.⁵ The most recent meta-analysis on yogurt and diabetes included 11 studies and found a nonlinear association of consumption of 80 g of yogurt per day with a 14% reduced risk of T2D compared with no yogurt.⁴ The study also examined 5 studies on fermented dairy, but no association was observed unless high-fat fermented dairy from 1 of the studies was included.⁴

Using the GRADE evidence appraisal,⁴⁹ an inverse association between yogurt consumption and T2D risk was ranked as favorable, with a high-quality evidence rating based on 5 meta-analyses of prospective cohort studies.³ The association between fermented dairy and T2D was reported as neutral with a moderate quality evidence rating.³ The differences observed between fermented dairy and yogurt are likely due to the smaller number of studies and a greater diversity of foods included in the fermented dairy group compared with yogurt. Although these appraisals are based on observational or indirect data, they have received the highest evidence rating that can reasonably be attributed to nutritional studies in the absence of long-term randomized controlled trials. It is also important to note, for T2D, yogurt and low-fat dairy were the only dairy groups that were given favorable ratings and were appraised with high-quality evidence.³ Additionally, moderate yogurt consumption (1 to <3 servings per week) was associated with lower risk of developing prediabetes during a 12-year follow-up study in the Framingham Heart Study Offspring Cohort.²⁵

Based on the consistent strength and direction of the current evidence, there are probable positive health benefits of yogurt consumption on prevention of T2D that would have important global implications for clinical practice guidelines, public health nutrition recommendations, and the functional food market. Thus, research to elucidate the mechanisms of action of yogurt consumption on T2D prevention should be intensified so that specific guidelines on physiological doses of yogurt consumption can be proposed. Additionally, details about the lactic acid bacteria (LAB) species used in commercial yogurt fermentation would help identify whether there is a species and strain specificity implicated in potential beneficial effects of yogurt on T2D incidence.

METABOLIC SYNDROME

Few studies have examined the relationship between fermented milk and MetS. Nevertheless, at least 5 prospective cohort studies have been conducted in a wide range of populations, including American young adults (n = 3157; aged 18–30 y with a 10-y follow-up),²⁶ Dutch older adults (n = 2484; aged 50–75 y with a 6.4-y follow-up),²⁷ Spanish older adults (n = 1868; aged 55–80 y with a mean follow-up of 3.2 y),²⁸ Spanish adults (n = 1997; aged 20–90 y with a 6-y follow-up),²⁹ and Korean adults (n = 5510; aged 40–69 y with a 10-y follow-up).³⁰ Of these studies, favorable associations were found between yogurt consumption and MetS among Korean³⁰ and Spanish²⁸ cohorts and inverse or neutral associations in other cohorts.^26,27,29 Different age groups, length of follow-up, and MetS criteria could explain the lack of consistency among studies. One meta-analysis on dairy and MetS was conducted in 2015; however, yogurt subgroup analyses were only based on a single study and found a nonsignificant inverse association between yogurt consumption and MetS (P ≥ 0.06).³¹ At this time, fermented milks appear to have a beneficial relationship with MetS incidence; however, not enough evidence is available to draw conclusions. Future studies should examine associations between fermented milks and MetS according to standard diagnostic criteria, as well as according to individual MetS components.

CARDIOVASCULAR AND CORONARY HEART DISEASE

Despite 3 meta-analyses investigating cardiovascular diseases and fermented dairy product consumption, there appears to be too much heterogeneity in fermented dairy and too few studies on yogurt alone to draw a consistent conclusion.^32–34 A meta-analysis of 9 studies found that high yogurt intake (and other fermented dairy) compared with low intake was not associated with cardiovascular disease (CVD) risk (relative risk [RR], 1.01; 95%CI, 0.95–1.08).³² Another meta-analysis with 11 studies found a marginally reduced relative risk of CVD (RR, 0.98; 95%CI, 0.97–0.99) and no association with coronary heart disease (CHD) risk (RR, 0.99; 95%CI, 0.98–1.01) for every 20 g of fermented dairy consumed per day, but there was a high level of inconsistency among studies. The meta-analysis was also performed on 3 studies that included yogurt; it found no significant association with CVD (P = 0.499) or CHD risk (P = 0.685).³³ In a previous meta-analysis, no associations were found between yogurt consumption and CVD or CHD based on 3 and 4 studies, respectively.³⁴ The authors concluded that additional studies are required to adequately assess the relationship between yogurt and CVD.³⁴

