We’re all about breaking down scientific findings, especially when the research focuses on nutrition. Most of the time, we are providing important perspective to particularly popular studies, working to ensure that the totality of science is promoted rather than one-off, fringe findings. So instead of running with the trending headlines that somehow convince us that “X food will give us X disease”, we are working to support and promote sound science (especially since these types of findings rarely generate sensationalistic and saucy headlines).
What’s the study?
This meta-analysis from PLOS Medicine examined over 100 randomized control trials (RCTs), the gold standard of study design, to examine the effects of macronutrient intake, specifically saturated fat, mono-unsaturated fat, poly-unsaturated fat, and carbohydrates, on a variety of on health markers that are relevant for indicating the development of diabetes, such as blood sugar, insulin sensitivity, and insulin production in adults over the age of 18.
Why is this study interesting?
This is one of the first studies that systematically evaluated the totality of evidence (that is, using data available up to Nov 2015!) to examine the effects of saturated fat, mono-unsaturated fat, poly-unsaturated fat, and carbohydrates markers relevant for indicating the development of diabetes. Why the interest in diabetes? Diabetes is one of the most common non-communicable disease- in 2014, 387 million people had diabetes, and in addition to pharmaceutical intervention, modifying dietary habits has proven to be a successful strategy in both the prevention and management diabetes.
A chronic disorder, diabetes develops when the pancreas cannot produce enough insulin or when the body can no longer make use of the produced insulin. Not sure where sugar (glucose) comes in? Insulin is a key hormone that allows for the cells in your body to eat up the sugar in your blood, using it for energy. Long-term, improper production or function of insulin raises blood sugar levels (hyperglycemia) and can result in the development of diabetes.
What were the methods?
Broadly, the authors used multiple-treatment meta-regression, which is a fancy way of saying that the authors used statistical methods to analyze and synthesize studies with different interventions. More specifically, the authors analyzed the findings from 102 RCTs. In these studies, 4,660 participants were enrolled to evaluate how macronutrients such as saturated fat, mono-unsaturated fat, polyunsaturated fat, and carbohydrates impact blood sugar regulation as well as insulin sensitivity and production.
What were the overall findings?
Exchanging either carbohydrates or saturated fats for unsaturated fats, especially poly-unsaturated fats, was linked to improved blood glucose levels, insulin resistance, and insulin secretion.
Did the findings have clinical significance?
With more and more meta-analyses being published and significance being determined by p values, it’s important to assess if the “significant” findings have actual clinical significance. The authors found that increasing unsaturated fat intake (mono- or poly-unsaturated) improved three month average glucose levels found in a person’s blood by 0.1%. It has been previously shown that a 0.1% improvement could reduce the incidence of diabetes by 22%, which the authors deem as “clinically meaningful especially given the current global pandemic of diabetes”.
Were there any limitations to the study?
Like all studies, there were quite a few limitations to note: there was a small number of trials for some measured endpoints. Additionally, the authors note that there may be some “potential issues of blinding, compliance, generalisability, heterogeneity due to unmeasured factors, and publication bias”. Additionally, since most of the trials included in the analysis were in North America and Europe, the findings may not be applicable to other populations and regions.
How does the findings relate to the body of evidence?
These findings are largely in alignment with the body of evidence and are an extension of these findings. The USDA’s Nutrition Evidence Library (NEL) evaluates, synthesizes, and grades scientific studies on established methodology and criteria. The Scientific Report of the 2010 Dietary Guidelines Advisory Committee assessed the “effect of replacing a high carbohydrate diet with a high monounsaturated fatty acid (MUFA) diet in persons with type 2 diabetes” and founds that there is moderate evidence to support that increased mono-unsaturated fat intake, rather than high carbohydrate intake, may be beneficial for those with diabetes.
What’s the take home message?
As a whole, these findings may help inform health care professionals, policy makers, and the general public about dietary strategies to improve metabolic health. This research highlights focusing on healthy unsaturated fats found in vegetable oils and spreads, nuts, fish, and vegetables rich in unsaturated fats (e.g., avocado). In addition, the authors point out that their findings should not be extrapolated to carbohydrates found in “fruit, legumes, or minimally processed grains”. Swapping out saturated fat and/or carbohydrates for foods rich in unsaturated fat, especially poly-unsaturated fat can help in controlling blood glucose levels, especially for those who may be prediabetic or for those who have diabetes.