Effects of Saturated Fat, Polyunsaturated Fat, Monounsaturated Fat, and Carbohydrate on Glucose Insulin Homeostasis: A Systematic Review and Meta analysis of Randomised Controlled Feeding Trials
Background Effects of major dietary macronutrients on glucose insulin homeostasis remain controversial and may vary by the clinical measures examined. We aimed to assess how saturated fat (SFA) monounsaturated fat (MUFA) polyunsaturated fat (PUFA) and carbohydrate affect key metrics of glucose insulin homeostasis. Methods and Findings We systematically searched multiple databases (PubMed, EMBASE, OVID, BIOSIS, Web of Knowledge, CAB, CINAHL, Cochrane Library, SIGLE, Faculty1000) for randomised controlled feeding trials published by 26 Nov 2015 that tested effects of macronutrient intake on blood glucose, insulin, HbA1c, insulin sensitivity, and insulin secretion in adults aged =18 years. We excluded trials with non isocaloric comparisons and trials providing dietary advice or supplements rather than meals. Studies were reviewed and data extracted independently in duplicate. Among 6,124 abstracts, 102 trials, including 239 diet arms and 4,220 adults, met eligibility requirements. Using multiple treatment meta regression, we estimated dose response effects of isocaloric replacements between SFA, MUFA, PUFA, and carbohydrate, adjusted for protein, trans fat, and dietary fibre. Replacing 5% energy from carbohydrate with SFA had no significant effect on fasting glucose +0.02 mmol/L, 95% CI 0.01, +0.04; n trials 99) but lowered fasting insulin 1.1 pmol/L; 1.7, 0.5; n 90) Replacing carbohydrate with MUFA lowered HbA1c 0.09% 0.12, 0.05; n 23) 2 h post challenge insulin 20.3 pmol/L; 32.2, 8.4; n 11) and homeostasis model assessment for insulin resistance (HOMA IR) 2.4% 4.6, 0.3; n 30) Replacing carbohydrate with PUFA significantly lowered HbA1c 0.11% 0.17, 0.05) and fasting insulin 1.6 pmol/L; 2.8, 0.4) Replacing SFA with PUFA significantly lowered glucose, HbA1c, C peptide, and HOMA. Based on gold standard acute insulin response in ten trials, PUFA significantly improved insulin secretion capacity +0.5 pmol/L/min; 0.2, 0.8) whether replacing carbohydrate, SFA, or even MUFA. No significant effects of any macronutrient replacements were observed for 2 h post challenge glucose or insulin sensitivity (minimal model index) Limitations included a small number of trials for some outcomes and potential issues of blinding, compliance, generalisability, heterogeneity due to unmeasured factors, and publication bias. Conclusions This meta analysis of randomised controlled feeding trials provides evidence that dietary macronutrients have diverse effects on glucose insulin homeostasis. In comparison to carbohydrate, SFA, or MUFA, most consistent favourable effects were seen with PUFA, which was linked to improved glycaemia, insulin resistance, and insulin secretion capacity.
PLoS medicine