Scientists are now saying that high fat, low carbohydrate diets can reduce energy to the heart, and they suspect this could be damaging.
In a new study an Oxford University team say it is unclear at this stage whether this could have a damaging impact on health and more research is needed.
Although many people have lost weight quickly by following such diets, scientists have expressed concern that in the longer term they may not be good for health.
In this study the Oxford team monitored 19 people over a two week period and they found that the energy stored in the heart was reduced by an average of 16% among those who followed a high fat, low carbohydrate diet, and in some people the energy reduction was as much as a third.
It was also found that their hearts became slightly 'stiffer' and not relaxing quite as well as before the diet.
Apparently one of the participants could not manage his daily run while on the diet.
These changes were however reversed within two weeks after returning to a normal diet.
Lead researcher Professor Kieran Clarke says that people with illnesses such as diabetes, obesity and heart disease also had lower than normal heart energy levels.
He says a more severe form of energy depletion is one of the features of heart failure, where patients struggle with physical exertion because they have little in reserve.
Professor Clarke says the study provided some interesting insights into how extreme diets might be affecting people, but further research needs to be carried out on a larger sample over a longer period.
Clarke says the body is remarkably adaptive and it is not known whether the hearts would have gradually returned to normal had the 19 remained on the diet long-term.
The British Heart Foundation (BHF), which funded the work, said the results reinforced their view that people should not follow high fat, low carb diets to lose weight.
They would not recommend high fat-low carb diets to anyone who wanted to lose weight and look after their heart.
Professor Peter Weissberg, BHF medical director, says the research does not suggests that the high fat-low carb regime is going to give people heart failure, but people should be aware that such unbalanced diets are a major insult on the bodies' metabolism and may have a direct effect on the heart, particularly since they tend to be high in saturated fat.
He says that a balanced diet and regular exercise is sustainable for life and, for most people, is the safest way for the heart.
The study will be presented at an American Heart Association meeting.