A probe into the multi-million pound slimming industry has found that many healthy foods and weight loss drugs are not worth paying for.

The results of the investigation will come as a blow to the thousands of Northern Ireland people planning to shed this year’s Christmas bulge with low fat meals and slimming pills.

And the findings of the investigation have the backing of the Food Standards Agency, which advocates a sensible and balanced diet as the best way to lose and keep off unwanted weight.

Ruth Balmer, Food Standards Agency Northern Ireland dietition, said: “Rather than going on an extreme diet that doesn't contain the balance of nutrients we need to be healthy, there is a simple way to maintain a healthy weight by only eating as much food as you need, eating a healthy balanced diet, drinking more water and starting to get active.

“Don’t be tempted by the promise of super-fast results. The way to reach a healthy weight — and stay there — is to adapt your lifestyle gradually to reduce the amount you eat, improve your diet and be more active.

“Crash diets aren't good for your health and they don't work in the longer term.”

According to a survey carried out by Which, over half of the 2,500 members it questioned admitted they had bought food or drinks to aid weight loss in the last year. However, an analysis of calorie, fat and sugar levels of some so-called diet and light brands by the consumer organisation has shown they provide little benefit over standard versions.

Kellogg’s Special K, often perceived as a slimming cereal and marketed as a tool to aid weight loss, has the same calories (171kcal per 30g) as Kellogg’s Cornflakes and even more than Kellogg’s Branflakes (157kcal).

Weight Watchers thick-sliced white bread (68kcal per 29g slice) is nutritionally so similar to Warburtons Toastie sliced white (69kcal per 29g slice) and Asda Danish white bread (63kcal per 25g slice) that Which? recommends buying the one you prefer.

And while some products have less fat than others, the investigation found they have a high salt or sugar content, making little overall difference in terms of health.

McVitie’s Lights digestive biscuits have less fat than McVitie’s Original biscuits, but actually have more sugar (2.9g rather than 2.5g per 15g biscuit), so the difference between biscuits is 4kcal.

The investigation also found huge differences in nutritional values according to the store — M&S’s Count on Us lasagne has 440 calories, less than the M&S standard range, which has 620kcal. However, a standard lasagne from Morrisons has 464kcal, and the Waitrose version has 468kcal — not that different to the M&S healthy option.

Which? also looked at over-the-counter weight-loss supplements and did not think that any were proven to help lose weight in the long-term.

Body Perfect Fat Burner Capsules, Bio Synergy provided no evidence of clinically significant weight loss and one expert felt that Lipobind, Goldshield was just “an expensive bulking agent”.

As part of the investigation to test the weight-loss claims of so-called healthy foods, Which? bought a range of items from Asda, Morrisons, Sainsbury’s, Tesco, M&S and Waitrose, then compared the nutritional information on the packaging of standard and healthy ranges.

The organisation also visited high-street pharmacies and health shops and bought readily available over-the-counter weight-loss supplements including Body Perfect Fat Burner Capsules, Bio Synergy; Lipobind, Goldshield; Adios Tablets, Diomed Herbals; Hoodia Gordonii Capsules, Bio Synergy; Slimmers Aid, Gerard House; Slimthru Oil Sachets, Melbrosin UK Ltd; and Zotrim, Natures Remedies Ltd.

Researchers contacted the manufacturers and asked them to |provide the scientific evidence for the claims made on their packaging. The evidence was reviewed by an expert panel including a pharmacist, an obesity specialist and a dietitian.

Nikki Ratcliff, head of services research at Which?, said: “If you’re looking for a New Year quick-fix to shed a few pounds, weight-loss products aren’t the answer.

“The harsh reality is that exercise coupled with a healthy balanced diet is the only effective way to lose weight.

“Just because foods are labelled as light or advertised as diet brands, it doesn’t mean that they’re the lowest calorie option.

“Look at other similar products on the shelf — you might find some that don’t brand themselves as light actually have fewer calories or less fat or less sugar, so you’d be better off buying them instead.”