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Using findings from 100 RCTs, researchers summarize the effects of different dietary fats and carbohydrate on insulin resistance and type 2 diabetes.
With rates of insulin resistance and type 2 diabetes mellitus (T2DM) rising, there is a need for new, evidence-based preventive strategies. A healthy diet is the cornerstone, but the effects of different dietary fats and carbohydrate on metabolic health have led to confusion about specific dietary guidelines and priorities. A new study shows that eating healthy fats instead of carbohydrates or saturated fats improves risk factors for patients with T2DM.
“The world faces an epidemic of insulin resistance and diabetes. Our findings support preventing and treating these diseases by eating more fat-rich foods like walnuts, sunflower seeds, soybeans, flaxseed, fish, and other vegetable oils and spreads, in place of refined grains, starches, sugars, and animal fats,” said senior author Dariush Mozaffarian, MD, DrPH, dean of the Friedman School of Nutrition Science and Policy at Tufts University.
The researchers conducted 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.
Their review identified and summarized the findings of 102 randomized controlled trials, including a total of 4,660 participants who were provided meals varying in the types and levels of fat and carbohydrate.
The researchers found that replacing 5% energy from carbohydrate with saturated fat had no significant effect on fasting glucose, but lowered fasting insulin. Replacing carbohydrate with monounsaturated fat lowered hemoglobin A1c (HbA1c), 2-hour post-challenge insulin, and homeostasis model assessment for insulin resistance (HOMA-IR).
Replacing carbohydrate with polyunsaturated fat significantly lowered HbA1c and fasting insulin. Replacing saturated fat with polyunsaturated fat significantly lowered glucose, HbA1c, C-peptide, and HOMA.
Based on gold-standard acute insulin response in 10 trials, polyunsaturated fat significantly improved insulin secretion capacity whether replacing carbohydrate, saturated fat, or even monounsaturated fat. No significant effects of any macronutrient replacements were observed for 2-hour post-challenge glucose or insulin sensitivity.
For each 5% of dietary energy switched from carbohydrates or saturated fats to mono- or polyunsaturated fats, there was an approximately 0.1% reduction in HbA1c level. The researchers note that prior research has shown that each 0.1% reduction in HbA1c is estimated to reduce the incidence of type 2 DM by 22% and cardiovascular diseases by 6.8%.
“Until now, our understanding of how dietary fats and carbohydrate influence glucose, insulin, and related risk factors has been based on individual studies with inconsistent findings,” said lead author Fumiaki Imamura, PhD, at the Medical Research Council Epidemiology Unit at the University of Cambridge in Cambridge, United Kingdom. “By combining results from more than 100 trials, we provide the strongest evidence to-date on how major nutrients alter these risks.”
The researchers stated their findings suggest 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.”
The researchers published their results in PLoS Medicine.
Reference: Imamura F, et al. 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 Medicine. July 19, 2016.