In light of the obesity epidemic, dietary guidance focuses on reduction of overall caloric intake from any source. At the same time, sugar is targeted by many groups as a potential food component to limit, while others suggest that inclusion of some sugars makes other foods more palatable (for example, high-fiber cereals and chocolate milk). Sugars appear in many forms in foods: sucrose (table sugar), comprised of fructose and glucose linked together; high-fructose corn syrup (HFCS), which contains fructose and glucose as single molecules; honey; agave nectar (primarily fructose); and crystalline fructose, which is added to a few foods in small amounts, are some examples.
Of the many forms of sugars, fructose currently is in the spotlight more than others. In part, this is related to a perceived increase in the prevalence of fructose malabsorption or intolerance. Fructose malabsorption refers to a low capacity to absorb fructose by the gastrointestinal tract. As a result of malabsorption, an individual may (or may not) experience symptoms of intolerance such as bloating, flatulence, and loose stools. The ability to absorb fructose varies widely, but controlled clinical studies suggest that up to 50 percent of Americans may be unable to absorb 25 grams of pure fructose. This number rises to 80 percent when individuals in these studies are asked to drink 50 grams of fructose at one time. Intimidating as these numbers may seem, they should be interpreted in light of two factors: (1) the way fructose malabsorption is defined in a clinical study and (2) the amounts of fructose consumed in a typical diet. A recent article reviewed over 20 clinical human studies examining fructose malabsorption and intolerance with these considerations in mind.
The test to diagnose fructose malabsorption measures the amount of hydrogen in expired breath after an individual drinks a fructose solution on an empty stomach. If the fructose is not absorbed, it is fermented in the large intestine to produce gases which cause the symptoms described above. Outside of a clinical study, fructose is not consumed as a single beverage, but rather it is a component of other sugars, foods, or part of the total diet. This makes a difference when clinical information is considered. Of particular importance is that glucose is known to significantly enhance fructose absorption, and glucose occurs with fructose in most foods that contain sugar. When the amounts of glucose and fructose in a food are about equal, fructose should be completely absorbed. In some foods, the number of fructose molecules is greater than the number of glucose molecules. The amount of fructose over that of glucose can be considered the “net fructose.” Consumption of larger quantities of foods that have a net fructose greater than zero could result in symptoms of intolerance. Results of clinical studies suggest that in individuals with a normal absorption capacity, 25 grams of “net fructose” should still be tolerated without a problem. When the amount of fructose and glucose in foods is examined, it is apparent that consuming more than 25 grams of net fructose at one time would occur only rarely. For example, in order to reach this level of net fructose, one would have to drink over 3 cups of apple juice or 1.5 liters of HFCS-sweetened soda in one sitting.
HFCS, by its name, sounds as though it provides a large amount of fructose, but in fact a 12-oz soda made with HFCS has about the same amount of net fructose as a medium apple. This is because the two most common types of HFCS are made up of 55 or 42 percent fructose – the other portion being glucose. Agave syrup, a popular new sweetener, has a higher net fructose compared to other sweeteners.
Knowing that it is the amount of net fructose that can result in problems with malabsorption, it is helpful to compare amounts given in research settings with what is consumed on a daily basis. The estimated average amount of fructose consumed in the U.S. population is 49 grams per day per person. Adolescent males have the highest estimated average fructose intake at 75 grams per day. The majority of this fructose is consumed in the form of sucrose and HFCS, which contain approximately equal amounts of fructose and glucose. The likelihood of malabsorption and intolerance should be low when fructose at these levels is consumed over the course of a day, in combination with glucose and other macronutrients.
Certainly, there remain some individuals with a very low capacity to absorb fructose. For those who do suffer from fructose intolerance, the American Gastroenterological Association suggests limiting fruits, honey, and alcohol. Another approach is to eliminate all fructose-containing foods, then add these back into the diet gradually as tolerated.
While scientific studies of fructose malabsorption and intolerance provide interesting information about the human absorption capacity, application to practical consumption is limited. As scientists and regulators continue to debate the role of sugars (including fructose) in the diet, it is important to consider how foods are consumed outside of the laboratory, and their contributions to the total diet. Healthcare practitioners should evaluate each case and make recommendations suitable to the individual. Certainly, recommendations for daily intake of sugars should assist Americans with making informed and healthful choices that meet the Dietary Guidelines for Americans, 2010.