HYPERTENSION

Unlike other cardiometabolic conditions, there is weak epidemiological evidence, but a strong clinical basis, to support the potential antihypertensive effects of fermented dairy consumption on hypertension. Bioactive tripeptides (valine-proline-proline [Val-Pro-Pro] and isoleucine-proline-proline [Ile-Pro-Pro]) derived from milk proteins are found in high concentrations in certain types of fermented milks and may have blood pressure–lowering properties through angiotensin-converting enzyme (ACE)–inhibitory activity.⁵⁰ A daily dose for 10 weeks of 150 mL of milk fermented with Lactobacillus helveticus that had high concentrations of bioactive peptides (7.5 mg of Ile-Pro-Pro and 10 mg of Val-Pro-Pro in 100 g of fermented milk) resulted in mean lower systolic (−4.1 ± 0.9 mm Hg; P < 0.001) and diastolic (−1.7 ± 0.8 mm Hg; P < 0.048) blood pressure compared with a control.⁵¹ A Cochrane collaboration review of 15 randomized controlled trials of milk or casein fermented with several bacterial strands (including probiotics) concluded that there was a modest effect of fermented milk on lower systolic blood pressure (−2.45 mm Hg; 95%CI, −4.30 to −6.0), but no effect on lower diastolic blood pressure (−0.67 mm Hg; 95%CI, −1.48 to 0.14).⁵² A meta-analyses of 13 randomized controlled trials concluded that probiotic fermented milks had a lowering effect on both systolic and diastolic blood pressure compared with a placebo, and the effects were greater in hypertensive individuals.⁵³ Clinical studies confirm the plausibility that fermented dairy has antihypertensive-lowering properties, particularly probiotics; however, these results cannot be generalized to all fermented milks or to real-world settings where there is a mismatch between probiotic benefits in trials and food products.⁵⁴ Commercial products are highly variable in terms of probiotic strains and dosage of colony-forming units.⁵⁴

A single meta-analysis of prospective cohort studies did not find any association between fermented dairy or yogurt intake and hypertension based on 4 and 5 studies, respectively.³⁵ For fermented dairy, the pooled relative risk (n = 7641) was 0.99 (95%CI, 0.97–1.04) for each 150-g portion per day. For yogurt, the pooled relative risk (n = 45 088) was 0.99 (95%CI, 0.96–1.01) per 50-g portion per day. Using the GRADE evidence appraisal based on the evaluation of the single meta-analysis, the relationship between fermented dairy and yogurt and hypertension was reported as neutral, supported by moderate quality evidence.³ Since these meta-analyses were conducted, however, 2 additional studies have found promising results. In a Spanish prospective cohort, the highest tertile of whole-fat yogurt intake was significantly associated with a lower risk for high blood pressure (hazard ratio [HR], 0.62; 95%CI, 0.44–0.86, P_trend = 0.001) compared with the lowest tertile; however, low-fat and total yogurt consumption were not associated.²⁸ A prospective cohort (n = 2 636) from the Framingham Heart Study Offspring Cohort found a significant 6% lower risk (95%CI, 1%–10%; P = 0.02) of incident hypertension for every additional 227-g serving of yogurt per week. Similarly, there was a significant 3% lower risk (95%CI, 1%–5%; P = 0.01) of incident hypertension for every additional serving per week of fermented milk products (sour cream, yogurt, cottage cheese, ricotta cheese, cream cheese, and other cheese).³⁶ This study also examined incremental annualized changes in blood pressure and found that both yogurt and fermented dairy intake were inversely associated with changes in systolic blood pressure, but not diastolic,³⁶ which is concordant with results from the Cochrane collaboration review.⁵² A recent study examined CVD risk among already hypertensive participants in 2 large American cohorts (Nurses’ Health Study, n = 55 898 women; and Health Professional Follow-up Study, n = 18 232 men) and found yogurt consumption was inversely associated with CVD risk. Furthermore, hypertensive participants who consumed yogurt regularly and had higher DASH diet scores had even lower risk for CVD, indicating a potential synergistic effect of consuming yogurt in addition to following a healthy dietary pattern.³⁷

