BOSTON (July 19, 2016)--Eating more unsaturated fats, especially polyunsaturated fats, in place of either dietary carbohydrate or saturated fats lowers blood sugar levels and improves insulin resistance and secretion, according to a new meta-analysis of data from 102 randomised controlled feeding trials in adults.
The study, led by Dariush Mozaffarian M.D., Dr.P.H., dean of the Friedman School of Nutrition Science and Policy at Tufts University, and Fumiaki Imamura, Ph.D., at the Medical Research Council (MRC) Epidemiology Unit, University of Cambridge, provides novel quantitative evidence for the effects of dietary fats and carbohydrate on the regulation of glucose and insulin levels and several other metrics linked to type 2 diabetes.
The results were published in PLOS Medicine on July 19.
Rates of insulin resistance and type 2 diabetes are rising sharply worldwide, 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.
"The world faces an epidemic of insulin resistance and diabetes," said Mozaffarian, 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 other vegetable oils and spreads, in place of refined grains, starches, sugars, and animal fats."
"This is a positive message for the public," he added. "Don't fear healthy fats."
In their study, Imamura, Mozaffarian and colleagues performed the first systematic evaluation of all available evidence from trials to quantify the effects of different types of dietary fat (saturated, monounsaturated and polyunsaturated) and carbohydrate on key biological markers of glucose and insulin control that are linked to development of type 2 diabetes.
The team identified and summarized findings from 102 randomised controlled trials, involving a total of 4,660 adult participants, which provided meals that varied in the types and amounts of fat and carbohydrate. The team then evaluated how such variations in diet affected measures of metabolic health, including blood sugar, blood insulin, insulin resistance and sensitivity, and ability to produce insulin in response to blood sugar.
The researchers found that exchanging dietary carbohydrate or saturated fat with a diet rich in monounsaturated fat or polyunsaturated fat had a beneficial effect on key markers of blood glucose control. For example, for each five percent of dietary energy switched from carbohydrates or saturated fats to mono- or polyunsaturated fats, there is an approximately 0.1 percent reduction in HbA1c, a blood marker of long-term glucose control. The authors note that based on prior research, each 0.1 percent reduction in HbA1c is estimated to reduce the incidence of type 2 diabetes by 22 percent and cardiovascular diseases by 6.8 percent.
"Among different fats, the most consistent benefits were seen for increasing polyunsaturated fats, in place of either carbohydrates or saturated fat," said Imamura, who is first author on the study.
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," Imamura said. "By combining results from more than 100 trials, we provide the strongest evidence to-date on how major nutrients alter these risks."
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Additional authors of the study are Renata Micha, Ph.D., research assistant professor at the Friedman School of Nutrition Science and Policy; Jason H. Y. Wu, Ph.D., M.Sc., senior research fellow at the Food Policy group at The George Institute for Global Health, The University of Sydney, Sydney Medical School; Marcia C. de Oliveira Otto, Ph.D., M.S., assistant professor of Epidemiology, Human Genetics, and Environmental Sciences at the University of Texas Health Science Center at Houston; Fadar O. Otite, M.D., neurology resident at University of Miami Miller School of Medicine/Jackson Memorial Hospital; and Ajibola I. Abioye, M.D., M.P.H., research fellow in the Department of Global Health and Population at the Harvard T.H. Chan School of Public Health.
The study was supported by the National Heart, Lung, and Blood Institute of the National Institutes of Health under award number HL085710 and the Medical Research Council Epidemiology Unit Core Support (MC_UU_12015/5). For competing interests disclosure, please see the study.
Imamura F, Micha R, Wu JHY, Otto MC de O, Fadar O. Otite, Abioye AI, et al. Effects of Saturated Fat, Polyunsaturated Fat, Monounsaturated Fat, and Carbohydrate on Glucose- Insulin Homeostasis: A Systematic Review and Metaanalysis of Randomised Controlled Feeding Trials. PLoS Med. 2016;13(7):e1002087. doi:10.1371/journal.pmed.1002087
About the Friedman School of Nutrition Science and Policy
The Gerald J. and Dorothy R. Friedman School of Nutrition Science and Policy at Tufts University is the only independent school of nutrition in the United States. The school's eight degree programs - which focus on questions relating to nutrition and chronic diseases, molecular nutrition, agriculture and sustainability, food security, humanitarian assistance, public health nutrition, and food policy and economics - are renowned for the application of scientific research to national and international policy.
About the MRC Epidemiology Unit
The MRC Epidemiology Unit is a department at the University of Cambridge. It studies the genetic, developmental and environmental factors that cause obesity, type 2 diabetes and related metabolic disorders. The outcomes from these studies are then used to develop strategies for the prevention of these diseases in the general population. http://www.mrc-epid.cam.ac.uk
About the Medical Research Council
The Medical Research Council (MRC) is at the forefront of scientific discovery to improve human health. Founded in 1913 to tackle tuberculosis, the MRC now invests taxpayers' money in some of the best medical research in the world across every area of health. Thirty-one MRC-funded researchers have won Nobel prizes in a wide range of disciplines, and MRC scientists have been behind such diverse discoveries as vitamins, the structure of DNA and the link between smoking and cancer, as well as achievements such as pioneering the use of randomised controlled trials, the invention of MRI scanning, and the development of a group of antibodies used in the making of some of the most successful drugs ever developed. Today, MRC-funded scientists tackle some of the greatest health problems facing humanity in the 21st century, from the rising tide of chronic diseases associated with ageing to the threats posed by rapidly mutating micro-organisms. http://www.mrc.ac.uk
About the University of Cambridge
The mission of the University of Cambridge is to contribute to society through the pursuit of education, learning and research at the highest international levels of excellence. To date, 90 affiliates of the University have won the Nobel Prize.
Founded in 1209, the University comprises 31 autonomous Colleges, which admit undergraduates and provide small-group tuition, and 150 departments, faculties and institutions.
Cambridge is a global university. Its 19,000 student body includes 3,700 international students from 120 countries. Cambridge researchers collaborate with colleagues worldwide, and the University has established larger-scale partnerships in Asia, Africa and America.
The University sits at the heart of one of the world's largest technology clusters. The 'Cambridge Phenomenon' has created 1,500 hi-tech companies, 14 of them valued at over US$1 billion and two at over US$10 billion. Cambridge promotes the interface between academia and business, and has a global reputation for innovation. http://www.cam.ac.uk
Journal
PLOS Medicine