Reaching for a diet soda may actually hinder weight loss efforts, a new study done in mice suggests.

In experiments, researchers found that the artificial sweetener aspartame, which is found in some diet drinks, may contribute to the development of a condition called "metabolic syndrome," which involves a cluster of symptoms, including high blood pressure, high cholesterol levels and a large waist size. People with metabolic syndrome face an increased risk of heart disease, stroke and diabetes.

The researchers found how aspartame could be linked with metabolic syndrome: Aspartame may stop a key gut enzyme from performing its work in breaking down fat during digestion. [7 Biggest Diet Myths]

"This is the novel mechanistic insight," and it may explain why diet drinks can be ineffective at helping people to lose weight, Dr. Richard Hodin, senior author of the study and a professor of surgery at Harvard Medical School, told Live Science.

The study included three separate experiments. In the first, researchers added the gut enzyme to solutions of diet soda and regular soda. They found that the activity of the enzyme was significantly lower in the solutions of diet soda as compared to the solutions of regular soda.

Normally, the enzyme — intestinal alkaline phosphatase, or IAP — works in the intestines to break down cholesterol and fatty acids. Previous studies by the researchers showed that levels of IAP may be linked with people's risk of obesity, diabetes and metabolic syndrome.

In the second experiment, researchers looked at the effects of aspartame in the small intestines of male mice. They injected either a solution containing aspartame or a placebo solution of salt water into the small intestines of the mice, and then measured the same gut enzyme's activity. They found that 3 hours later, the enzyme's activity was 50 percent lower in mice that were injected with the aspartame solution as compared to the mice injected with the salt water.

In the final experiment, the researchers looked at four groups of male mice over a period of 18 weeks: The mice were all allowed to eat as much food as they wanted, but two groups were given normal food and two groups were given high-fat food. Within each division, one group received water to drink that contained aspartame, while the other received regular water to drink. [5 Experts Answer: Is Diet Soda Bad for You?]

At the end of this experiment, the researchers found that the mice given the high-fat food and the aspartame-infused water gained more weight than the mice fed a high-fat food and the regular water.

However, the aspartame did not seem to make a difference for the mice on the normal diet: There was no difference in weight gain between the group given aspartame water and the group given regular water.

In addition to the difference in weight gain among the mice on the high-fat diet, the researchers also found that the mice on both diets that drank the aspartame-infused water had significantly higher levels of glucose intolerance than mice that drank the regular water. Glucose intolerance is a condition in which the body has trouble using the glucose in the bloodstream, leading to higher levels of glucose in the blood, and which may lead to diabetes.

Also, both of the groups of mice that drank the aspartame-infused water showed increased markers of inflammation, which previously has been linked to the development of the metabolic syndrome, the researchers said.

The new results should be confirmed in experiments with a larger sample size, the researchers said. In addition, only male mice were used in the experiment, which is significant because male and female mice tend to react differently to dietary changes; for example, male mice tend to have higher levels of diet-induced inflammation, and female mice have better insulin sensitivity.

It remains unclear whether the findings may apply to people.

The effects of aspartame may be more complicated in humans, due to a "number of factors related to human behavior," Hodin told Live Science. However, he said, all else being equal, "diet drinks are probably not a great alternative to sugary sodas. Water is probably better."

Original on Live Science.