ROLE OF MICROBES AND FERMENTATION BY PRODUCTS

Fermentation is a traditional form of food preservation that has the practical purpose of extending the life of foods by inhibiting pathogen growth. In addition to preservation, fermentation enhances nutritional value, increases functionality, and imparts unique and pleasant organoleptic properties to foods.⁵⁵ It is the enhanced nutritional value and functionality that is of interest to many consumers. Fermented foods are being positioned as health foods with added value to improve digestion, improve immunity, and reduce cholesterol.⁵⁴ However, it is debatable whether all commercial products deliver sufficient quantities of live microbes.⁵⁴ Nevertheless, emerging research has indicated that bacteria do not necessarily need to be viable to impart health benefits. Furthermore, the fermentation process itself may improve the nutritional quality of a food by increasing the quantity of bioactive peptides available, the amount of conjugated linoleic acid (CLA), and the bioavailability of certain nutrients.⁷

Many large cohort studies have found favorable associations between the consumption of fermented dairy or yogurt and reduced incidence of cardiometabolic diseases such as T2D. These associations are thought to be driven by the unique properties of fermented foods. Fermented foods contribute live microorganisms that interact directly with the host through the gut microbiota via possible probiotic effects and provide metabolites produced during the fermentation process that have a biogenic effect.⁵⁶ For example, lactate produced during LAB fermentation can reduce pro-inflammatory cytokine production and reduce the production of reactive oxygen species in enterocytes within the intestine.⁵⁷ Fermentation may also increase the concentration of bioactive molecules (eg, peptides, exopolysaccharides, CLA) naturally present in dairy foods. Furthermore, the fermented dairy food matrix may enhance viability of live microorganisms and provide substrates that directly or indirectly benefit gut microbial health and facilitate digestion of nutrients.¹⁰ Perhaps the most notable health benefit for dairy products exists for yogurt. Specifically, the European Food Safety Association considers the evidence to be sufficient such that yogurt cultures (Lactobacillus bulgaricus and Streptococcus thermophilus) can be claimed to improve lactose digestion in lactose maldigesters.⁵⁸ The mechanism occurs via the release of β-galactosidases by yogurt cultures.⁵⁹ The bacteria delivered through fermented foods may also influence the microbial composition of the gut microbiota, potentially influencing metabolism and gene expression of key regulatory pathways. These concepts will be explored in the following sections.

GUT DYSBIOSIS

Gut dysbiosis is defined by an altered gut microbiota composition that coincides with alterations in microbial transcriptomes, proteomes, or metabolomes.⁶⁰ These changes stem from shifts in relative bacterial populations associated with a variety of diseases, including obesity, inflammation, immune deficiency, infection, or exposure to antibiotics and toxins.⁶¹ The microbiome of obese individuals can be described by altered ratios of dominant colonic microbial phyla, Bacteroidetes and Firmicutes, compared with lean individuals.⁶² This altered microbiome is more efficient at harvesting energy from food in the gastrointestinal tract, thereby resulting in increased storage of excess energy in adipose tissues, impacting energy balance and contributing to obesity.⁶³ In ob/ob mice, gut dysbiosis is associated with increased circulating concentrations of bacterial lipopolysaccharide (LPS), triggering an inflammatory response that has been linked with the development of insulin resistance.⁶⁴ Indeed, elevated LPS levels in obesity promote metabolic endotoxemia and metabolic disorders.⁶⁴ Besides obesity, a dysbiotic microbiome can promote a variety of disease states, including inflammatory bowel disease,⁶⁵ hypertension, T2D,^66,67 cancer,⁶⁸ depression, and central nervous system disorders.^69,70

