Low-fat foods 'could cause weight gain'

Behind the Headlines

Monday December 5 2011

Researchers said some low-fat foods may confuse the body

Fat-free foods could “work against dieters”, The Daily Telegraph has reported. The newspaper said that fat substitutes may “confuse the body, gearing it up to receive calories that are never delivered”.

The news is based on research in rats fed various combinations of full-fat and reduced-fat Pringles crisps over four weeks. The diet crisps contained a controversial artificial fat substitute called olestra, which mimics the taste and sensation of eating fat but cannot be absorbed during digestion. Olestra is used in many foods in the US, but isn’t available in the UK.

When all the rats were later put on a high-fat feed, those that had previously popped the diet crisps just could not stop putting on weight and fat. In fact, they put on even more weight than rats who had eaten full-fat crisps. The researchers suggested that this is because fat substitutes interfere with the body’s ability to predict a particular food’s calorie content based on texture, taste and smell.

While this news may interest crisp-loving rodents looking to lose weight, it can be difficult to apply these findings to humans, who may behave differently when dieting or choosing food. However, dieters can always modify their diet to one naturally low in fat, rather than switching to foods containing fat substitutes.


Where did the story come from?

The study was carried out by researchers from Purdue University and was funded by the US National Institutes for Health. It was published in the peer-reviewed journal Behavioural Neuroscience.

This study was originally published in June 2011 and reported on today by The Daily Telegraph. The coverage was accurate.


What kind of research was this?

This was an animal study examining whether consuming fat-substituted potato crisps would interfere with the learned relationship between sensory cues of fat and calories, and whether this would in turn cause them to gain weight and fat.

The researchers said that animal research has shown that sensory cues from foods that are rich in sugar or fat can trigger certain physical processes, such as the release of hormones or changes in the metabolism. However, they added that low-calorie ingredients that mimic sugar and fat may interfere with these responses and undermine the learned behaviour that sweet or fatty foods are a rich source of calories.

As it is easier to control the diets of laboratory animals, it can be difficult to extrapolate findings in rats to humans. Ideally, a randomised controlled trial would be performed in humans to test whether this is relevant to human weight loss or dieting.


What did the research involve?

The researchers took rats and divided them into four groups, each of which was given one of the following dietary courses:

  • high-fat feed supplemented by a diet containing full-fat crisps
  • high-fat feed supplemented by a diet containing both full-fat and low-fat crisps (given in a random order)
  • normal feed supplemented by a diet containing full-fat crisps
  • normal feed supplemented by a diet containing both full-fat and low-fat crisps

The rats received 5g of crisps a day for 28 days, after which all four groups were switched to high-fat feed (without crisps) for a further 16 days.

The potato crisps used were regular (full-fat) and light (reduced-fat) versions of Pringles original and sour cream and onion flavours. In the US, the lighter crisps contain olestra, a fat substitute which cannot be absorbed and therefore has no calorific or nutritional value. Olestra is not currently used in foods in the UK.

The reason some mice were given a mixture of full-fat and reduced-fat crisps was to see whether it would weaken the association between the sensation of tasting fat and receiving calories.

Body weight, body composition and food consumption were monitored throughout the study.


What were the basic results?

Among the two groups of rats who started on a high-fat diet, there was greater food intake, body weight gain and proportion of the body made up by fat among rats who received the mixed crisps than among rats fed full-fat crisps.

When the rats were given normal feed there was no difference in food intake, weight gain or body composition, regardless of what type of crisp they received. However, when the mice stopped receiving crisps, and were switched to the high-fat feed, those that had previously been on the normal feed and receiving a combination of crisps put on more weight and were fatter than the mice that had been given the high-fat crisps.


How did the researchers interpret the results?

The researchers concluded that these results support the theory that animals use “the sensory properties of food to predict the consequences of consuming food”. They say that these findings call into question the conventional wisdom that low-calorie and no-calorie substitutes that mimic sweetness and fat can be used to reduce food intake, weight gain and body fat.



In this study, it was found that rats receiving crisps with varying fat content ate more, gained more weight and were fatter than rats that only consumed full-fat crisps when given alongside a high-fat diet. An effect on food intake, weight and fat was also observed if rats were switched to high-fat feed from normal feed after the rats stopping receiving reduced-fat crisps containing olestra.

These findings, like many findings from rat trials, cannot be directly applied to humans. Among other factors, the rats were not trying to lose weight. Anyone’s attempts to lose weight are likely to involve a complex mix of willpower, knowledge of the calories in food, the ability to realise that certain foods will not make you feel full and active choices over what to eat. However, dieters can always modify their diet to one naturally low in fat, rather than switching to diet foods containing fat substitutes.

Analysis by Bazian

Edited by NHS Choices

Links to the headlines

'Fat-free' foods could work against dieters. The Daily Telegraph, December 5 2011

Links to the science

Swithers SE, Ogden SB, Davidson TL. Fat substitutes promote weight gain in rats consuming high-fat diets. Behavioral Neuroscience, 125, 2011, 512-518.


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