Diets high in protein or saturated fat do not affect insulin sensitivity or plasma concentrations of lipids and lipoproteins in overweight and obese adults

Impact of saturated compared with unsaturated dietary fat on insulin sensitivity, pancreatic β-cell function and glucose tolerance: a systematic review and meta-analysis of randomized, controlled trials

BACKGROUND

The impact of the dietary fat type on type 2 diabetes (T2D) remains unclear.

OBJECTIVES

We aimed to evaluate the effects of replacing dietary saturated fatty acids (SFA) with mono- or poly-unsaturated fatty acids (MUFA and PUFA, respectively) on insulin sensitivity, pancreatic β-cell function, and glucose tolerance, as surrogate endpoints for T2D.

METHODS

We conducted a systematic review and meta-analysis of randomized controlled trials that replaced ≥5% of total energy intake provided by SFA with MUFA or PUFA and reported indexes of insulin sensitivity, β-cell function, and/or glucose tolerance. We searched MEDLINE, Scopus, and the Cochrane Library (CENTRAL) up to 9 January, 2023. Eligible interventions had to be isocaloric, with no significant difference in other macronutrients. Data were synthesized using random-effects model meta-analysis.

RESULTS

Of 6355 records identified, 10 parallel and 20 crossover trials with 1586 participants were included. The mean age of the participants was 42 years, 47% were male, mean body mass index (BMI; in kg/m2) was 26.8, median baseline fasting glucose was 5.13 mmol/L, and the median duration of interventions was 5 weeks. Replacing SFA with MUFA or PUFA had no significant effects on insulin sensitivity [standardized mean difference (SMD) SFA compared with MUFA: 0.01, 95% confidence interval (CI): -0.06 to 0.09, I2 = 0% and SMD SFA compared with PUFA: 0, 95% CI: -0.15 to 0.14, I2 = 0%]. Replacing SFA with MUFA did not significantly impact the β-cell function, evaluated by the disposition index (mean difference: -12, 95% CI: -158 to 133, I2=0%). Evidence on glucose tolerance (SFA compared with MUFA or PUFA) and on β-cell function when SFA were replaced with PUFA was scant.

CONCLUSIONS

Short-term substitution of saturated with unsaturated fat does not significantly affect insulin sensitivity nor β-cell function (the latter in the SFA compared with MUFA comparison). Future studies are needed to elucidate longer term effects of dietary fat saturation on glucose homeostasis. This trial was registered at PROSPERO as CRD42020178382.

Dietary fat intakes, lipid profiles, adiposity, inflammation, and glucose in women and men in the Framingham Offspring Cohort

Introduction: The role of dietary fat in the evolution of cardiometabolic disorders is highly controversial. As both dietary intake and the development of cardiometabolic risk differ by sex, we evaluated sex-specific differences in the associations between dietary fats (saturated and unsaturated) and four key cardiometabolic risk factors-lipid profiles, body fat, inflammation, and glucose regulation. Methods: We included 2391 women and men aged ≥30 years in the prospective Framingham Offspring Cohort. Weight-adjusted dietary fats (saturated, monounsaturated, and polyunsaturated fats, including omega-3 and omega-6) were derived from 3-day dietary records. Analysis of covariance was used to derive adjusted mean levels of all outcomes. Results: In both men and women, intakes of saturated and monounsaturated fats were inversely associated with TG:HDL ratio (p < 0.02 for both types of fat). In women, higher omega-3 and omega-6 PUFAs were also inversely associated with TG:HDL (p < 0.05 for both), but for men, only omega-3 PUFAs were associated (p = 0.026). All types of dietary fat were beneficially associated with larger HDL particle sizes in both men and women, while only saturated and monounsaturated fats were associated with larger LDL particles in men. In addition, saturated and monounsaturated fats were associated with statistically significantly higher concentrations of HDL and lower concentrations of LDL and VLDL particles in both sexes, while polyunsaturated fat had favorable associations in women only. Saturated fat also had beneficial associations with three measures of body fat. For example, women with the highest (vs. lowest) saturated fat intake had a lower BMI (27.7 ± 0.25 vs. 26.2 ± 0.36 kg/m², p = 0.001); findings were similar in men (28.2 ± 0.25 vs. 27.1 ± 0.20, p = 0.002). Unsaturated fats had beneficial associations with body fat primarily in women. Finally, omega-3 PUFAs among women were inversely associated with interleukin-6 levels. There was no association between dietary fat intake and fasting glucose levels in either women or men. Discussion: In sum, we found no evidence of an adverse association between dietary fats and several surrogate markers of cardiometabolic health. This study suggests that different dietary fats may have divergent associations with cardiometabolic risk in women and men, perhaps owing to differences in food sources of the same dietary fats.

