Weight loss surgery can help you lose weight, but it’s also likely to leave you unable to absorb iron, a new study suggests: Iron deficiency is a common problem after stomach bypass surgery to treat severe obesity - and standard iron supplements may not be enough to prevent it in some patients.

Researchers found that among 67 Chilean women who had undergone the most common form of weight-loss surgery, 39 percent developed low blood counts, also known as anemia, within 18 months of surgery. That anemia was most often due to a deficiency in iron, which the body needs to produce healthy red blood cells that carry oxygen.

In contrast, less than two percent of the women had been anemic before surgery, the researchers report in the American Journal of Clinical Nutrition.

It’s well known that nutritional deficiencies are a risk after the type of surgery examined in the trial, known as Roux-en-Y gastric bypass, the most common and most effective form of weight- loss surgery for severe obesity.

The procedure involves stapling off the upper portion of the stomach to create a small pouch that restricts the amount of food a person can eat at one time. The surgeon also makes a bypass from the pouch that skirts around the rest of the stomach and a portion of the small intestine, limiting the body’s absorption of nutrients.

The new findings suggest that impaired iron absorption, rather than reduced iron intake, is the major cause of long-term deficiency after gastric bypass, according to the researchers.

Tests done six months after surgery showed that, on average, women were absorbing just one-third of the iron from food that they had before surgery.

What’s more, their absorption of iron from supplements showed nearly as great a decline. And many women became deficient in iron despite taking supplements after surgery, according to the researchers, led by Manuel Ruz of the University of Chile in Santiago.

All of the women in the study were put on vitamin and mineral supplements after surgery, though not all took iron pills. Those who did were prescribed 18 milligrams per day - the standard recommended iron intake for women younger than 50.

That amount, Ruz and his colleagues write, appears “largely insufficient to prevent iron deficiency and iron deficiency anemia.”

The researchers say that some patients may need to take newer, more readily absorbed iron formulations, or receive infusions of the mineral rather than pills, to prevent a deficiency.

According to the American Society for Metabolic & Bariatric Surgery, about 220,000 Americans had some form of weight-loss surgery in 2008, with gastric bypass accounting for the majority.

SOURCE: American Journal of Clinical Nutrition, September 2009.