This article was originally published in the November 2016 edition of the Tufts Health & Nutrition Letter, a publication of the Friedman School of Nutrition Science & Policy at Tufts University.
Substituting unsaturated fats for carbs or saturated fats improves blood sugar control.
Swapping healthy unsaturated fats for carbohydrates or saturated fats may reduce your risk of diabetes, according to a new analysis of 102 randomized trials totaling 4,660 participants. The analysis found that eating more unsaturated fats, especially polyunsaturated fats, in place of either carbohydrates or saturated fats lowers blood sugar levels and improves insulin resistance and secretion.
“This is a positive message for the public,” Dr. Mozaffarian says. “Don’t fear healthy fats.”
“The world faces an epidemic of insulin resistance and diabetes,” says Dariush Mozaffarian, MD, DrPH, dean of the Friedman School of Nutrition Science and Policy at Tufts University and editor-in-chief of the Tufts Health & Nutrition Letter, who is senior author on the study. “Our findings support preventing and treating these diseases by eating more fat-rich foods like walnuts, sunflower seeds, soybeans, flaxseed, fish and vegetable oils and spreads, in place of refined grains, starches, sugars and animal fats.”
The findings reinforce the idea that, contrary to the low-fat craze that began in the 1980s, not all fats are equally bad for you. “This is a positive message for the public,” Dr. Mozaffarian says. “Don’t fear healthy fats.”
The study, published in PLOS Medicine, provides novel quantitative evidence for the effects of dietary fats and carbohydrate on the regulation of glucose and insulin levels and several other factors linked to type 2 diabetes. Rates of insulin resistance and type 2 diabetes are rising sharply worldwide, researchers noted, highlighting the need for new, evidence-based preventive strategies. While a healthy diet is clearly a cornerstone of such efforts, the effects of different dietary fats and carbohydrate on metabolic health have been controversial, leading to confusion about specific dietary guidelines and priorities.
To help clear up that confusion, Dr. Mozaffarian and colleagues identified and combined findings from previously completed randomized controlled “feeding trials” that provided meals varying in the types and amounts of fats and carbohydrates. These studies each evaluated how such variations in diet affected measures of metabolic health, such as blood sugar, blood insulin, insulin resistance and sensitivity, and ability to produce insulin in response to blood sugar. Because these trials directly provided food, and were randomized and controlled, they provide rigorous, robust evidence on how dietary fats and carbohydrates affect these risk factors.
By pooling these 102 trials, Dr. Mozaffarian and colleagues found that exchanging dietary carbohydrates or saturated fats with a diet rich in monounsaturated fats or polyunsaturated fats had a beneficial effect on key markers of blood glucose control. For example, for each 5% of calories switched from carbohydrates or saturated fats to mono- or polyunsaturated fats, participants saw an approximately 0.1 unit improvement in HbA1c, a blood marker of long-term glucose control. Based on prior research, each 0.1 percent reduction in HbA1c should reduce the incidence of type 2 diabetes by 22%, and of cardiovascular diseases by 6.8%.
Among different fats, the most consistent benefits were seen for increasing polyunsaturated fats, in place of either carbohydrates or saturated fat. Replacing 5% of calories from carbohydrates with polyunsaturated fats significantly lowered not only HbA1c but also fasting insulin levels. Similarly, replacing saturated fats with polyunsaturated fats significantly lowered glucose, HbA1c, C-peptide (created when insulin is produced and released) and HOMA-IR (a measure of insulin resistance). Eating more polyunsaturated fats in place of carbohydrate, saturated fat or even monounsaturated fat also significantly improved the body’s ability to secrete insulin - a finding never before confirmed in any dietary intervention.
Given the current global pandemic of type 2 diabetes, the authors hope that these findings will help inform scientists, clinicians and the public on dietary priorities related to dietary fats and carbohydrates and metabolic health.
“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,” says study first author Fumiaki Imamura, PhD, of the University of Cambridge. “By combining results from more than 100 trials, we provide the strongest evidence to-date on how major nutrients alter these risks.”
The findings suggest that exchanging dietary carbohydrate with saturated fat does not appreciably influence markers of blood glucose control. The researchers added that solely emphasizing either lowering consumption of carbohydrates or saturated fats is not the best strategy for reducing diabetes risk.
Instead, keep in mind that some fats are actually healthy, and substitute foods containing those unsaturated fats for less-healthy animal fats and refined carbohydrates, starches and sugars.
Foods high in mono- and polyunsaturated fats include:
Liquid vegetable oils, such as olive, peanut, canola, sunflower, corn and soybean oils, and spreads made from these oils
Nuts such as walnuts, almonds, hazelnuts, cashews, pistachios, peanuts and pecans
Seeds such as pumpkin, flax, chia, sunflower and sesame seeds
Soy foods such as tofu and tempeh
Fish, particularly fatty fish such as salmon, mackerel, herring, lake trout, sardines and albacore tuna
For more information on this and other nutrition science and policy topics, read the Tufts Health & Nutrition Letter, a publication of the Friedman School of Nutrition Science and Policy at Tufts University: Editor-in-Chief, Dariush Mozaffarian, MD, DrPH, and Executive Editor, Alice H Lichtenstein, DSc.
The Gerald J. and Dorothy R. Friedman School of Nutrition Science and Policy is the only graduate school of nutrition in the United States. The mission of the school is to generate trusted science, educate future leaders, and produce real world impact.