Sugar Consumption, Metabolic Disease and Obesity: The State of the Controversy

A review article published in Clinical Reviews in Clinical Laboratory Sciences discusses the current scientific evidence which fosters an environment of controversy about the health effects of sugar consumption.  The reviewers evaluated research which examined both direct and indirect effects of added sugars on the development of metabolic disease which, for the case of this review, refers to the presence of cardiovascular disease (CVD), type 2 diabetes (T2DM), and non-alcoholic fatty liver disease (NAFLD). Reviewers opted not to review the evidence regarding added sugars, hypertension, and central obesity.

Reviewers asserted the current literature has identified two pathways in which added sugars contribute to metabolic disease. As noted in the review, “Directly, the fructose component in sugar causes dysregulation of lipid and carbohydrate metabolism. Indirectly, sugar promotes positive energy balance, thus body weight gain and fat gain, which also cause dysregulation of lipid and carbohydrate metabolism.”  The reviewers also noted,  “considerable epidemiological evidence suggesting intake of added sugars and/or sugar-sweetened beverages are associated with the presence of unfavorable lipid levels, insulin resistance, fatty liver, T2DM, CVD, metabolic syndrome, visceral adiposity, and hyperuricemia” which generally demonstrates an association even after adjustment for body mass index (BMI) and total energy intake. Reviewers pointed specifically to a CVD risk assessment from the National Health and Nutrition Examination Survey III (NHANES), which suggested that consuming 15% of daily calories from added sugars, the average consumption in the United States, was associated with an 18% increased risk for CVD.  Reviewers cited a number of plausible mechanisms by which consumption of added sugars may contribute to both CVD and T2DM. These mechanisms included increased de novo lipogenesis, dyslipidemia (specifically very low density lipoproteins and triglycerides), higher circulating uric acid levels, reduced fatty acid oxidation, and reduced insulin sensitivity.

Reviewers then looked at various diet intervention studies which had investigated the effects of added sugars on various blood lipid measures. However, researchers noted that the “obvious limitation to these studies is that the sugar-sweetened beverages were consumed with the subjects’ own usual ad libitum diets for all or part of the study, thus the total amount of added sugar that the participants consumed is unknown.” Later in the review, similar studies are compared but again, because sugar was consumed as a part of the subjects’ ad libitum diet, there could be a number of confounders which could influence the result.

Throughout the review, the authors noted a number of studies which studied the effects of sugar consumption under isocaloric circumstances. It appears from these studies that added sugar consumption contributes to metabolic syndrome; however, reviewers asserted that there are numerous limitations to these studies which reduce their generalizability. Reviewers quote Kahn and Sievenpiper, noting, “there is no clear or convincing evidence that any dietary or added sugar has a unique or detrimental impact relative to any other source of calories on the development of obesity or diabetes. Sugar is purely a highly palatable source of energy.” Reviewers then highlighted three industry funded studies; all of which concluded that there are no adverse health effects associated with consumption of added sugar. When reviewers looked at recent meta-analyses, they found the results to “range from yes to equivocal to no regarding the effects of fructose or sugar consumption on the risk factors for metabolic disease.”

Due to inconclusiveness of the existing literature, researchers suggested initiation of “clinical trials, lasting 4 weeks (longer would be better) in which healthy subjects consume added sugar as part of energy-balanced diets that are prepared/provided as per experimental protocol throughout the entire study.” Reviewers briefly discussed the policy and economic implications of these studies, referring to the recent city-mandated taxes on sugar sweetened beverages. Reviewers stated their belief that well-controlled studies are essential to “make progress in implementing the policies that will change the food environment into one that does not promote the development of obesity and metabolic disease; especially for implementing policies that may threaten the profits of the sugar and beverage industries.”