Q&A: Is High-Fructose Corn Syrup Just Sugar?

Reading time: 4 – 6 minutes

Question: I’ve heard high-fructose corn syrup is bad, but I’ve also seen ads that say it’s just sugar. What’s the truth?

Answer: There are many different kinds of sugar. The most familiar one is table sugar, which has the chemical name sucrose. Sucrose is a disaccharide, which means that it’s made up of two sugar units. Each sugar unit is called a monosaccharide (“single sugar unit”). The two monosaccharides that make up sucrose are glucose (the most common sugar in nature) and fructose (“fruit sugar”). The cells can use both glucose and fructose as sources of energy, but the digestive tract can’t absorb sucrose. Instead, a digestive enzyme called sucrase breaks sucrose down into its glucose and fructose components, which are absorbed into the bloodstream.

Sucrose is a popular table sweetener and component of foods, but for a variety of reasons, it’s more expensive than certain sources of pure glucose. This makes glucose syrup — especially corn-derived glucose syrup (“corn syrup”) — a cheap and attractive sweetener in processed foods. While corn doesn’t taste sweet on the tongue, it’s full of starch. Starch — the chemical amylose — is made up of long chains of glucose molecules. The amylose molecule is much too large to bind to the sweetness receptor in the human mouth, but the digestive tract uses an enzyme called amylase to break amylose into its glucose units, which are then absorbed. In essence, amylose (even though it isn’t sweet) is really just glucose. It’s possible to break down the amylose in corn into pure glucose, which is how corn syrup is made. However, glucose isn’t as sweet on the tongue as sucrose, which means it’s not desirable to use corn syrup in foods in place of sucrose; the foods won’t taste quite the same. By processing glucose syrup with another enzyme called invertase, manufacturers convert about half of the glucose into fructose, which is much sweeter than either glucose or sucrose. A mix of 50:50 glucose and fructose is approximately as sweet as sucrose. This mixture is called high-fructose corn syrup (HFCS).

HFCS is very similar to but not chemically identical to sucrose. While each is made up of 50:50 (or nearly 50:50) glucose and fructose, in HFCS, the glucose and fructose units aren’t chemically bonded together. In sucrose, by comparison, they are. Of course, this difference disappears as soon as the digestive enzyme sucrase breaks sucrose down into its constituent monosaccharides, but some scientists feel that there is evidence that despite their great similarities, HFCS and sucrose don’t act identically in the body. This observation has yet to be explained. Indeed, a recent study found that high-fructose corn syrup causes more, and faster, weight gain than table sugar. Another study suggests that HFCS promotes more weight gain per calorie consumed than other sugars, including sucrose [1]. And a third study suggests that the fat gained as a result of HFCS consumption is particularly likely to concentrate in the abdominal region, which is associated with increased cardiovascular disease risk [2].

Another hypothesis regarding HFCS is that, while it might not be any worse than sucrose in its effects on the body, it’s not found in nature. Instead, sources of HFCS include fast food and highly processed food, which are lower in nutritive value and higher in fat — including unhealthy saturated fat and trans fat — than whole food. This hypothesis holds that HFCS is a marker of unhealthy food in general, and therefore those who consume the most HFCS are those consuming the least healthy diets.

Q&A is a biweekly series answering YOUR questions about the science of health. Have a question? Use the contact form to ask us!

References

1. Light et al. The type of caloric sweetener added to water influences weight gain, fat mass, and reproduction in growing Sprague-Dawley female rats. Exp Biol Med (Maywood). 2009 Jun;234(6):651-61.
   View abstract
2. Bocarsly et al. High-fructose corn syrup causes characteristics of obesity in rats: Increased body weight, body fat and triglyceride levels. Pharmacol Biochem Behav. 2010 Nov;97(1):101-6.
   Link to abstract