UK researchers says anyone with an underlying heart condition should be cautious about going on a 'crash' diet.

Crash diets such as popular meal replacement programs can cause short term deterioration in heart function, a UK study has found.

Researchers warn anyone with an underlying heart condition should seek medical advice before adopting a very low calorie diet.

Dr Jennifer Rayner, a clinical research fellow at the University of Oxford says these diets have a very low calorie content and can be effective for losing weight, reducing blood pressure, and reversing diabetes.

"But the effects on the heart have not been studied until now," said Dr Rayner.

The University of Oxford study used magnetic resonance imaging (MRI) to investigate the impact of a calorie restricted meal replacement program on heart function and the distribution of fat in the abdomen, liver, and heart muscle.

Presented at the CMR 2018 meeting in Barcelona, the study involved 21 obese men and women who consumed 600 to 800 kcal per day for eight weeks.

An MRI was performed at the start of the study and again at one and eight weeks.

After one week, total body fat, visceral (deep tissue) fat and liver fat had all significantly fallen. This was accompanied by significant improvements in insulin resistance, fasting total cholesterol, triglycerides, glucose and blood pressure.

However, heart fat content had risen by 44 per cent. This was associated with a deterioration in heart function, including the heart's ability to pump blood.

By eight weeks, heart fat content and function had improved and all other measurements including body fat and cholesterol were continuing to improve.

"The metabolic improvements with a very low calorie diet, such as a reduction in liver fat and reversal of diabetes, would be expected to improve heart function. Instead, heart function got worse in the first week before starting to improve," said Dr Rayner.

Dr Rayner says otherwise healthy people may not notice the initial temporary change in heart function but "caution" is needed in people with heart disease.

"If you have heart problems, you need to check with your doctor before embarking on a very low calorie diet or fasting. People with a cardiac problem could well experience more symptoms at this early time point, so the diet should be supervised."

Australian cardiologist Professor Bob Graham, Executive Director of the Victor Chang Cardiac Research Institute says it appears such a low calorie diet causes fat to be released from the liver and other parts of the body and this "overwhelms" the heart in the short term.

Professor Graham supports the call for caution but is not alarmed by the study.

"There are very very few people who can sustain a diet of 600 to 800 calories for eight weeks, that's a very strict diet and I am surprised they were able to get anyone to stick to it, so I don't think it's a lot of people that we need to worry about who go on to such a severe diet," said Prof Graham.