Love a sugar hit? Your sweet tooth may hail from an unlikely source: your liver. A hormone made by the organ appears to control how much carbohydrate and sugar we want to eat, and helps slow us down when we are overindulging.
The hormone, called FGF21, has already been found to help obese mice lose weight and regain their sensitivity to insulin. A modified form is currently in clinical trials to test whether it has the same effect in people with diabetes.
Our bodies break down carbohydrates into sugars such as sucrose, glucose and fructose. Recent genetic studies have suggested that people with altered levels of FGF21 consume more carbohydrates.
To find out more, a team co-led by Matthew Potthoff at the University of Iowa observed the eating habits of mice with either abnormally high or low levels of the hormone. They found that mice genetically modified to lack the hormone chose to drink much higher levels of sugar-sweetened drinks than normal mice. Those given an extra dose of the hormone, on the other hand, reduced their sugar intake.
"The hormone, called FGF21, has already been found to help obese mice lose weight and regain their sensitivity to insulin."
The team also showed that the hormone is produced in response to high carbohydrate levels; it then enters the bloodstream, where it sends a signal to the brain to suppress our sugar intake. In people, blood levels of FGF21 triple 24 hours after a spike in blood sugar levels.
When monkeys were given the synthetic version of the hormone being tested in clinical trials, they also opted for a diet low in sugar, according to a separate study by Steven Kliewer at the University of Texas Southwestern Medical Center at Dallas and colleagues. The team also found that these monkeys consumed less alcohol than those that weren’t given the compound.
Interestingly, FGF21 only seems to affect the intake of simple carbohydrates – those found in cakes and biscuits, for example, that get broken down into sugars quickly. It has no effect on complex carbs that take longer to break down.
Other appetite regulating hormones exist, such as ghrelin and leptin made by gut and fat cells respectively, but these act more broadly. FGF21 is the first hormone found to act on a specific nutrient.
Potthoff thinks that the hormone might affect the brain’s reward pathways, dialling down the appeal of otherwise enjoyable sweets and alcohol. The same might apply to addictive drugs, he speculates.
If FGF21 is the simple carbohydrate regulator, the next step is to determine whether other unidentified hormones regulate our intake of complex carbohydrates, proteins and fats. If such compounds exist, a combination could potentially be used to encourage people to eat healthy, balanced diets.