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.

Citation: Imamura F, Micha R, Wu JHY, de Oliveira Otto MC, Otite FO, Abioye AI, et al. (2016) 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. PLoS Med 13(7): e1002087. doi:10.1371/journal.pmed.1002087
Published: July 19, 2016

Abstract
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.

Author Summary
Why Was This Study Done?

  • Effects of dietary fat and carbohydrate on metabolic health have been controversial, leading to confusion about specific dietary guidelines and priorities.
  • To date there has not been a systematic evaluation of all available evidence to quantify the effects of dietary fat (saturated, monounsaturated, and polyunsaturated fat), and carbohydrate on various markers mediating the development of diabetes, including blood sugar, insulin sensitivity, and ability to produce insulin.

What Did the Researchers Do and Find?

  • We systematically identified and summarized findings of 102 randomised controlled trials, including a total of 4,660 participants, that provided meals varying in the types and levels of fat and carbohydrate to study participants and evaluated how such variations affected various measures of blood glucose control, insulin sensitivity, and ability to produce insulin.
  • The findings suggest exchanging dietary carbohydrate with saturated fat does not appreciably influence markers of blood glucose control.
  • On the other hand, substituting carbohydrate and saturated fat with a diet rich in unsaturated fat, particularly polyunsaturated fat, was beneficial for the regulation of blood sugar.

Wheat_productsWhat Do These Findings Mean?

  • These findings may help inform scientists, clinicians, and the public on dietary priorities related to dietary fats and carbohydrates and metabolic health.
  • This investigation suggests that consuming more unsaturated fats in place of either carbohydrates or saturated fats will help improve blood glucose control. Sole emphasis on lowering consumption of carbohydrates or saturated fats would not be optimal.
  • Translated to foods, these findings support benefits of increasing consumption of vegetable oils and spreads, nuts, fish, and vegetables rich in unsaturated fats (e.g., avocado), in place of either animal fats or refined grains, starches, and sugars.

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