When you're trying to jump-start your weight loss, it's tempting to go on an ultra-low-calorie “crash diet”. These types of diets, which usually entail eating an incredibly restrictive 800 calories a day, have been shown to help people drop weight, lower blood pressure, and reverse diabetes—but should only be attempted under medical supervision.

One reason for that? Restrictive meal plans may cause damage to the heart, according to new research from the University of Oxford in the U.K. presented at a recent meeting of the European Society of Cardiology.

For the study, researchers put 21 obese, middle-aged patients on a low-calorie diet (600–800 calories a day) for eight weeks. Only one week in, the participants showed a 6% drop in body fat, an 11% drop in visceral fat, and a 42% drop in liver fat. The only caveat? The participants' heart fat jumped by 44%, which hindered how well the cardiac muscle could pump blood. By the end of the study period, heart fat content and functioning had recovered and surpassed what it was before the crash diet.

"The sudden drop in calories causes fat to be released from different parts of the body into the blood and be taken up by the heart muscle," said lead author Jennifer Rayner, M.D., clinical research fellow, Oxford Centre for Magnetic Resonance, University of Oxford. "The heart muscle prefers to choose between fat or sugar as fuel, and being swamped by fat worsens its function. After the acute period in which the body is adjusting to dramatic calorie restriction, the fat content and function of the heart improved."

Looking for a slightly more reasonable diet plan that can help reduce your caloric intake without starving yourself? Here's our guide to intermittent fasting, and a primer on the three main types of intermittent fasting.