Effect of Consuming Beef with Varying Fatty Acid Compositions as a Major Source of Protein in Volunteers under a Personalized Nutritional Program

Beef is an excellent source of nutrients; unfortunately, most nutritional recommendations suggest limiting or even avoiding it. Studies have shown that the fatty acid composition of meat influences weight loss. This randomized controlled clinical trial evaluated the anthropometric and serum lipid changes after a food intervention that included frequent beef consumption (120 g consumed four days/week for four weeks). Volunteers were randomly assigned to the commercial or Wagyu-Cross beef groups, with the latter beef possessing higher fat and MUFA contents. Both groups exhibited reductions in body measurements and lipid profiles; however, the Wagyu-Cross group exhibited greater changes in weight (−3.75 vs. −2.90 kg) and BMI (−1.49 vs. −1.03) than the commercial group, without a significant difference between them. No significant group differences in lipid profiles were observed; however, the Wagyu-Cross group exhibited a more favorable change in decreasing the TC concentration (−7.00 mg/dL) and LDL-C concentration (−12.5 mg/dL). We suggest that high MUFA beef could be included in weight-loss programs since it does not affect weight loss and hasn’t a negative influence on lipid metabolism.

The Measurement of Insulin Clearance

Insulin clearance has recently been highlighted as a fundamental aspect of glucose metabolism, as it has been hypothesized that its impairment could be related to an increased risk of developing type 2 diabetes. This review focuses on methods used to calculate insulin clearance: from the early surrogate indices employing C-peptide:insulin molar ratio, to direct measurement methods used in animal models, to modeling-based techniques to estimate the components of insulin clearance (hepatic versus extrahepatic). The methods are explored and interpreted by critically highlighting advantages and limitations.

Substituting Lean Beef for Carbohydrate in a Healthy Dietary Pattern Does Not Adversely Affect the Cardiometabolic Risk Factor Profile in Men and Women at Risk for Type 2 Diabetes

BACKGROUND

Observational evidence suggests that red meat intake is associated with type 2 diabetes (T2D) and cardiovascular disease incidence, but few randomized controlled trials have assessed effects of lean, unprocessed red meat intake on insulin sensitivity and other cardiometabolic risk factors.

OBJECTIVE

This study compared the USDA Healthy US-Style Eating Pattern, low in saturated fat and red meat (<40 g/d red meat; USDA-CON), with a modified version with an additional 150 g/d lean beef as an isocaloric replacement for carbohydrate (USDA-LB) on insulin sensitivity and cardiometabolic risk markers.

METHODS

Participants (7 men, 26 women; 44.4 y old) with overweight/obesity [BMI (kg/m2) = 31.3] and prediabetes and/or metabolic syndrome completed this randomized, crossover, controlled-feeding trial consisting of two 28-d treatments (USDA-CON and USDA-LB) separated by a ≥14-day washout. Insulin sensitivity (primary outcome variable), lipoprotein lipids, apolipoproteins (apoA-I and apoB), and high-sensitivity C-reactive protein (hs-CRP) (secondary outcome variables), in plasma or serum, and blood pressures were assessed at baseline and the end of each diet period.

RESULTS

USDA-LB and USDA-CON did not differ significantly in effects on whole-body insulin sensitivity and other indicators of carbohydrate metabolism, lipoprotein lipids, apoA-I and apoB, hs-CRP, and blood pressures. USDA-LB produced a shift toward less cholesterol carried by smaller LDL subfractions compared with USDA-CON [least-squares geometric mean ratios for LDL1+2 cholesterol of 1.20 (P = 0.016) and LDL3+4 cholesterol of 0.89 (P = 0.044)] and increased peak LDL time versus USDA-CON (1.01; P = 0.008).

