The health impacts of dietary fat and carbohydrate are hotly debated topics. Although health authorities are continuing to recommend that saturated fat (SFA) intake be reduced from current levels there has been recent controversy about the relationship between SFA and coronary heart disease (CHD) risk.
This controversy exists in part because it is now recognized that the macronutrient that replaces saturated fat greatly influences CHD risk. For example, when polyunsaturated fat (PUFA) replaced SFA, an analysis of over 130,000 men and women who were followed for as long as 32 years found that CHD risk was reduced by 25%. However, when refined carbohydrate and sugars replaced saturated fat, risk was not reduced.1 Unfortunately, in their attempt to decrease SFA intake, many Americans increased their intake of refined carbohydrate.
The coronary benefit of PUFA replacing SFA in the diet is largely attributed to the reduction in LDL-cholesterol that results. However, new research suggests that the benefit of PUFA is not limited to this one metric as a recently published meta-analysis found pronounced benefits of PUFA on glycemic control.2
The analysis in question involved 102 controlled trials, including 239 diet arms and 4,220 adults. Using multiple-treatment meta-regression, Imamura et al.2 estimated the dose-response effects of isocaloric replacements between SFA, monounsaturated fat (MUFA), PUFA and carbohydrate, adjusted for protein, trans fat, and dietary fiber.
The key findings from this analysis are highlighted below:
- Replacing 5% energy from carbohydrate with SFA had no significant effect on fasting blood glucose, but lowered fasting insulin.
- Replacing carbohydrate with MUFA lowered glycosylated hemoglobin and insulin resistance.
- Replacing carbohydrate with PUFA significantly lowered glycosylated hemoglobin and fasting insulin.
- Replacing SFA with PUFA significantly lowered glucose, glycosylated hemoglobin, C-peptide and insulin resistance.
- PUFA significantly improved insulin secretion capacity whether replacing carbohydrate, SFA or MUFA.
Not surprisingly, given the above findings, Imamura et al.2 concluded that their meta-analysis “provides evidence that dietary macronutrients have diverse effects on glucose-insulin homeostasis. In comparison to carbohydrate, SFA, or MUFA, most consistent favorable effects were seen with PUFA, which was linked to improved glycaemia, insulin resistance, and insulin secretion capacity.”
References
- Li, Y., Hruby, A., Bernstein, A.M., Ley, S.H., Wang, D.D., Chiuve, S.E., Sampson, L., Rexrode, K.M., Rimm, E.B., Willett, W.C., et al. Saturated fats compared with unsaturated fats and sources of carbohydrates in relation to risk of coronary heart disease: A prospective cohort study. J Am Coll Cardiol. 2015, 66, 1538-48.
- Imamura, F., Micha, R., Wu, J.H., de Oliveira Otto, M.C., Otite, F.O., Abioye, A.I., and Mozaffarian, D. 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. 2016, 13, e1002087.