So-called low-fat foods can contain a similar number of calories as the standard versions - and might have more sugar, experts have warned.
Consumer watchdog Which? found six out of 10 consumers eat low-fat and light foods several times a week thinking they are a healthier option.
But a "snapshot sample" of 12 low-fat, reduced and light products compared with their standard counterparts found some minimal differences in calorie content, the consumer watchdog said.
Among the foods singled out are;
A standard McVitie's chocolate digestive contained 85 calories and a light one had 77.
The difference of eight calories could be burned off in less than a minute of swimming or running, it found.
A Tesco low-fat yoghurt had more calories per pot at 130 than a standard Activia version at 123, while the Tesco option contained more sugar at 20.2g - more than four teaspoons - than the 16.9g in the Activia pot.
The high fat and saturated fat content of cheese meant Cathedral City lighter cheddar was still rated red under the traffic light labelling system.
Which? found misconceptions among consumers about the meaning of the terms reduced fat and light, with 16% of people correctly responding that products carrying the label had to contain 30% less fat than the standard alternative.
Labelling regulations define low fat as containing less than 3% fat, the terms reduced fat, light and lite mean 30% less fat than the standard or original product, and more than 20g of fat per 100g makes a product high in fat.
Which? has campaigned for clearer food labelling and is calling on Morrisons and Iceland, the two remaining supermarkets yet to adopt the traffic light system, to do so as soon as possible.
United Biscuits, which makes the McVitie's chocolate digestive, said the report was misleading:
A Tesco spokeswoman said:
A Department of Health spokesman added: