Scientists have found high fat, low carbohydrate diets can reduce energy to the heart.
There are concerns about a low-carb diet
The Oxford University team say it is unclear at this stage whether this could have a damaging impact on health - but say more research is needed.
Many people have lost weight quickly by following such diets - but scientists fear they may not be good for health in the longer term.
The new study is being presented to an American Heart Association meeting.
The Oxford team monitored 19 people over a two week period.
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.
In some people the energy reduction was as much as a third.
Their hearts also became slightly 'stiffer' - not relaxing quite as well as before the diet.
One of the participants even noticed he could not manage his daily run while on the diet.
The changes were reversed within two weeks after returning to a normal diet.
Associated with illness
Lead researcher Professor Kieran Clarke told BBC News that people with illnesses such as diabetes, obesity and heart disease also had lower than normal heart energy levels.
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 the tank'.
Professor Clarke said further research should be carried out on a larger sample over a longer period.
"Our study gives us some interesting insights into how extreme diets might be affecting us.
"Of course, this is a small and short-term piece of work. The body is remarkably adaptive and what we don't know is whether our hearts would have gradually returned to normal had we stuck to the diet long-term."
The British Heart Foundation, which funded the work, said the results reinforced their view that people should not follow high fat, low carb diets to lose weight.
Professor Peter Weissberg, BHF medical director, said: "Diet devotees can be reassured that this research in no way suggests that the high fat-low carb regime is going to give them heart failure.
"However, they should be aware that such unbalanced diets are a major insult on their bodies' metabolism and, as this study shows, may be having direct effects on their hearts, particularly since they tend to be high in saturated fat.
"We would certainly not recommend high fat-low carb diets to anyone who wants to lose weight and look after their heart.
"Achieving this with a balanced diet and regular exercise is sustainable for life and, for most people, is the safest way for your heart."