For decades, diet gurus have advised slimmers to simply 'eat less' but while that mantra seems easy, it turns out that it's harder than it sounds.

However, Emma Brown, a nutritionist says that broadly one portion counts as 120g of steak, one biscuit and just half a scone.

Here she explains how much of each food group you should really be eating each day.

A portion of red meat is 120g and has 162 calories and 5.4g of fat. Although small amounts of protein regularly throughout the day are good for you, try not to have red meat too often.

A portion of fish is 125g which contains 271 calories and 18.8g of fat. A healthy diet should include at least two portions of fish a week, including one of oily fish.

A portion of potatoes should be 235g with 172 calories per portion, Emma also advised eating the skin for extra nutrients.

Vegetables should be in portions of 80g, there's 27 calories in these carrots and they count as one of your five a day.

Fruit is also 80g per portion, with 28 calories in these blue berries, and Emma warned even overeating fruit or vegetables can lead to weight gain.

A portion of crisps is 30g, roughly equal to a normal packet of crisps, which contains 153 calories and 7.6g of fat.

The portion size for biscuits is 14g - which works out at just one biscuit if your snack of choice is a Bourbon.