CONCLUSIONS

Substituting lean, unprocessed beef for carbohydrate in a Healthy US-Style Eating Pattern resulted in a shift toward larger, more buoyant LDL subfractions, but otherwise had no significant effects on the cardiometabolic risk factor profile in men and women with prediabetes and/or metabolic syndrome.This trial was registered at clinicaltrials.gov as NCT03202680.

Effects of substituting eggs for high-carbohydrate breakfast foods on the cardiometabolic risk-factor profile in adults at risk for type 2 diabetes mellitus

Objectives

To assess effects of egg-based versus non-egg, higher-carbohydrate (CHO) breakfast meals on cardiometabolic health markers in overweight or obese adults with prediabetes and/or metabolic syndrome.

Methods

This randomized, crossover study included two 4-week dietary interventions, separated by a ≥4-week washout. Subjects incorporated into their habitual diets breakfast meals containing either 2 eggs/day for 6 days/week (Egg condition), or energy-matched, non-egg, higher-CHO-based foods (Non-Egg condition). Dietary intakes, insulin sensitivity, and other CHO metabolism indices, lipid biomarkers, high-sensitivity C-reactive protein, and blood pressures were measured.

Results

Thirty men and women with mean age 54.1 ± 1.9 years and body mass index 31.9 ± 0.7 kg/m² provided data. Neither diet condition significantly altered insulin sensitivity indices, but the homeostasis model assessment for insulin resistance was significantly (p = 0.028) higher after the Non-Egg vs. the Egg condition. Low-density lipoprotein cholesterol (LDL-C) was decreased from baseline (119 mg/dL) by 2.9 and 6.0% with Egg and Non-Egg breakfasts, respectively (p = 0.023). Systolic blood pressure was reduced from baseline (127 mm Hg) by 2.7 and 0.0% with Egg and Non-Egg, respectively (p = 0.018). Diet records indicated 149 kcal/day higher (p = 0.008) energy intake from non-study foods during the Egg condition; however, weight change from baseline did not differ between conditions.

Conclusion

Compared with the baseline diet, consumption of 12 eggs/week for 4 weeks at breakfast was associated with less reduction in LDL-C, and more lowering of systolic blood pressure, than observed with non-egg-based, energy-matched, control foods higher in CHO.

Risk of gestational diabetes mellitus in relation to plasma concentrations of amino acids and acylcarnitines: A nested case-control study

AIMS

Gestational diabetes mellitus (GDM) affects between 5 and 10% of all pregnancies in Canada and can lead to adverse health outcomes in both the mother and fetus. Amino acids (AA) and acylcarnitines (AC) have been identified as early biomarkers of type 2 diabetes but their usefulness in screening for GDM has yet to be demonstrated.

METHODS

We conducted a nested case-control study involving 50 controls and 50 GDM cases diagnosed between the 24th and 28th week of gestation. Heparinized plasma samples were obtained during the first and early second trimester of pregnancy. Case and controls were matched according to date of recruitment, maternal age, gestational age at blood sampling as well as pre-pregnancy body mass index. Eight AA and eight AC were quantified using an ultra-high pressure liquid-chromatography quadrupole time-of-flight mass spectrometry platform. Conditional regression analyses adjusted for matching factors and smoking habits during pregnancy were performed to identify plasma metabolites associated with GDM risk.

RESULTS

Odds ratio (OR) and 95% confidence interval (CI) for the prediction of GDM per one standard deviation increase of AA or AC in plasma levels were 0.25 (0.08-0.79) for butyrylcarnitine, 0.31 (0.12-0.79) for glutamic acid, 2.5 (1.2-5.3) for acetylcarnitine, 2.9 (1.3-6.8) for isobutyrylcarnitine and 5.3 (1.7-17.0) for leucine. These five metabolites were selected by stepwise conditional logistic regression to create a predictive model with an OR of 2.7 (1.5-4.9).

CONCLUSION

Whether the identified metabolites can predict the risk of developing GDM requires additional studies in a larger sample of pregnant women.

Substitution of dietary ω-6 polyunsaturated fatty acids for saturated fatty acids decreases LDL apolipoprotein B-100 production rate in men with dyslipidemia associated with insulin resistance: a randomized controlled trial

Background

The substitution of omega (ω)-6 (n-6) polyunsaturated fatty acids (PUFAs) for saturated fatty acids (SFAs) is advocated in cardiovascular disease prevention. The impact of this substitution on lipoprotein metabolism in subjects with dyslipidemia associated with insulin resistance (IR) remains unknown.

