If you're going to overindulge and gain weight, at least try to make sure the extra calories come from unsaturated fats, a new study suggests.

When lean people pack on even a few extra pounds, heart disease risk factors in the bloodstream change – some for the better if the excess food contains unsaturated fats, versus saturated fats, researchers found.

Even a moderate weight gain of about three pounds for lean, young people clearly increased markers of heart disease risk factors like insulin resistance as well as signs of impaired vascular function, said senior author Dr. Ulf Riserus of the Unit for Clinical Nutrition and Metabolism at Uppsala Science Park in Sweden.

But unsaturated fats in the diet improved cholesterol levels despite the extra calories and weight gain, which is surprising, Riserus told Reuters Health by email.

For seven weeks, two groups of healthy, relatively lean adults ages 20 to 38 were told to keep to their habitual exercise level and daily diets, adding three to four muffins to their diets each day.

The researchers provided the 240-calorie muffins, with half their energy from fats. One group of 19 adults received muffins made with sunflower oil, polyunsaturated fatty acids (PUFA), while the other group of 20 people ate muffins made with palm oil, a saturated fat. The muffins were otherwise identical.

After seven weeks, each group had gained between two and three percent of their body weight, about 3.5 pounds (1.5 kilos) each, and waist girth increased by about one percent, but blood pressure did not change significantly.

This level of weight gain in the short term is probably not dangerous at all, Riserus said, but if weight accumulates over time, especially abdominal fat, there can be health consequences.

Based on blood tests, the sunflower oil group had lower cholesterol and lipid levels at the end of the study than they had at the beginning of the study. For the palm oil group, cholesterol went up, according to the results in the Journal of the American Heart Association.

Both groups showed signs of increased insulin resistance, a diminished ability to process blood sugar that can be a warning sign for diabetes onset.

Riserus and his team had previously found that the type of fat in the diet determined how much of the excess calories were stored as abdominal fat and liver fat, he said.

“If the high-caloric diet was based on unsaturated fats rather than saturated fats, very little fat was stored as liver and abdominal fat, whereas the opposite was true for the diet high in saturated fats,” he said.

Saturated and unsaturated fats have different molecular effects on the liver, he said. Unsaturated fats signal the liver to take up cholesterol from the blood, he said.

“We believe our results are very relevant considering that a large part of most populations are in caloric excess and gradually gain weight over time,” Riserus said. “Although weight gain should be avoided, the results basically tell us that we may benefit from having enough unsaturated fats in our diets, irrespectively of how many calories we eat.”

The results support the American Heart Association recommendation to replace some saturated fats in the diet, like fatty beef, butter and cheese, with unsaturated fats like vegetable oils and nuts, he said.

“I do not think people usually plan for weight gain, but, as we know, it just happens quite commonly,” said Ursula Schwab, an associate professor of nutrition therapy at the University of Eastern Finland in Kuopio.

“So, by following the guidelines regarding dietary fat, unintentional weight gain can be less harmful than in cases when the recommendations on the quality of dietary fat is not followed,” Schwab told Reuters Health by email. She was not involved in the new study.

All dietary polyunsaturated fats are beneficial, she said, but that is not necessarily the case for supplements.

In addition to sunflower oil, rapeseed oil and canola oil are good sources of polyunsaturated fats, Riserus said.