Diet is thought to have a considerable influence on the gut microbial balance. Excessive high-fat feeding creates changes in the gut microbiota, which increase intestinal permeability, resulting in greater absorption of LPS. The Western diet in particular, high in sugar and fat, increases the number of Clostridium innocuum, Eurobacterium dolichum, Catenibacterium mitsuokai, and Enterococcus spp. and decreases Bifidobacteria and Bacterioidetes, altering the composition of the gut microbiota, ultimately leading to dysbiosis and aberrant immune responses.^71–74 Using diet to promote microbes that can prevent or control inflammatory-mediated cardiometabolic diseases presents a promising case for diet-based interventions.⁷³ Fermented dairy products are thought to act on cardiometabolic disease prevention by contributing large numbers of live microbes to the gastrointestinal tract and helping to restore gut microbial imbalances.⁷⁵ However, due to mainly observational evidence, it is difficult to separate the beneficial effects of the microbes from the dairy matrix.⁷⁶ Nevertheless, in a general manner, live or active dairy cultures found in yogurt (eg, L. bulgaricus and S. thermophilus) could contribute to a healthy microbiome through mechanisms that include promoting short-chain fatty-acid production, decreasing pH, regulating intestinal transit, normalizing disturbed microbiota, increasing enterocyte turnover, and competing with pathogens for space and nutrients.^56,76 Furthermore, other Lactobacillus, Bacidobacterium, and specific probiotic strains may be added to commercial yogurts. These cultures may be more viable during gastrointestinal transit or have a greater capacity to colonize the gut, providing additional health benefits.¹² Although a proven strain-specific health effect and a viable cell count through the digestive tract are necessary for cultures to be considered probiotic, experts have reported that Lactobacillus and Bacidobacterium species may be classified as probiotics despite strain-level differences because they are likely to impart general health benefits by supporting a healthy gut microbiota.^57,76 Consuming foods rich in LAB, such as cheese and yogurt, enhances LAB populations in the gut, where they play an important role in preventing colonization of exogenous bacteria and reducing toxigenic and mutagenic effects of endogenous bacteria.⁷⁷

BIOACTIVE METABOLITES

Fermented milk products supply substrates to the organisms that reside in the gut; however, the nature of the metabolites produced from these substrates is often strain dependent.⁵⁷ The most notable metabolites are produced during LAB fermentation and include exopolysaccharides (EPS), CLA, and bioactive peptides.⁷⁸ In addition, CLA, short-chain fatty acids, neurotransmitters, selenoproteins,⁷⁷ and some vitamins can also be produced through de novo synthesis (eg, riboflavin, vitamin B12, vitamin K).⁷⁹ The bioavailability and physiological activity of these metabolites, however, are not well-established. Nevertheless, these bioactive metabolites largely differentiate fermented milks from nonfermented milks and are thought to be a major contributing factor to yogurt’s inverse association with T2D.

Bioactive peptides

Milk naturally contains bioactive peptides that are released during gastrointestinal digestion; however, the concentration of these peptides is higher in fermented dairy. This is due to fermentation with proteolytic starter cultures such as LAB, in addition to proteolytic enzymes released in the dairy matrix that are capable of further breaking down proteins into bioactive peptides.⁷⁷ Casein serves as the primary substrate for the release of bioactive peptides.⁵⁵ Dairy peptides have been characterized with ACE-inhibitory, antioxidant, glucose-lowering, cholesterol-lowering, anticarcinogenic, immunomodulatory, antimicrobial, antithrombotic, mucin-stimulating, cell-cycle, and apoptosis modulatory activity.⁷⁷ Specific peptides with ACE-inibitory, antioxidant, opioid, and immumomodulatory properties have been characterized in yogurt made with traditional cultures (L. bulgaricus and S. thermophilus),^80–82 as well as probiotic cultures.⁸³ Although the bioactivity of peptides has been demonstrated in many studies, it is difficult to comprehend the full range of activity and applicability of general LAB when the properties vary according to strain. Nevertheless, there may be common pathways that are similar among dairy peptides regardless of strain. One such pathway involves regulation of various metabolic and immune system genes. Indeed, it has recently been demonstrated that the ingestion of both probiotic yogurt and acidified milk resulted in modulation of inflammatory and glycolytic genes. After dairy product ingestion, there was a differential enrichment of gene expression between yogurt and acidified milk that depended on the timing of the postprandial response (2 h, 4 h ,or 6 h), which could, in theory, lead to different physiological responses. Furthermore, gene expression was consistent with measured biomarkers.⁸⁴ Another study demonstrated that peptide β-CN(94–123) from fermented milk induced greater intestinal expression of mucins (Muc2 and Muc4) and antibacterial factors (lysoxyme and rdefa5) in rat pups. These findings imply that yogurt peptides may contribute to immune regulation of the microbiota and epithelial dynamics of the gut structure.⁸² Despite nascent evidence based on animal studies, the authors of this review hypothesize that certain peptides made available through yogurt fermentation have antidiabetic activity through mechanisms that regulate the expression of genes directly involved in glucose uptake and insulin secretion, as well as those involved in intestinal homeostasis and the inflammatory response.

