Study: 'Pill' may cancel some bone-building benefits of exercise

June 20, 2001

WEST LAFAYETTE, Ind. -- Young women who take oral contraceptives and exercise may have increased risks of bone loss, according to recent research at Purdue University.

A two-year study headed by Connie Weaver, head and distinguished professor of foods and nutrition, suggests women ages 18 to 30 who exercise while using oral contraceptives may lose bone density in the hip and spine, even more than women who take the pill and are sedentary. This loss of density could make a women susceptible to bone fractures later in life.

However, the study also indicates that women in this group who get the minimum daily requirements for calcium may be able to ward off bone loss.

"The study showed that, overall, exercise had positive affects on whole body bone mineral content for everyone," Weaver says. "Only the spine and hip were compromised if subjects who were on oral contraceptives exercised, and only then if calcium intakes were inadequate."

The purpose of the study, funded through the National Institutes of Health, was to investigate the effects of exercise on bone mineral content and bone mineral density in women 18 to 30 years old who either were or were not taking birth control pills. The study started with 180 women. At the end of the two-year period, 55 women remained.

The study, published in the June issue of the scientific journal Medicine and Science in Sports and Exercise, had several unexpected results, Weaver says.

"One big surprise was that for women who use birth control pills, exercise had a negative impact on the spine and the hip, especially in the first six months," she says. "By the end of the two years, that group fell below the groups of non-exercisers or non-pill users in terms of bone mineral content and bone mineral density."

Bone mineral content and bone mineral density are both indicators of bone mass. It is widely held that women with higher bone mass early in adulthood are at a lower risk for developing osteoporosis-related fractures after menopause.

Previous research has demonstrated that oral contraceptive use can prevent the increases in bone mass that normally occur in young adult women. Because exercise has been shown to help accumulate bone mass, women on birth control pills usually are advised to exercise in order to maintain bone health.

Weaver's study suggests the solution to maintaining bone health in young women is more complicated. "This negative interaction is very scary because we want women to exercise for all kinds of beneficial reasons," she says. "But we don't want bone, especially of the hip, to be compromised because that's the worst kind of fracture you can have."

Only three of the women in the study who both took birth control pills and exercised did not experience a loss in bone density. These women differed from the others in the study because their diets included higher levels of calcium, Weaver says.

"They were the three that came close to the required levels of calcium intake," she says. Weaver emphasized the role of calcium in maintaining bone health in young women who take birth control pills. "If you're going to be on birth control pills and exercise, you have to get enough calcium," she says. "You either need to get calcium through foods -- and that could be dairy or fortified foods, such as juices or cereals -- or you need to supplement."

The study found that the control group of women who did not exercise also lost bone density throughout the two years. "It wasn't like the non-exercising group maintained bone throughout and the exercising group went up," Weaver says. "These young, college-age women were already losing bone if they weren't exercising."

Weaver suggests this may change the way researchers look at the development of osteoporosis. "We didn't expect 18- to 30-year-old women to already be losing bone," she says. "Doctors aren't even doing bone scans, for the most part, on these young people, so they haven't known. Even before they go through menopause, they can already be in trouble, which is what we showed."

The study, conducted by an interdisciplinary group made up of 12 experts in food science, health, mechanical engineering and medicine, differs in several ways from other studies that have assessed the role of exercise on bone health in women.

First, none of the participants regularly participated in any form of exercise prior to the study. They were randomly assigned to a non-exercise control group or to a group that exercised three times weekly. This is an important distinction, Weaver says, noting, "Quantitatively, you can't assess the effects on bone if you don't randomize."

In addition, this study focused on a younger age group than most earlier research. "Data in this age group from exercise intervention studies is very scarce," Weaver says.

In the future, Weaver plans to study whether bone mass acquired in early adulthood may act as a depository later in life.

"Where the positive impact of exercise on bone mineral density was the greatest was the arms and the legs," Weaver says. "Our next question is, can that increase in the limbs, the long bones of the legs and arms, move over to the hip and the spine later in life when the body needs it? No one knows the answer to that."

ABSTRACT

Impact of exercise on bone health and contraindication of oral contraceptive use in young women

Connie Weaver, Dorothy Teegarden, Roseann Lyle, George McCabe, Linda McCabe, William Proulx, Mark Kern, Darlene Sedlock, David Anderson, Ben Hillberry, Munro Peacock, and C. Conrad Johnston

The effect of quantified resistance and high impact exercise training on bone mass as modified by age and oral contraceptive use in young women was studied. Methods: Women were categorized by age (18-23 vs. 24-31 years) and oral contraceptive use, and were then randomized into either three sessions of resistance exercise plus 60 minutes/week of jumping rope or a control group for 24 months. Total body, spine, femoral neck, greater trochanter, Ward's area and radial bone mineral density (BMD) and/or bone mineral content (BMC), biochemical markers of bone turnover, dietary intake of calcium, lean body mass, maximal oxygen uptake, and strength were determined at baseline and every six months. Results: Total body BMC percent change from baseline was higher in exercisers compared with non-exercisers at six and 24 months. Oral contraceptive users had lower bone turnover at baseline and a decrease in total body bone mineral content from baseline compared with non-oral contraceptive users at 24 months. Spine bone mineral content and bone mineral density decreased in the exercise and oral contraceptive user group at six months and remained significantly below non-exercisers who used oral contraceptives at two years. Femoral neck bone mineral density also decreased in the exercise and oral contraceptive group at six months. Conclusions: Exercise prevented a decline in total body bone mineral content seen in the non-exercisers. On the other hand, exercise in oral contraceptive users prevented the benefits observed in the spine of the non-exercise plus oral contraceptive group.
-end-
NOTE TO JOURNALISTS: Copies of the journal article are available from Susan Gaidos, Purdue News Service, 765-494-2081.

Sources: Connie Weaver, 765-494-8231, weavercm@cfs.purdue.edu
Dorothy Teegarden, 765-494-8246, teegarden@cfs.purdue.edu

Writer: Jenny Cutraro, 765-494-2096, jenootz@yahoo.com

Intern coordinator: Susan Gaidos, 765-494-2081, sgaidos@purdue.edu

Other source: Roseann Lyle, 765-494-3158; rlyle@purdue.edu

Purdue University

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