Posted by:

Weight loss supplements loosen dietary control

People who take weight loss supplements are less likely to control other areas of their diet – a well-described psychological effect known as moral licensing.

Weight loss supplements are big business despite most of them having little evidence to support their miraculous weight-loss claims. Fat burners, fat blockers, metabolism boosters, and appetite suppressants – these are the popular categories of pills and potions that fly off pharmacy and health food store shelves.

The most a person could hope for from the myriad of weight-loss supplements available is a placebo effect driving initial weight loss. Putting weight loss effects aside, medical researchers are now asking the question if a fixation on using a ‘pill’ to solve a weight problem can come with some unintended side-effects.

Borrowing from psychological theory, the presence of one perceived positive health behaviour may give license to follow a less healthy behaviour. It is called moral licensing where a ‘good’ act boosts the likelihood that more liberal morals are applied to other behaviours.  A classic example would be someone who goes to the gym and then allows themselves unhealthy reward treats after.

Could turning to weight loss supplements which are a clear statement of a weight loss goal behaviour, mean a person is more likely to let other aspects of their diet slip? To answer this, 74 healthy adults, who were unaware of the research question, were randomly allocated to take either a weight loss supplement or a placebo pill. What the participants didn’t know, was that every pill was a placebo pill.

After giving ratings on the size, shape, colour and texture of the pill they took, each person then completed a questionnaire measuring how they thought they were making progress in their weight loss goals. After the questionnaire, each person was offered a reward drink (bubble cup tea) where they could choose how much sugar they wanted in it.

The final stage of the experiment involved each person taking part in a ‘taste test’ of different types of confectionery where they could consume as much as they wished.

So how did the people who were told they were given a ‘weight loss supplement’ fare compared to those who knew they were taking a placebo? People taking the weight loss supplement consumed 29% more of the confectionery in the taste test and asked for almost double the amount of sugar in their drink. This same group of people though were more likely to report that they were making greater progress towards their weight loss goals.

Breaking the results down further, the researchers could see that the knowledge that they were taking a weight loss supplement was driving the participants’ perceived progress towards weight loss goals. And the further people reported that they were moving towards their goals, the more confectionery they ate in the taste test. The full study was published in the journal Appetite.

The results of this study do not stand alone. Previous research has found that people taking multivitamins are more likely to slacken off in other important health behaviour areas.

What it all means
Dieters beware: taking part in actions that are perceived as healthy can give psychological licence for self-indulgence. People taking weight loss supplements are best advised to keep their main goal of weight loss in mind and make consistent choices across all their lifestyle habits.


Confused about the mixed soup of nutrition messages being stirred through the media? Tim maintains an active nutrition blog at where you’ll find the latest nutrition research and controversies discussed in straight forward language, distilling out what you need to know for your better health.

Leave a Reply