Many bacterial strains used in food fermentation produce bioactive peptides that are of particular interest because of their antimicrobial properties, especially S. thermophilus t3D1 and Lactobacillus. delbrueckii subsp. bulgaricus b38, b12, and b24 because of their utilization in yogurt making. These peptides tend to be stable in foods, acting as preservatives, and are also stable in blood and serum. Although the specific mechanisms of LAB proteolysis and peptide antimicrobial activity have yet to be exposed,⁷⁷ numerous peptides with antimicrobial activity have been reported, regardless of strain specificity. Investigation into strain-specific effects would help optimize the selection of the most efficient strains. The authors of this review hypothesize that antibacterial peptides derived from proteins during milk fermentation are a contributing factor in helping to maintain a healthy gut microbiome, thereby reducing diet-induced gut dysbiosis and thereby the cardiometabolic diseases associated with systemic inflammation. The premise for this hypothesis is that, regardless of strain, LAB-derived peptides have antimicrobial activity that works through various mechanisms.

Release of ACE-inhibitory peptides from milk is well documented, with >50 different sequences having been identified.^11,85 The activity of these peptides is also known to be enhanced through fermentation with commercial LAB starters.⁸⁶ Although ACE-inhibitory peptides have been identified and their activity has been characterized in vitro, their applicability in clinical settings remains uncertain. ACE-inhibitory peptides have yet to be quantified in commercial fermented milks; thus it remains to be determined whether physiological doses of this peptide can be obtained through dietary consumption.

Conjugated linoleic acid

Lactobacillus and Streptococcus species used in yogurt production are thought to be the most effective genera to produce the bioactive fatty acid CLA.⁷⁷ Conjugated linoleic acid is one of the most widely studied bioactive fatty acids; however, its activity has been primarily reported in vitro and in animal models using supplemental doses rather than amounts that would be representative of regular dietary intake. Compared with milk, LAB fermentation can increase the concentrations of this fatty acid, making products like yogurt or kefir a potential source of dietary CLA. Still, the content of CLA in milk can be highly variable and is dependent upon ruminant diet and breed, and increased concentration through fermentation is dependent on bacterial strains used.^87–89 Nevertheless, CLA has the potential to be useful in cardiometabolic disease prevention, namely for prevention of obesity and T2D.^90–92 However, quantification of CLA in commercial yogurts needs to be performed to determine whether physiological doses can be obtained through consumption of fermented milks.

