Danish obesity expert Dr. Arne Astrup, whose survey is published this week in The Lancet medical journal, concludes that headaches, muscle weakness and either diarrhea or constipation are reported more often by Atkins dieters than people on conventional diets. Those side effects may be signs that the eating plan isn't healthy in the long run, he says.
However, other experts said the diet remains a viable option for some because the side effects aren't bad enough to throw them off the eating plan famous for its shunning of bread, pasta and many fruits.
"More people stayed in the low-carb group than in the low-fat group, so you've got to wonder how severe those side effects were if more people kept to the low-carb diet," said William Yancy, a Duke University researcher who conducted one of the major studies that Astrup reviewed.
The Atkins diet, which allows unlimited consumption of protein and fat but drastically limits carbohydrates and does not restrict calories, has had a following for decades but only recently has come under serious scientific scrutiny. It has been embraced by an estimated 20 million people worldwide.
In his review, Astrup, director of research in the department of human nutrition at the Royal Veterinary and Agricultural University in Frederiksberg, Denmark, examined the evidence from about 60 studies on the Atkins and other low-carb diets.
Several small studies in the last year or two have surprised the experts by showing that people lose more weight on the Atkins diet than on the standard low-calorie, low-fat diet, at least in the short term, with even better cholesterol improvements.
Longer studies have since shown that when dieters are followed for a year, the total weight loss ends up almost the same with the two approaches. The long-term effect on cholesterol has not been studied yet.
Experts have suspected that the weight loss on a low-carb diet may be largely due to water loss, because lots of fluid is bound up in the body's carbohydrate stores that are depleted. However, Astrup said body composition studies indicated the weight loss is a real fat loss, not just water.
The most frequent complaints with low-carb diets are constipation and headache, which are readily explained by the lack of fruit, vegetables and whole grains, Astrup said.
Also, bad breath, muscle cramps, diarrhea, general weakness and rashes are more often reported on low-carb diets than on low-fat diets, Astrup found.
"The majority had some of these side effects in the Atkins group. In the control group, almost nothing," he said.
These side effects are consistent with carbohydrate deficiency, because the brain and muscle do not get enough sugar from carbohydrates to maintain their normal function, Astrup said.
"We have known for many years that there is a minimum intake of carbohydrate necessary to maintain the normal function of your body and that is approximately 150 grams a day," he said. "But, if on the Atkins diet you go down to 20 to 30 grams in the induction phase, then maybe go up to 100 grams, still you are far below what your body needs."
The body can coast along for a while with the carbohydrate stores in the liver and the muscles, but eventually problems start to occur, Astrup said.
"I think these symptoms are signs that something is wrong," Astrup said.
But Yancey, the U.S. researcher, said side effects were seen not just after six months but also at the beginning and could have been from dehydration, which is easily overcome.
"We know that it works," Yancy said. "I think it's good that people are acknowledging that this diet might be an option for people. ...At this point, we need options for people and when we're looking for options we need to consider even things that go against our judgment sometimes."