A North Dakota physiologist has found an unexpected reason why American women are unusually prone to osteoporosis, the bone disease: When they lose weight they also lose bone, even if they exercise regularly and consume adequate amounts of calcium while dieting.
In a five-month study of 14 overweight women from 20 to 40 years old, Dr. Henry C. Lukaski found that the women, who lost an average of 18 pounds, also lost 2 to 3 percent of their bone mass.
Mr. Lukaski said he was very surprised by the finding, since the weight-reduction program was designed to prevent bone loss. The women participated in a supervised aerobic exercise program, which helps to maintain bone mass, and they consumed more than the recommended 800 milligrams of calcium each day.
Mr. Lukaski, a physiologist at the Grand Forks Human Nutrition Research Center, a division of the Federal Agricultural Research Service, speculated that women dieting on their own might lose even more bone. That is because few would be likely to consume this much calcium and because many dieters fail to exercise regularly.
After one month on a maintenance diet, the study participants' caloric intake was reduced by 25 percent for the next month and by 50 percent for the last three months.
Even though their diets contained adequate amounts of all essential vitamins and minerals, they lost bone faster and formed new bone more slowly than normal when their caloric intake was cut in half. In another surprising finding, the study showed that the women retained less than normal amounts of magnesium, a mineral involved in bone growth.
It is not known whether bone mass is restored when women regain lost weight, but Mr. Lukaski said that if it was not, those who "lose and gain weight in a roller-coaster cycle could be at a greater risk for osteoporosis than previously thought."
Osteoporosis, which is characterized by a reduction of bone density and extreme brittleness, is especially prevalent among older women, for whom it can be life-threatening when it results in hip fractures.