Concern over the impact of sugar on health has grown over the past several decades. The World Health Organization’s 2015 report on sugar consumption recommends limiting free sugar intake (which does not occur naturally in food) to below 10% of daily calories, based in part on data showing that increasing free sugar consumption commonly results in an energy surplus and consequential weight gain that may contribute to obesity. Some researchers have even proposed warning labels for sugar-sweetened beverages.
Added sugars go by many names, but, with few exceptions, all are either glucose, fructose, or some combination of the two. Glucose is obtained from the diet and produced in the body. It can be considered a universal energy source for all cells. Blood glucose levels are tightly regulated because levels that are too high or too low have pathologic consequences. By contrast, fructose is obtained almost exclusively from the diet and is quickly removed from circulation to be metabolized primarily by the liver and, to a lesser extent, fat cells and muscle tissue.
Much of the discussion over the health impact of added sugars has focused on fructose. The central issue with fructose, as illustrated in Figure 1, is that it bypasses the main rate-limiting steps of glycolysis in the liver and therefore acts as an unregulated substrate for fat synthesis and uric acid production. There is no shortage of research publications blaming fructose for causing insulin resistance and related cardiometabolic diseases such as type 2 diabetes because of the way it is handled in the body. Further evidence implicating a role of fructose in obesity and diabetes comes from genetic evidence that suggests humans developed a mutation that enhances the effect of fructose to increase fat stores, which may at one time have provided a survival advantage, but is now working against us in the modern world (although this “thrifty gene” hypothesis is not without controversy).
Reference: Khitan et al. J Nutr Metab. 2013 May.
The fructose hypothesis is not a settled matter, though. Meta-analyses of observational data have shown that although sugar-sweetened beverages are associated with an increased risk of type 2 diabetes, other sources of fructose, such as 100% fruit juices, are not. Another recent meta-analysis reported that total sugar intake and fructose intake were not associated with increased incidence of type 2 diabetes. If fructose was uniquely detrimental to health, then one could reasonably expect it to be associated with disease regardless of its dietary source. However, this does not appear to be the case. Furthermore, the fructose hypothesis has been criticized for its heavy reliance on unrealistic and circumstantial evidence derived from animal models that use extremely high doses of pure fructose.
Clinical trials in humans also do not support a link between fructose and diabetes. One meta-analysis evaluated how chronically consuming small doses of fructose (22-36 grams per day, which is below the average American intake of 49 grams) in place of an equal amount of other carbohydrates positively impacted glycemic control, with fructose significantly lowering HbA1c and fasting blood glucose but not fasting insulin. Another meta-analysis restricted to people with type 2 diabetes reported similar findings even though the median intake of fructose was substantially higher at 60 grams per day, with a range of 22-137 grams per day. For reference, a standard 12-ounce can of soda has about 22 grams of fructose.
The most common comparator carbohydrate in these meta-analyses was starch. Realistically, however, people at risk for developing diabetes or concerned with managing their blood sugar levels are likely to swap out other added sugars for fructose, rather than other carbohydrates in general. Additionally, neither meta-analysis evaluated the acute effects of fructose on post-meal glycemia, which has been linked to an increased risk of all-cause mortality, diabetes, and cardiovascular disease, in both people with and without impaired glucose tolerance.
To address these knowledge gaps, a group of researchers published two companion meta-analyses focusing on the effect of substituting fructose for an equal amount of glucose or sucrose. The first meta-analysis evaluated the acute effects on post-meal glycemia while the second evaluated the chronic effects on glycemic control.
Concern over the health effects of added sugars has grown over the past several decades, and much of this attention has been given to fructose as being particularly detrimental because of the unique way the body handles it metabolically. However, the role of fructose in cardiometabolic diseases such as type 2 diabetes remains controversial. Current evidence suggests that it may benefit long-term glycemic control compared to other carbohydrates. Two companion meta-analyses add to the evidence by comparing the effects of fructose to other sugars, rather than carbohydrates in general, on post-meal glycemia and long-term glycemic control.
Other Articles in Issue #33 (July 2017)
Remember what you see with vitamin D
Low levels of vitamin D are correlated with cognitive problems. But can taking vitamin D improve cognition in people who are healthy?
One whey to go for exercise performance recovery
It’s well known that adding protein supplementation to strength training improves gains over the long term. Supplementation’s effects in the short term are less well studied.
Can fasted exercise increase fat oxidation in women?
Recent findings suggest that fasted aerobic exercise makes the body use relatively more fat for fuel over course of a day. Until now, though, this research was done mainly in men.
Interview: Denise Minger
In this issue, we chat with author, health consultant, and public speaker Denise Minger about a host of topics, ranging from veganism to her experience writing about nutrition.
Do probiotics improve quality of life in seasonal allergies?
Bacteria in the gut can influence the immune system, and the immune system plays a large role in seasonal allergies. So can probiotics influence allergic symptoms?
Are probiotics effective for treating small intestinal bacterial overgrowth?
Bacteria can cause trouble when they grow too much in places they shouldn’t, like the small intestine. Could probiotics prevent or reverse this process?
Interview: Marie Bragg, PhD
Dr. Bragg received her doctorate in clinical psychology before moving on to research obesity and food policy. In this interview, we discuss food marketing, the psychology of weight loss and maintenance, and more.