CHICAGO -- Researchers are nearing completion of a multi-year study that they hope will further our understanding of the physiological impact of fructose and glucose in the human diet.
In an interim report at the annual scientific sessions of the American Diabetes Association, Kimber Stanhope, PhD, RD, said the new study is designed to test the impact of the 2010 recommendations of the US Department of Agriculture's Dietary Guidelines Advisory Committee, which advised that people consume no more than 25% of an individual's total energy from sweetened beverages.
The hopes it will also shed light on the interconnection between sugar intake and lipid changes, and will further illuminate the differences between added fructose and glucose in beverages.
A prior iteration of this study compared 48 young, generally healthy subjects who consumed 25% of their daily energy intake in the form of beverages sweetened with high fructose corn syrup to those who consumed glucose. The results of that study, published in the October 2011 issue of the Journal of Clinical Endocrinology and Metabolism, found that high-fructose corn syrup had a far worse impact on health than did glucose (Stanhope KL, et al. J Clin Endocrinol Metab. 2011; 96(10): 1596-605).
Higher Risk in Two Weeks
Specifically, those who consumed large quantities of high fructose corn syrup beverages experienced a 40% increase in postprandial triglycerides. The high-fructose beverages increased risk factors in healthy adults in as little as two weeks.
"The fact that our studies show consistent postprandial changes in those who consume sugar is a very important finding, as doctors never measure prandial triglycerides. Patients are told to fast before they show up at the doctor's office, and their blood is taken in a fasting state," said Dr. Stanhope, PhD, RD, associate professor of nutritional biology at UC Davis, who has been at the forefront of research on the health effects of various sweeteners.
The current study involves 200 healthy adults taking part in a two-week protocol during which they consumed either high fructose corn syrup or non-caloric sweetened beverages with an artificial agent such as aspartame, Dr. Stanhope explained.
Though the study is not yet completed, she shared some preliminary findings. Subjects who drank the fructose-sweetened beverages experienced an increase in LDL cholesterol by 21%, when they consumed 25% of their daily energy intake as high fructose corn syrup, Stanhope said.
She expects that the study will be completed within the year, with full results to be presented at next year's ADA meeting.
DGAC Recs: Hardly Healthy
The main motive behind the original study was to test the recommendations made in the 2010 report from the Dietary Guidelines Advisory Committee, and co-sponsored by the Institute of Medicine, which suggested a maximal intake of 25% or less of total energy from added sugars.
While the "25% or less" guideline is certainly an improvement over the heavy added-sugar loads consumed by many Americans, a number of researchers have questioned whether the recommendation would really have any meaningful public health impact.
Getting one quarter of one's daily calories from sweetened beverages is still a large glycemic load. Further, researchers are coming to realize that various types of sugar are not necessarily equivalent in terms of their physiological impact.
Dr. Stanhope's earlier study found that postprandial triglycerides increased by 150% or more in those who consumed 25% of their daily energy as high-fructose corn syrup beverages. There was no such increase in triglycerides among those on diets high in complex carbohydrates but little added sugar, she said.
Those who consumed 25% of their energy as glucose experienced a 30% decrease in postprandial triglyceride levels, indicating that glucose and fructose are metabolically quite different.
She stressed that the adverse effects of fructose on the cardiovascular and hepatic systems may be offset when the sugar is found in fruit, as opposed to artificially sweetened beverages.
"An apple has a larger amount of fructose than any other fruit, but in comparison to a beverage, the apple also has fiber and hundreds of other micronutrients. We don't know how all the micronutrients are affecting the pathways by which fructose turns into fat, because no-one has ever done a study as to the difference."
Dr. Stanhope recently collaborated with researchers at Laval University, Quebec, in a study of 32 overweight/obese adults who consumed glucose- or fructose-sweetened beverages (25% energy requirement) with their ad libitum diets for 8 weeks. This was followed by a 2-week period during which they drank the sweetened beverages in conjunction with a standardized, energy-balanced diet.
The investigators measured plasma variables including plasma acylation stimulating protein (ASP), adiponectin, and leptin concentrations at baseline, 2, 8, and 10 weeks, and assessed the measurements relative to energy intake, body weight, adiposity, circulating triglycerides, and insulin sensitivity.
Among the key findings they noted that at 10 weeks, fasting adiponectin levels decreased in both the glucose and the fructose-consuming groups. These decreases were inversely associated with baseline intra-abdominal fat volume. Sugar consumption increased fasting leptin, which was associated with body weight changes.
However, 24-hour leptin profiles increased during glucose consumption and decreased during fructose consumption. The changes correlated with changes of 24-h insulin levels (Rezvani R, et al. Obesity. 2013; Epub ahead of print).
Fructose vs Aspartame
In contrast to the earlier study, which took place between 2005 and 2008, the new protocol assigns some participants to a steady intake of beverages containing non-caloric artificial sweeteners.
A number of prominent researchers and many health advocates have questioned the safety of sweeteners like aspartame. Dr. Stanhope stressed that her study is not designed to assess the safety of artificial sweeteners, but rather to compare them against high-fructose corn syrup and glucose in terms of short-term impact on cardiometabolic risk profiles.
She acknowledged that there are "a couple of epidemiological studies that show that people who consumed the most aspartame are the most overweight," but added that this observation may have been confounded by the fact that, "people who are overweight are usually taking aspartame in an active attempt to lose weight."
Some opponents of aspartame claim that long-term use can increase the risk of cancer. Dr. Stanhope said she had never seen an article in the scientific literature associating aspartame with cancer, adding that "cancer would never show up in a 6 month study anyway."
She noted that the Dietary Guidelines Advisory Committee is expected to update its recommendations in 2014.