'LOW-FAT' foods can contain a similar number of calories as the standard versions and might have more sugar.
A new study found three-in-five consumers eat low-fat and light foods several times a week thinking they are a healthier option.
But a "snapshot sample" of 12 products labelled as low-fat, reduced and light found some minimal differences in calorie content.
A standard McVitie's chocolate digestive contained 85 calories and a light one had 77 -- the lighter one contains less fat but more sugar.
And 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.
And while Kelloggs Special K is marketed as a diet food, it contains more calories than Bran Flakes -- which also has a higher fibre content that would help you feel fuller for longer.
UK consumer watchdog 'Which?' found misconceptions among consumers about the meaning of the terms 'reduced fat' and 'light'.
Identical rules on nutritional labelling apply in both Ireland and the UK under EU rules.
These require 'low fat' foods to contain less than 3pc fat, while 'reduced' 'light' or 'lite' products must have 30pc less fat or saturated fat than the standard product.
United Biscuits, which makes the McVitie's chocolate digestive, said the report was misleading as it focused on calories and "ignored the fact that the light version had 30pc less fat".
A Tesco spokeswoman said: "We take seriously our responsibility to help our customers make healthy choices. Our products display clear nutritional information on the front of the packaging."