Objective

In men with dyslipidemia and IR, we evaluated the impact of substituting ω-6 PUFAs for SFAs on the in vivo kinetics of apolipoprotein (apo) B-containing lipoproteins and on the intestinal expression of key genes involved in lipoprotein metabolism.

Design

Dyslipidemic and IR men (n = 36) were recruited for this double-blind, randomized, crossover, controlled trial. Subjects consumed, in a random order, a fully controlled diet rich in SFAs (SFAs: 13.4% of energy; ω-6 PUFAs: 4.0%) and a fully controlled diet rich in ω-6 PUFAs (SFAs: 6.0%; ω-6 PUFAs: 11.3%) for periods of 4 wk, separated by a 4-wk washout period. At the end of each diet, the in vivo kinetics of apoB-containing lipoproteins were measured and the intestinal expression of key genes involved in lipoprotein metabolism was quantified in duodenal biopsies taken from each participant.

Results

The substitution of ω-6 PUFAs for SFAs had no impact on TRL apoB-48 fractional catabolic rate (Δ = -3.8%, P = 0.7) and production rate (Δ = +1.2%, P = 0.9), although it downregulated the intestinal expression of the microsomal triglyceride transfer protein (Δ = -18.4%, P = 0.006) and apoB (Δ = -16.6%, P = 0.005). The substitution of ω-6 PUFAs for SFAs decreased the LDL apoB-100 pool size (Δ = -7.8%; P = 0.005). This difference was attributed to a reduction in the LDL apoB-100 production rate after the substitution of ω-6 PUFAs for SFAs (Δ = -10.0%; P = 0.003).

Conclusions

This study demonstrates that the substitution of dietary ω-6 PUFAs for SFAs decreases the production and number of LDL particles in men with dyslipidemia and IR. This trial was registered at clinicaltrials.gov as NCT01934543.

The Effect of a Diet Moderately High in Protein and Fiber on Insulin Sensitivity Measured Using the Dynamic Insulin Sensitivity and Secretion Test (DISST)

Evidence shows that weight loss improves insulin sensitivity but few studies have examined the effect of macronutrient composition independently of weight loss on direct measures of insulin sensitivity. We randomised 89 overweight or obese women to either a standard diet (StdD), that was intended to be low in fat and relatively high in carbohydrate (n = 42) or to a relatively high protein (up to 30% of energy), relatively high fibre (>30 g/day) diet (HPHFib) (n = 47) for 10 weeks. Advice regarding strict adherence to energy intake goals was not given. Insulin sensitivity and secretion was assessed by a novel method—the Dynamic Insulin Sensitivity and Secretion Test (DISST). Although there were significant improvements in body composition and most cardiometabolic risk factors on HPHFib, insulin sensitivity was reduced by 19.3% (95% CI: 31.8%, 4.5%; p = 0.013) in comparison with StdD. We conclude that the reduction in insulin sensitivity after a diet relatively high in both protein and fibre, despite cardiometabolic improvements, suggests insulin sensitivity may reflect metabolic adaptations to dietary composition for maintenance of glucose homeostasis, rather than impaired metabolism.

Impact of Dietary Resistant Starch on the Human Gut Microbiome, Metaproteome, and Metabolome

Diet can influence the composition of the human microbiome, and yet relatively few dietary ingredients have been systematically investigated with respect to their impact on the functional potential of the microbiome. Dietary resistant starch (RS) has been shown to have health benefits, but we lack a mechanistic understanding of the metabolic processes that occur in the gut during digestion of RS. Here, we collected samples during a dietary crossover study with diets containing large or small amounts of RS. We determined the impact of RS on the gut microbiome and metabolic pathways in the gut, using a combination of “omics” approaches, including 16S rRNA gene sequencing, metaproteomics, and metabolomics. This multiomics approach captured changes in the abundance of specific bacterial species, proteins, and metabolites after a diet high in resistant starch (HRS), providing key insights into the influence of dietary interventions on the gut microbiome. The combined data showed that a high-RS diet caused an increase in the ratio of Firmicutes to Bacteroidetes, including increases in relative abundances of some specific members of the Firmicutes and concurrent increases in enzymatic pathways and metabolites involved in lipid metabolism in the gut.

This work was undertaken to obtain a mechanistic understanding of the complex interplay between diet and the microorganisms residing in the intestine. Although it is known that gut microbes play a key role in digestion of the food that we consume, the specific contributions of different microorganisms are not well understood. In addition, the metabolic pathways and resultant products of metabolism during digestion are highly complex. To address these knowledge gaps, we used a combination of molecular approaches to determine the identities of the microorganisms in the gut during digestion of dietary starch as well as the metabolic pathways that they carry out. Together, these data provide a more complete picture of the function of the gut microbiome in digestion, including links between an RS diet and lipid metabolism and novel linkages between specific gut microbes and their metabolites and proteins produced in the gut.

Lipid Abnormalities and Inflammation in HIV Inflection

Infection with the human immunodeficiency virus (HIV), and subsequent treatment with antiretroviral therapy (ART), is often associated with perturbations in lipid profiles. Furthermore, persistent inflammation, in spite of suppression of viral replication by ART, likely contributes to modifications in lipid composition and function, exacerbating risk for development of cardiovascular disease (CVD). Increased levels of several pro-inflammatory lipid species, including oxidized low-density lipoprotein (LDL) and high-density lipoprotein (HDL), have been measured in HIV-infected persons and are associated with markers of immune activation. The mechanisms linked to this bidirectional relationship in which inflammation increases lipid levels and promotes their modification, and these modified lipid species perpetuate inflammatory processes, require further investigation. Treatment with statins and other lifestyle modifications, including improvement in dietary intake and exercise, are critical to reducing CVD risk. Well-designed clinical trials that take into account the complex relationships among lipids and inflammation within persons infected with HIV need to be considered.

Short term elevation in dietary protein intake does not worsen insulin resistance or lipids in older adults with metabolic syndrome: a randomized-controlled trial

Background

There is a great deal of controversy as to whether higher protein intake improves or worsens insulin sensitivity in humans. The purpose of the study was to determine the influence of a short-term elevation in dietary protein on hepatic and peripheral insulin sensitivity in twelve older subjects (51–70 yrs) with metabolic syndrome.

Methods

Individuals were randomly assigned to one of the dietary groups: recommended protein intake (RPI, 10% of daily calorie intake) or elevated protein intake (EPI, 20% of daily calorie intake) for 4 weeks. Prior to and immediately following the dietary intervention, subjects were studied with primed continuous infusion of [6,6-²H₂]glucose and [1-¹³C]glucose dissolved in drink during the dual tracer oral glucose tolerance test (DT OGTT) to determine hepatic and peripheral insulin sensitivity. Plasma lipids were measured pre- and post-dietary intervention.

Results

In both intervention groups: 1) hepatic insulin sensitivity as assessed by the endogenous glucose rate of appearance (glucose R_a), 2) peripheral insulin sensitivity as assessed by the metabolic clearance rate of glucose normalized to plasma glucose concentration (MCR) and/or the rate of glucose utilization (R_d) or 3) glucose/insulin AUC were unaffected by the diets. Moreover, fasting lipid was not affected by RPI or EPI.

Conclusion

Our findings suggest that a short-term elevation in EPI with correspondingly higher branched chain amino acid (BCAA) contents has no detrimental impact on hepatic and peripheral insulin sensitivity or plasma lipid parameters in older adults with metabolic syndrome.

Trial registration

ClinicalTrials.gov Identifier: NCT02885935; This trial was registered retrospectively (Study start date, April 01, 2013, date of registration, August 26, 2016).

Electronic supplementary material

The online version of this article (doi:10.1186/s40795-017-0152-4) contains supplementary material, which is available to authorized users.

