Sugars, added sugars, and sweeteners — some of the greatest threats to human health — are back on the discussion table, with a new twist. Results from a recently published study, which combined decision-making tests with brain imaging data, show that fructose helps our brains to find high-calorie foods more appealing — in other words, it leads us to overeat.
Fructose occurs naturally in fresh fruits — thus, many people think of fructose as a “healthier” alternative to other types of sugar. However, fruits contain very small amounts of this sugar. In contrast, fructose is present in much larger amounts in sweetened products such as soda.
The study was carried out with a group of 24 healthy volunteers — researchers first asked the volunteers to rate their hunger and desire to eat on a one-to-10 scale (from “not at all” to “very much”). Then, they asked the volunteers to drink cherry-flavored water containing either fructose or glucose. Researchers measured levels of glucose, fructose and insulin in blood samples collected from all volunteers. They also measured levels of leptin and ghrelin, which are enzymes involved in controlling hunger and satiety. In addition, they showed all volunteers images of food and neutral objects, and asked them to rate their desire to eat — not only, researchers at the same time performed functional magnetic resonance imaging brain scans on the volunteers. Finally, researchers showed images of high-calorie food to the volunteers, and asked them to choose between food (given at that time), or a monetary award (given a month later.)
The researchers found that, when compared to glucose, fructose resulted in increased activity in the brain’s reward regions, led to greater hunger and desire for food, and promoted a willingness to give up long-term monetary rewards to obtain immediate high-calorie foods.
Although there was no difference in the levels of hunger and satiety hormones between fructose and glucose drinkers, the insulin response was clearly lower in fructose drinkers. Kathleen Page, senior author of the study, told the New York Times that the lower insulin response may affect what we eat. “Insulin is released when we consume glucose,” she said. “The pancreas secretes insulin, and insulin drives glucose into cells so that it can be used for energy. But it also sends a signal to the brain that says ‘you’ve eaten.’ Fructose doesn’t stimulate insulin secretion, and if there’s no insulin, you don’t get the information that you’re full.”
All together, these results are not surprising — rather, they almost unequivocally confirm what already suspected. Robert Lustig, professor of pediatric endocrinology at University of California, San Francisco, explained a few years ago that instead of helping to sate us, fructose fools our brains into thinking we are not full, so we overeat.
Fructose is responsible for additional problems. According to Lustig, excess fructose cannot be converted into energy by the mitochondria inside our cells. Instead, our cells turn excess fructose into liver fat, starting a cascade of insulin resistance that leads to chronic metabolic disease, diabetes and heart disease.
How can we respond to the new findings? As Alice Walton says: “The real moral might be just to eat more natural foods, and worry less about what kind of added sugar is in our foods and drink – we shouldn’t be eating these anyway. In other words, if we’re really eating healthy, that question should be moot.”