Exopolysaccharides

Immunomodulatory potential of EPS produced by LAB have been demonstrated but are strain specific and not well understood.⁹³ Some LAB can synthesize EPS that have B-cell mitogen activity,^94,95 induce cytokine production,⁹⁶ and modify macrophage and splenocyte functions.^97,98 The exopolysaccharides produced by S. thermophilus and L. bulgaricus also have a structural role contributing to rheological and sensorial properties of yogurt and other fermented milks. The increased viscosity contributes to a food matrix that helps protect degradation of live bacteria during passage through the gastrointestinal system, enhancing the viability and delivery of ferments to the gut.⁹⁹

ADDITIONAL CONSIDERATIONS

Innovative and alternative fermented milks

The increased interest in fermented foods and their purported health properties has led to new innovations in a range of functional fermented foods, particularly with cow-milk alternative fermented foods and liquid/semisolid, nondairy, plant-based fermented products.^100,101 Alternatives to bovine milk, such as fermented goat milk, are of interest for their lipid and protein profiles. Although there are variations in protein (distinct peptide profiles)¹⁰² and milk-fat composition among different animal milks,¹⁰³ generally nonbovine milks from goats, sheep, buffalo, yaks, or camels are all nutrient-rich foods that can be included in any healthy dietary pattern.¹⁰⁴ Nevertheless, goat milk is thought to differ from bovine milk for its specific fatty-acid and protein profiles, which make it easier to digest than cow’s milk and less allergenic. In addition to its better digestibility, goat milk contains higher concentrations of short- and medium-chain fatty acids, minerals (zinc, iron, magnesium, and calcium), and oligosaccharides, making it more suitable for infant formula.¹⁰⁵ These favorable nutritional properties may be enhanced through fermentation, making goat-based fermented milks ideal functional products for consumers interested in therapeutic milks.¹⁰⁶ All other nonbovine dairy milks can be fermented with typical cultures used for cheeses, yogurts, or kefir, but goat milk is the most available milk throughout the world, and fermented products such as cheeses are already widely consumed.¹⁰⁴ Research on fermented milks should expand and include nonbovine dairy milks such as goat milk to determine the best functional and therapeutic products for consumers.

Plant-based nondairy products

Plant-based, fermented, nondairy products that are liquid or semisolid are also positioned as alternatives for consumers with lactose intolerance, milk allergies, and concerns over cholesterol.^101,107 Plant-based products include a large variety of cereal-, legume-, nut-, seed-, and pseudo-cereal–based milks.¹⁰⁸ These products differ greatly in their nutrient composition (protein and fatty-acid content) and nutrient density, depending on their source (oat, coconut, almond, soy, etc), nutrient fortification, and other ingredients added during processing. These products may be deficient in 1 or several nutrients—namely, calcium and protein—and high in simple carbohydrates.¹⁰⁰ Soy-based products, although not equal to animal milks, are a good source of protein and are nutritionally equivalent (when fortified), representing an appealing alternative to animal milk for consumers.^100,109 Other popular nondairy milks made from almonds, rice, or coconuts are very low in protein and can also be very low in energy.¹⁰⁹ Soymilk fermented with different combinations of bacterial strains typically used in dairy yogurt production yielded a product containing acceptable quantities of viable bacteria (10⁸ CRU/mL) and released bioactive peptides with ACE-inhibitory activity.¹¹⁰ In an animal study, a diet that included fermented soy milk prevented cholesterol elevation induced by ovariectomy in older rats.¹¹¹ There have also been studies in rats that found antiobesity activity in fermented soy as compared with unfermented soy.¹¹² In addition, soy consumption has been associated with an increased abundance of Bifidobacterium spp. and Lactobacillus spp., as well as the ratios of Bifidobacterium spp. and Lactobacillus spp. to Clostridium perfringens in humans.¹¹³ Although few studies have been performed on the health benefits of nondairy fermented products, fermented soy yogurt-like products may potentially offer similar health benefits to cow-milk fermented dairy products, and interest is likely to increase in the future.