Odd Chain Fatty Acids; New Insights of the Relationship Between the Gut Microbiota, Dietary Intake, Biosynthesis and Glucose Intolerance

Recent findings have shown an inverse association between circulating C15:0/C17:0 fatty acids with disease risk, therefore, their origin needs to be determined to understanding their role in these pathologies. Through combinations of both animal and human intervention studies, we comprehensively investigated all possible contributions of these fatty acids from the gut-microbiota, the diet, and novel endogenous biosynthesis. Investigations included an intestinal germ-free study and a C15:0/C17:0 diet dose response study. Endogenous production was assessed through: a stearic acid infusion, phytol supplementation, and a Hacl1^−/− mouse model. Two human dietary intervention studies were used to translate the results. Finally, a study comparing baseline C15:0/C17:0 with the prognosis of glucose intolerance. We found that circulating C15:0/C17:0 levels were not influenced by the gut-microbiota. The dose response study showed C15:0 had a linear response, however C17:0 was not directly correlated. The phytol supplementation only decreased C17:0. Stearic acid infusion only increased C17:0. Hacl1^−/− only decreased C17:0. The glucose intolerance study showed only C17:0 correlated with prognosis. To summarise, circulating C15:0 and C17:0 are independently derived; C15:0 correlates directly with dietary intake, while C17:0 is substantially biosynthesized, therefore, they are not homologous in the aetiology of metabolic disease. Our findings emphasize the importance of the biosynthesis of C17:0 and recognizing its link with metabolic disease.

Diets high in resistant starch increase plasma levels of trimethylamine-N-oxide, a gut microbiome metabolite associated with CVD risk

Production of trimethylamine-N-oxide (TMAO), a biomarker of CVD risk, is dependent on intestinal microbiota, but little is known of dietary conditions promoting changes in gut microbial communities. Resistant starches (RS) alter the human microbiota. We sought to determine whether diets varying in RS and carbohydrate (CHO) content affect plasma TMAO levels. We also assessed postprandial glucose and insulin responses and plasma lipid changes to diets high and low in RS. In a cross-over trial, fifty-two men and women consumed a 2-week baseline diet (41 percentage of energy (%E) CHO, 40 % fat, 19 % protein), followed by 2-week high- and low-RS diets separated by 2-week washouts. RS diets were assigned at random within the context of higher (51-53 %E) v. lower CHO (39-40 %E) intake. Measurements were obtained in the fasting state and, for glucose and insulin, during a meal test matching the composition of the assigned diet. With lower CHO intake, plasma TMAO, carnitine, betaine and γ-butyrobetaine concentrations were higher after the high- v. low-RS diet (P<0·01 each). These metabolites were not differentially affected by high v. low RS when CHO intake was high. Although the high-RS meal reduced postprandial insulin and glucose responses when CHO intake was low (P<0·01 each), RS did not affect fasting lipids, lipoproteins, glucose or insulin irrespective of dietary CHO content. In conclusion, a lower-CHO diet high in RS was associated with higher plasma TMAO levels. These findings, together with the absence of change in fasting lipids, suggest that short-term high-RS diets do not improve markers of cardiometabolic health.

Modified Atkins diet induces subacute selective ragged-red-fiber lysis in mitochondrial myopathy patients

Mitochondrial myopathy (MM) with progressive external ophthalmoplegia (PEO) is a common manifestation of mitochondrial disease in adulthood, for which there is no curative therapy. In mice with MM, ketogenic diet significantly delayed progression of the disease. We asked in this pilot study what effects high-fat, low-carbohydrate "modified Atkins" diet (mAD) had for PEO/MM patients and control subjects and followed up the effects by clinical, morphological, transcriptomic, and metabolomic analyses. All of our five patients, irrespective of genotype, showed a subacute response after 1.5-2 weeks of diet, with progressive muscle pain and leakage of muscle enzymes, leading to premature discontinuation of the diet. Analysis of muscle ultrastructure revealed selective fiber damage, especially in the ragged-red-fibers (RRFs), a MM hallmark. Two years of follow-up showed improvement of muscle strength, suggesting activation of muscle regeneration. Our results indicate that (i) nutrition can modify mitochondrial disease progression, (ii) dietary counseling should be part of MM care, (iii) short mAD is a tool to induce targeted RRF lysis, and (iv) mAD, a common weight-loss method, may induce muscle damage in a population subgroup.

Dairy food products: good or bad for cardiometabolic disease?

