Skip Artificial Sweeteners

They may aggravate a weight problem.

People usually eat food containing artificial sweeteners to keep their weight down, but it often doesn’t help. Now we know more about why. Mice studies suggest that these sweeteners can actually promote weight gain. They may boost the bacteria in our gut, with the deceptively charming name of “Firmicutes,” that seem more likely to trigger fat storage — the same bacteria abundant in the guts of obese mice. In other words, even though the sweetener in a soda doesn’t have a big calorie count, it potentially makes the soda more fattening.

In the latest study, 10-week-old mice ate a daily dose of aspartame, sucralose, or saccharin. Another group of mice drank water laced with either glucose or sucrose, two forms of ordinary sugar. It turned out that after only 11 weeks, the mice on the artificial sweeteners were showing signs of high blood sugar, a precursor to diabetes.

In order to test whether a change in gut bacteria was linked to the problem, the researchers gave the mice broad-spectrum antibiotics that killed their gut bacteria temporarily. This brought the mice back to ground zero. The gut bacteria eventually returned to the pre-sweetener balance and the mice’s blood sugar fell. Eran Segal of the Weizmann Institute of Science in Rehovot, Israel, and co-author of the study, was impressed enough by this result to give up using artificial sweetener in his morning coffee.

More than 90 percent of the bacterial species in the gut come from two subgroups — Bacteroidetes and Firmicutes. The ratio of the two changes with age: we have much less Firmicutes as infants and in our later years. Studies have not consistently shown that overweight or obese people have an unusually unbalanced microbiome; however, in mice there seems to be a clear connection. Obese mice that have a genetic defect that makes them unable to produce leptin, a hormone regulating appetite, have half as many Bacteriodetes and that much more Firmicutes than other mice. When researchers put some of the Firmicutes from obese mice into normal mice, they gained weight. There may be two mechanisms: the Firmicutes seem to lead to more production of the enzymes used to extract calories and also trigger the storage of fat.

Mice studies don’t translate directly to predictions about humans: People are genetically diverse and have specific eating habits that vary in their lifetime. Mice are much less individual and complex.

For example, artificial sweeteners may not be bad for you in particular. In another study, when seven lean volunteers who normally didn’t eat artificial sweeteners consumed saccharin over five days, three of them didn’t experience any change in their gut bacteria or blood sugar. However, four of them did end up with more Firmicutes and higher blood sugar. New York University gastroenterologist Ilseung Cho, who studies human gut bacteria, argues that in some people, as in mice, too many Firmicutes may affect the production of leptin and thus lead people to overeat. It works the other way, too: There is evidence that when people lose weight, they end up with more of the good guys, the Bacteroidetes.

How can you change your diet to modify your gut bacteria in ways that might help you lose weight or keep it off? Try staying away from artificial sweeteners. You also might eat more cereals, beans, and vegetables. One study compared the microbe balance (in feces, which reflects the guts) of children from Burkina Faso and a comparison group of Italian kids. The Burkina Faso children had a higher proportion of Bacteroidetes, which the authors attributed to the fact that their rural diet was much higher in fiber. Bacteroidetes are experts at making tough fibers useful. In fact, when researchers fed humans a cracker made of a wheat starch that was modified to be completely non-digestible, the participants got a boost in Bacteroiodetes. The crackers contained a product now widely used in commercial bakery products, processed meats, and pastas. According to the manufacturer, which helped fund the study, retail consumers can look for products that are touted as “high fiber” and list “modified wheat starch” in the ingredients.   

Meanwhile, drug companies are pouring money into research looking for ways to change gut bacteria for desired results. Pfizer, for example, has teamed up with San Francisco biotech firm Second Genome to study the microbiomes of about 900 people, including those with metabolic disorders that lead to obesity.