Shopping at Lidl may provide fat discounts but is also associated with pounds that are better shed than saved, a study has shown.

Customers of cut price supermarkets are likely to be heavier and fatter than those who shop at expensive city centre stores, say researchers.

The French study, which named Lidl as an example of a "hard discount" supermarket, found a similar trend among people who visited stores far from where they lived.

Discount shopping was associated with higher body mass index (BMI) and waist circumference (WC) even after adjusting for social background and distance from the store. However, the link was stronger among shoppers with a poorer education.

Efforts to improve eating habits, such as promoting healthy foods, should target specific supermarkets, the scientists suggest.

The study, published in the online journal Public Library of Science ONE, was conducted in Paris between 2007 an d 2008.

A total of 7,131 people shopping in more than 1,000 different supermarkets were surveyed.

Researchers compared customers from a range of Parisian supermarkets, including the upmarket Monoprix chain, large "hypermarkets" such as Cora, and the "hard discount" stores Aldi, Ed and Lidl.

The team led by Dr Basile Chaix, from the INSERM research institute in Paris, wrote: "After controlling for individual and residential neighbourhood SES (socio-economic status) and distance to the supermarket, and using the Monoprix brand (expensive citymarkets located in city centres) as the referent, participants shopping in certain supermarket brands, especially hypermarkets such as Cora or in hard discount supermarkets such as Ed or Lidl, had greater BMI and WC."

There was a "strong interaction" between education levels and discount shopping. The association between shopping in a hard discount store and greater body weight was "markedly stronger for lower education levels" said the researchers.

Conversely, people who shopped in organic stores were much more likely to have a lower BMI and slimmer waists.

The study also found that just 11.4% of participants went food shopping mainly in their own neighbourhood.

This was significant because previous research on shopping habits tended to assume people bought food near where they lived.

The researchers added: "Causal effects of supermarket brand, type, and SES (in support of which our observational data do not provide solid evidence) may stem from the differential availability of healthy foods such as fruits/vegetables or fish, from the availability of low-cost energy-dense foods in hard discount supermarkets, or from the differential advertisement, showcasing, or nutritional labelling of these products in the different supermarkets, all of which may constrain or influence individual purchasing behaviour".

Strategies targeting food-buying behaviour in specific supermarkets may be an "efficient strategy" because supermarkets "are the very place where dietary preferences are concretely materialised and translated into a definite set of purchased foods", they said.

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