New research examines the impact of so-called crash diets, also known as very low-calorie diets, on heart function. The findings call for more careful cardiac monitoring in people trying to lose weight.

Crash diets are very appealing to those wishing to lose weight fast—and that is most people.

Are they healthful? And how do they affect the body? The National Institutes of Health (NIH) warn that they could be dangerous, depriving the body of essential nutrients, and that these effects are particularly worrisome in children and teenagers.

Other adverse health effects that scientists have warned about include the slowing down of the metabolism, the weakening of the immune system, and the increasing chances of dehydration and arrhythmia.

Now, new research—presented at this year's CMR conference, held in Barcelona, Spain—looks specifically at the effects of crash diets on heart health.

Dr. Jennifer Rayner, who is a clinical research fellow in the Oxford Centre for Magnetic Resonance at the University of Oxford in the United Kingdom, is the lead author of the new study.

She explains the motivation for the research and offers more context around this dieting practice, saying, "Crash diets, also called meal replacement programs, have become increasingly fashionable in the past few years."

"These diets have a very low-calorie content of 600 to 800 [calories] per day and can be effective for losing weight, reducing blood pressure, and reversing diabetes," Dr. Rayner adds.

"But, the effects on the heart have not been studied until now," she says. So, Dr. Rayner and team used MRI to examine the impact of crash diets on fat distribution and the good functioning of the heart.

Crash diets: The good and the bad

To this end, the researchers recruited 21 participants who were obese and aged 52, on average. For 8 weeks, they were fed a diet restricted to 600–800 calories per day.

Dr. Rayner and her colleagues performed MRI scans at the beginning of the study, 1 week later, and at the end of the study.

After just a week, body fat levels had decreased. Specifically: the amount of total body fat fell by an average of 6%; visceral fat, or the fat around our internal organs, fell by 11%; and liver fat decreased by 42%.

The crash diet revealed some important health benefits after just 1 week: better insulin resistance and healthier levels of total cholesterol, blood sugar, and blood pressure.

But surprisingly, heart fat levels rose by 44%. This change, write the authors, correlated with dysfunctions in the heart's ability to pump blood.

By week 8, however, this returned to normal—as did heart function. In fact, these aspects of heart health improved even more than what they used to be before the diet started.

"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."

Dr. Jennifer Rayner

Caution needed in those with heart disease

Dr. Rayner explains the changes induced by the diet, saying, "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."

"The heart muscle," she adds, "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."

As a consequence, Dr. Rayner warns about the extra caution that those with a heart condition need to take before starting a crash diet.

"If you have heart problems," she says, "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."

She also adds that very low-calorie diets need not be avoided altogether, as they do hold benefits. "[H]ealthy people may not notice the change in heart function in the early stages," she says. "But caution is needed in people with heart disease."