Low-calorie “crash” diets can impair a heart’s function before it begins to improve, according to research presented at CMR 2018, the joint meeting of the European Society for Cardiovascular Magnetic Resonance and the Society for Cardiovascular Magnetic Resonance.

A crash dieter’s heart becomes fatter in the short term, reported lead author Jennifer Rayner, BMBCh, and colleagues. They said fat released from other sources into the bloodstream can become acquired by the heart, affecting its ability to pump blood.

Rayner and coauthors studied 21 obese participants—with an average age of 52 and mean body mass index of 37—who agreed to follow a diet of 600 to 800 calories per day for eight weeks. MRI was performed after one and eight weeks to assess the diet’s impact on heart function as well as fat distribution in the abdomen, liver and heart muscle.

After one week, total body fat (6 percent), visceral fat (11 percent) and liver fat (42 percent) all decreased, but heart fat rose by 44 percent and researchers noted a decline in heart function. However, after eight weeks, heart function and fat were significantly improved compared to baseline levels.

“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,” Rayner said in a press release. “Instead, heart function got worse in the first week before starting to improve.

“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 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.”

Rayner suggested people with existing heart problems should be supervised when undertaking a low-calorie diet or fasting as it could put them at increased short-term risk. But healthy individuals may not notice the transient decline in heart function, she said.