Prevalence of type 2 diabetes mellitus (T2DM) is rapidly increasingly and is a key risk for CVD development, now recognised as the leading cause of death globally. Dietary strategies to reduce CVD development include reduction of saturated fat intake. Milk and dairy products are the largest contributors to dietary saturated fats in the UK and reduced consumption is often recommended as a strategy for risk reduction. However, overall evidence from prospective cohort studies does not confirm a detrimental association between dairy product consumption and CVD risk. The present review critically evaluates the current evidence on the association between milk and dairy products and risk of CVD, T2DM and the metabolic syndrome (collectively, cardiometabolic disease). The effects of total and individual dairy foods on cardiometabolic risk factors and new information on the effects of the food matrix on reducing fat digestion are also reviewed. It is concluded that a policy to lower SFA intake by reducing dairy food consumption to reduce cardiometabolic disease risk is likely to have limited or possibly negative effects. There remain many uncertainties, including differential effects of different dairy products and those of differing fat content. Focused and suitably designed and powered studies are needed to provide clearer evidence not only of the mechanisms involved, but how they may be beneficially influenced during milk production and processing.

Branched-Chain Amino Acids and Insulin Metabolism: The Insulin Resistance Atherosclerosis Study (IRAS)

OBJECTIVE

Recent studies using untargeted metabolomics approaches have suggested that plasma branched-chain amino acids (BCAAs) are associated with incident diabetes. However, little is known about the role of plasma BCAAs in metabolic abnormalities underlying diabetes and whether these relationships are consistent across ethnic populations at high risk for diabetes. We investigated the associations of BCAAs with insulin sensitivity (SI), acute insulin response (AIR), and metabolic clearance of insulin (MCRI) in a multiethnic cohort.

RESEARCH DESIGN AND METHODS

In 685 participants without diabetes of the Insulin Resistance Atherosclerosis Study (IRAS) (290 Caucasians, 165 African Americans, and 230 Hispanics), we measured plasma BCAAs (sum of valine, leucine, and isoleucine) by mass spectrometry and SI, AIR, and MCRI by frequently sampled intravenous glucose tolerance tests.

RESULTS

Elevated plasma BCAAs were inversely associated with SI and MCRI and positively associated with fasting insulin in regression models adjusted for potential confounders (β = -0.0012 [95% CI -0.0018, -0.00059], P < 0.001 for SI; β = -0.0013 [95% CI -0.0018, -0.00082], P < 0.001 for MCRI; and β = 0.0015 [95% CI 0.0008, 0.0023], P < 0.001 for fasting insulin). The association of BCAA with SI was significantly modified by ethnicity, with the association only being significant in Caucasians and Hispanics. Elevated plasma BCAAs were associated with incident diabetes in Caucasians and Hispanics (multivariable-adjusted odds ratio per 1-SD increase in plasma BCAAs: 1.67 [95% CI 1.21, 2.29], P = 0.002) but not in African Americans. Plasma BCAAs were not associated with SI-adjusted AIR.

CONCLUSIONS

Plasma BCAAs are associated with incident diabetes and underlying metabolic abnormalities, although the associations were generally stronger in Caucasians and Hispanics.

Effect of whey protein on blood lipid profiles: a meta-analysis of randomized controlled trials

Previous studies have suggested that whey supplementation may have beneficial effects on lipid profiles, although results were inconsistent. A literature search was performed in March 2015 for randomized controlled trials observing the effects of whey protein and its derivatives on circulating levels of triacylglycerol (TG), total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C) and high-density lipoprotein cholesterol (HDL-C). A meta-analysis was subsequently conducted. The meta-analysis results of 13 trials showed that whey supplementation significantly reduced the circulating TG level by 0.11 mmol/l (95% CI: -0.21, 0 mmol/l), whereas the whey protein had no effects on circulating TC (-0.11 mmol/l, 95% CI: -0.27, 0.05 mmol/l), LDL-C (-0.08 mmol/l, 95% CI: -0.23, 0.07 mmol/l) and HDL-C (0.01 mmol/l, 95% CI: -0.04, 0.05 mmol/l). Subgroup analysis showed that significant TG reduction disappeared in participants with low body mass index, low supplemental whey dose or under exercise training/energy restriction during the trial. No evidence of heterogeneity across studies and publication bias was observed. In conclusion, our findings demonstrated that the effects of whey protein supplementation were modest, with an overall lowering effect on TG but no effect on TC, LDL-C and HDL-C.

