Eating too many cakes, crisps and burgers can start damaging your body in just three weeks, research suggests.

Foods like this that are high in saturated fat are dangerous for the heart even when people do not gain weight, a study by Oxford University found.

In the tests, one group of people were asked to follow a diet high in saturated fat or polyunsaturated fat and another group were asked to eat foods containing healthier polyunsaturated fats - like oily fish and nuts - for up to 24 days.

At the end, neither group had gained weight but those who ate more saturated fat showed a sharp deterioration in basic markers of health and had scan results linked to an increased risk of heart disease.

They also had a 20 per cent increase in the amount of fat stored in livers, heightening risk of type 2 diabetes and saw their levels of total and 'bad' cholesterol rise by roughly 10 per cent.

In contrast, those who ate diets high in polyunsaturated fats saw their health significantly improve.

They saw a drop in total blood cholesterol and 'bad' cholesterol levels of around 10 per cent and an increase in energy reserves in their heart muscle.

Nikola Srnic, the lead researcher from Oxford University, said: 'It's the type of fat, not the amount of fat, that's important.

'It is interesting how you can see such a change so quickly, within three weeks,' the researcher said.

'For diets that were higher in saturated fat we asked people to increase consumption of butter, whole fat dairy products, cakes and pastries.

'The polyunsaturated fat diet is higher in fats from fish, certain vegetable oils and certain nuts and seeds.

'The results suggest that a diet high in saturated fat may negatively change cardiovascular disease risk factors even when a person does not gain weight,' the PhD student said.

'On the other hand, we saw protective effects if a person ate a diet high in polyunsaturated fat.'

WHAT SHOULD A BALANCED DIET LOOK LIKE?

• Eat at least 5 portions of a variety of fruit and vegetables every day. All fresh, frozen, dried and canned fruit and vegetables count

• Base meals on potatoes, bread, rice, pasta or other starchy carbohydrates, ideally wholegrain

• 30 grams of fibre a day: This is the same as eating all of the following: 5 portions of fruit and vegetables, 2 whole-wheat cereal biscuits, 2 thick slices of wholemeal bread and large baked potato with the skin on

• Have some dairy or dairy alternatives (such as soya drinks) choosing lower fat and lower sugar options

• Eat some beans, pulses, fish, eggs, meat and other proteins (including 2 portions of fish every week, one of which should be oily)

• Choose unsaturated oils and spreads and consuming in small amounts

• Drink 6-8 cups/glasses of water a day

• Adults should have less than 6g of salt and 20g of saturated fat for women or 30g for men a day

Source: NHS Eatwell Guide