Sex differences

Greater yogurt consumption is consistently associated with women compared with men in the majority of populations worldwide and across age groups. This has been observed in American,³⁷ Spanish,⁴² Mediterranean,³⁸ Irish,¹¹⁴ and Canadian²¹ populations. These differences in yogurt or fermented dairy consumption can easily be attributed to different dietary behaviors practiced by women and the overall tendency for women to follow more a healthful diet than men. However, there are also biological sex differences that may result in notable differences in the potential physiological impacts of yogurt consumption on various health parameters. A recent prospective cohort study in a hypertensive population observed significant inverse associations between yogurt consumption and risk of major CHD in women (P for linear trend < 0.001), but not men (P for linear trend = 0.06). Conversely, there were significant inverse associations between yogurt consumption and risk of stroke in men (P for linear trend = 0.04), but not women (P for linear trend = 0.44).³⁷ These differences could be attributed to sex differences in gut microbial populations or differential effects of yogurt bacteria on existing microbiota. A recent study found that, although yogurt consumption has a positive linear association to bacterial fecal count of Lactobacillus and Lactobacillus gasseri in both men and women, there was a negative association with Lactobacillus sakei, Enterobacteriaceae, and Staphylococcus in men, and a positive association with Lactobacillus casei in women.¹¹⁵ Because sex-specific interactions between diet and the microbiota are common across various species (fish, mice, and humans), this suggests that dietary therapies to treat dysbiotic-related conditions such as obesity and T2D may need to be sex specific.¹¹⁶ Currently, differences in health parameters between men and women who consume yogurt are based on observational studies; however, future research needs to take into account biological sex when examining mechanisms of action of yogurt in preclinical animal and clinical studies.¹¹⁷ Based on findings to date, it is fair to suggest that diet–microbiome interactions differ by host sex, potentially impacting the influence of yogurt on health and disease.

Fermented milk mechanistic research: proof of concept challenges

To defend the hypothesis that fermented milks are beneficial to cardiometabolic disease outcomes and bridge the gaps between observational and mechanistic studies, 4 assumptions are often made: 1) fermented dairy has unique properties that are distinct from nonfermented dairy products; 2) bioactive metabolites are released from all dairy products fermented with LAB; 3) yogurt contributes live bacteria to the gut microbiota regardless of viability in the feces; and 4) a physiological dose is equivalent to as little as 1–2 portions or less of yogurt per day. Unfortunately, studies involving animals or humans to date have focused on high doses of specific strains, often probiotic strains rather than fermentation cultures, and generalizations cannot be made across all bacterial cultures. Future studies should focus on commercially available products in amounts that would normally be consumed in a typical diet and take advantage of advances in metabolomics to test the effects of long-term yogurt consumption on gene expression and shifts in microbiota diversity and gut integrity. Table 2 summarizes future research needs.

CONCLUSION

Fermented milk consumption, primarily yogurt, has been studied in relation to various cardiometabolic disease states, and, for the most part, evidence shows favorable or neutral associations with health. Of note, yogurt consumption is consistently and favorably associated with reduced risk of T2D, but the mechanisms of action have yet to be identified. The presence of the microbial cultures is the most plausible explanation for the differential effects of yogurt on T2D compared with nonfermented milk. Fermented milks may contribute to cardiometabolic disease and T2D prevention through mechanisms that involve gut homeostasis (reduction or prevention of dysbiosis) and release of bioactive metabolites that change gene expression of key regulatory pathways for glucose metabolism, insulin secretion, and/or immunometabolism. Research into these mechanisms should be intensified to bridge the gap between observational and mechanistic studies and justify the development of clinical practice guidelines for fermented dairy consumption in T2D prevention. Such efforts may ultimately provide science-based evidence necessary to convince public health authorities to make recommendations for the inclusion of fermented dairy in food-based dietary guidelines.

Acknowledgments

We thank Dr. Robert Hutkins and Dr. Densie Webb for editing the manuscript.

Author contributions. Both authors wrote, revised, and edited the manuscript.

Funding. A.M. holds a Canadian Institutes of Health Research (CIHR)/Pfizer research chair on the pathogenesis of insulin resistance and cardiovascular diseases. M.A.F. is a CIHR Fellow (Funding Reference Number: MFE-152525).

Declaraton of interest. A.M. receives research grants from Danone Nutricia Research. He received travel support and honorarium from Yogurt in Nutrition Initiative (YINI) to present at the International Union of Nutritional Sciences International Congress on Nutrition in Buenos Aires, Argentina. M.A.F.’s research assistantship was supported, in part, through a research grant from Danone Nutricia Research.

References