For some time, nutritionists have suspected that artificial sweetener - often used as a substitute for sugar in coffee or added as an essential ingredient in diet sodas - does not help people lose weight. However, scientists have struggled to understand why this is the case.

Now, researchers from the Massachusetts General Hospital (MGH) have found a lead. The results of their study on this subject was published in the journal "Applied Physiology, Nutrition and Metabolism".

Richard Hodin's team investigated a sweetener called aspartame (which has the EU ingredients code E951). Along with the salt aspartame-acesulfame (E962), it is among the most commonly used sweeteners in the world.

Food producers add Aspartame to products that claim to contain "zero-sugar", such as soda drinks, sweets like bubble gum, ready-made dairy products, baked goods and instant coffee.

Switching off the diet-effect

Why does aspartame not aid weight loss? "We found that aspartame blocks a gut enzyme called intestinal alkaline phosphatase (IAP)," explains Professor Hodin, who teaches at Harvard Medical School.

IAP is produced in the small intestine. "We previously showed [this enzyme] can prevent obesity, diabetes and metabolic syndrome [a disease characterized by a combination of obesity, high blood pressure, a metabolic disorder and insulin resistence]. So, we think that aspartame might not work because, even as it is substituting for sugar, it blocks the beneficial aspects of IAP."

However, aspartame does not block the enzyme directly. It does so through one of its intestinal breakdown products called phenylalanine.

Watch out! The combination of fatty food and soda-drinks may cause obesity.

Sweetener causes higher blood sugar levels

The researchers confirmed their suspicions via a variety of tests on mice. In one case, they fed IAP directly to mice, who were also on a high-fat diet. It turned out that the IAP could effectively prevent the emergence of the metabolic syndrome. It also helped relieve symptoms in animals that were already suffering from the obesity-related illness.

The researchers then showed that the activity of IAP was reduced when an aspartame solution was injected directly into the small intestine. When this was done with a sugary or salty solution, the function of IAP remained active.

Subsequently, the researchers took four groups of mice. Two of them received a normal diet, the other two a high-fat diet. One of each of them received water to drink and the other an aspartame solution.

The amount of sweetener solution given to the normal diet group was equivalent to a human drinking three-and-a-half cans of soda a day. For the high-fat group it amounted to the equivalent of two cans.

Thirsty: it's better to have drinks that contain neither sugar nor sweetener!

In the end, there were hardly any noticeable differences between the two groups of mice that were fed a normal diet. But among the groups that received a high-fat diet, the mice that drank the sweetener solution became considerably heavier than those who drank plain water.

However, the blood sugar level was higher than normal in all of the mice that received sweetener solution. This is usually an indicator of glucose intolerance. All of these mice also experienced an increase of a protein called TNF-alpha in their blood. This protein is typically related to inflammations that are common in cases of metabolic syndrome.

What does that mean for humans? You'd be better off drinking plain water, or tea without sugar or sweetener, than grabbing a "zero-sugar" refreshment.