A review of potential metabolic etiologies of the observed association between red meat consumption and development of type 2 diabetes mellitus

Epidemiological studies suggest that red and processed meat consumption is related to an increased risk of type 2 diabetes. However, it is not clearly understood which components of red and processed meat contribute to this increased risk. This review examines potential mechanisms addressing the role of saturated fatty acid, sodium, advanced glycation end products (AGEs), nitrates/nitrites, heme iron, trimethylamine N-oxide (TMAO), branched amino acids (BCAAs) and endocrine disruptor chemicals (EDCs) in the development of type 2 diabetes based on data from published clinical trials and animal models. TMAO which is derived from dietary carnitine and choline by the action of bacterial enzymes followed by oxidation in the liver may be a strong candidate molecule mediating the risk of type 2 diabetes. BCAAs may induce insulin resistance via the mammalian target of rapamycin complex 1 (mTORC1) and ribosomal protein S6 kinase β 1 (S6k1)-associated pathways. The increased risk associated with processed meat compared with red meat suggests that there are interactions between the saturated fat, salt, and nitrates in processed meat and iron, AGEs and TMAO. Intervention studies are required to clarify potential mechanisms and explore interactions among components, in order to make firm recommendations on red and processed meat consumption.

Red meat, dairy, and insulin sensitivity: a randomized crossover intervention study

BACKGROUND

Epidemiologic studies have linked high consumption of red and processed meat with risk of developing type 2 diabetes, whereas high dairy consumption has been associated with decreased risk, but interventions have been limited.

OBJECTIVE

We compared the effects on insulin sensitivity of consuming a diet high in lean red meat with minimal dairy, a diet high in primarily low-fat dairy (from milk, yogurt, or custard) with no red meat, and a control diet that contained neither red meat nor dairy.

DESIGN

A randomized crossover study was undertaken with 47 overweight and obese men and women divided into 2 groups as follows: those with normal glucose tolerance and those with impaired fasting glucose or impaired glucose tolerance. Participants followed the 3 weight-stable dietary interventions for 4 wk with glucose, insulin, and C-peptide measured by using oral-glucose-tolerance tests at the end of each diet.

RESULTS

Fasting insulin was significantly higher after the dairy diet than after the red meat diet (P < 0.01) with no change in fasting glucose resulting in a decrease in insulin sensitivity after the high-dairy diet (P < 0.05) as assessed by homeostasis model assessment of insulin resistance (HOMA-IR). A significant interaction between diet and sex was observed such that, in women alone, HOMA-IR was significantly lower after the red meat diet than after the dairy diet (1.33 ± 0.8 compared with 1.71 ± 0.8, respectively; P < 0.01). Insulin sensitivity calculated by using the Matsuda method was 14.7% lower in women after the dairy diet than after the red meat diet (P < 0.01) with no difference between diets in men. C-peptide was not different between diets.

CONCLUSION

In contrast to some epidemiologic findings, these results suggest that high consumption of dairy reduces insulin sensitivity compared with a diet high in lean red meat in overweight and obese subjects, some of whom had glucose intolerance. This trial was registered at the Australian New Zealand Clinical Trials Registry as ACTRN12613000441718.

Saturated Fats Versus Polyunsaturated Fats Versus Carbohydrates for Cardiovascular Disease Prevention and Treatment

The effects of saturated fatty acids (SFAs) on cardiovascular disease (CVD) risk are modulated by the nutrients that replace them and their food matrices. Replacement of SFAs with polyunsaturated fatty acids has been associated with reduced CVD risk, although there is heterogeneity in both fatty acid categories. In contrast, replacement of SFAs with carbohydrates, particularly sugar, has been associated with no improvement or even a worsening of CVD risk, at least in part through effects on atherogenic dyslipidemia, a cluster of traits including small, dense low-density lipoprotein particles. The effects of dietary SFAs on insulin sensitivity, inflammation, vascular function, and thrombosis are less clear. There is growing evidence that SFAs in the context of dairy foods, particularly fermented dairy products, have neutral or inverse associations with CVD. Overall dietary patterns emphasizing vegetables, fish, nuts, and whole versus processed grains form the basis of heart-healthy eating and should supersede a focus on macronutrient composition.