Eating too much butter, cakes, biscuits and junk food for just three weeks can dangerously increase internal fat and cholesterol – even if you stay slim.
An Oxford Uni study saw 24 people follow a diet high in either saturated or polyunsaturated fat for up to 24 days.
Those who ate more saturated fat had a 20 percent increase in fat around their liver and 10 percent higher blood cholesterol, despite not gaining weight.
Study leader Nikola Srnic, a PhD student, said: “It is interesting how you can see such a change so quickly, within three weeks.
“Our health is a very complex picture, and although looking at the number on the scale is one measure we have used for many years, other factors influence our health in addition to our body weight.”
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People on the saturated fat diet were asked to increase their consumption of foods including whole fat dairy products, pastries and cakes.
Those on the polyunsaturated fat diet consumed more oily fish like mackerel and salmon, vegetable oils, nuts and seeds.
The latter group saw their total blood cholesterol and “bad” cholesterol levels fall by around 10 percent, and saw an increase in energy reserves in their heart muscle.
Mr Srnic said the findings highlighted the protective role that certain fats can play and the dangers of other fats even in the absence of weight gain.
He added: “It’s the type of fat, in addition to the amount of fat that’s important. Losing weight is the best way to reduce your metabolic disease risk.
“But what we’re seeing here is that you can still get some of the beneficial effects, although not to the same magnitude, if you just alter the type of fat that you consume. If you’re not losing weight, you shouldn’t lose hope.”
The researchers also studied heart muscle cells grown in conditions enriched with each type of fat.
Cells exposed to polyunsaturated fat became more active in taking up fat and breaking it down for energy, which could explain why consuming those foods may help lower the levels of fats in the blood.
Professor James Leiper, associate medical director at the British Heart Foundation, said: “Saturated fat has long been understood to cause a much higher risk of heart and circulatory disease.
“This study adds to this consensus and gives us evidence that saturated fat may silently start to pose a risk to heart health very quickly, without causing any changes to a person’s weight.”
Clare Thornton-Wood, paediatric dietitian at the British Dietetic Association, said: “This is a really interesting study that supports evidence already known that saturated fat intake is linked to an increase in heart disease.”
The research was presented at the European Society of Cardiology’s annual congress in London this weekend.
