Most so-called weight-loss food is often as fattening as standard fare and not worth buying, according to a consumer watchdog.
One healthy-option exception is low-fat supermarket sausages, which contain half as many calories, 81% less fat and 80% less saturated fat.
But a range of food from Asda, Morrisons, Sainsbury's, Tesco, M&S and Waitrose was found to offer little or no benefit over standard versions, Which? said.
Kellogg's Special K, marketed as a slimming cereal, had the same amount of calories (171 per 30g) as Cornflakes and even more than Branflakes.
Weight Watchers white bread (68 kcal per 29 g slice) was so similar to Warburtons (69 kcal per 29 g slice) and Asda Danish (63 kcal per 25 g slice), that Which? recommended consumers bought the one they preferred.
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."
But a Kellogg's spokeswoman said: "Consumers aren't stupid. The reason Special K is one of the UK's biggest selling cereals is because it works. Bran Flakes and Corn Flakes are an equally good choice if you're following a low calorie and low fat diet - which we've been saying for years."