Lean Body Mass Level Predicts Cardiac Output in Women

Lean body mass (LBM) appeared to correlate with cardiorespiratory fitness among women, independent of body fat percentage, researchers reported.

In a study that included 30 men and 30 women who were about 60 years old, although the women had lower cardiac capacity per unit of body surface area than the men, the difference was normalized by total LBM or leg LBM, said David Montero, PhD, of the University of Calgary in Alberta, Canada, in a presentation at the ObesityWeek meeting.

"Ultimately, LBM stands out as a strong, independent determinant of cardiac and aerobic capacities in women, regardless of body fat percentage, relationships that are not present in men," wrote Montero and colleagues in Obesity, where the study findings were published simultaneously.

"The main purpose of the present study was to determine the relationship of sex-specific body composition with cardiac and O₂ uptake responses to exercise in women and men matched by age and fitness status," the group explained.

Women in the study presented with lower peak right and left cardiac chamber dimensions and output normalized by body surface area compared with the men. The sex differences in cardiac structure and function, however, were abolished when normalized by total or leg LBM. Peak oxygen uptake (VO_2peak) was also similar in women and men after normalization by total or leg LBM, despite the fact that women usually have lower blood oxygen-carrying capacity, according to the researchers.

"These findings imply that the amount of most metabolically active tissues explain (or at least parallel) cardiac capacity in human individuals, irrespective of sex," Montero and coauthors wrote. "In consequence, the essentially higher women's percentage of less metabolically active tissue (i.e., body fat) may be considered as a weight-bearing handicap of the female cardiovascular system to power whole-body aerobic work, fundamental to the vast majority of human physical activities."

The researchers noted that there was evidence of a substantial (11%) increase in VO_2peak following resistance training in the women in the study, but not the men, despite similar increases in LBM that were induced in both groups.

"Nonetheless, a systematic review of the literature denotes the scarcity of studies specifically addressing such an intriguing hypothesis," the group wrote. "Until we gain further understanding, the observation remains that elite female endurance athletes with the highest levels of cardiorespiratory fitness are usually 'heavier' (relative to body height) than their male counterparts, a gap that may not be entirely explained by sex differences in body fat percentage."

Asked for his perspective, Mitchell Roslin, MD, chief of Bariatric Surgery at Lenox Hill Hospital in New York City, told MedPage Today: "There has been a lot of media attention to an active lifestyle, and the benefits of things like walking. But probably less discussion about doing some degree of resistance training -- which doesn't necessarily have to mean weights, but could be use of resistance bands, stretching, squats, and other things which can help build lean muscle mass or prevent the breakdown of muscle, which occurs during natural aging."

Roslin said that one big difference between men and women is that women have a higher fat distribution, which helps them regulate sex hormones.

"Menopause is related to [a] decrease in muscle and an increase in fat as fat replaces lost estrogen," Roslin said. "You will see the greatest amount of benefit for resistance training in adults because that's when they are having the greatest chance of muscle breakdown. The goal is to lose fat not muscle, but the truth is that with every successful weight loss program, people lose a lot of muscle. The goal in weight loss is to lose fat but at least break even in muscle loss," he said.

He said there should be more focus on resistance training. "It doesn't have to be in the gym and you don't have to use weights," Roslin noted. "Everything you need to do in resistance training can be done with some good resistance bands that cost $3.99 -- and a good instruction course."

For the study, Montero and co-authors recruited 60 healthy, moderately active women and men, matched by age and cardiorespiratory fitness. Body composition was determined via dual-energy x-ray absorptiometry, and transthoracic echocardiography and oxygen uptake were assessed at rest and throughout incremental exercise with established methods.

Major cardiac and pulmonary outcomes were normalized by body surface area, total LBM, or leg LBM.

Study limitations, the researchers said, included 1) because healthy individuals were selected to preclude the influence of disease-related confounding factors, further research will be needed to determine whether the findings can be extrapolated to particular clinical conditions with altered cardiovascular phenotypes or atypical body composition, and 2) three of the participants were taking hormone-replacement therapy (HRT), but the investigators said there were no apparent differences in the results for these individuals compared with the others, which is also in line with previous evidence that has indicated no effect of HRT on key cardiovascular variables during VO_2peak and exercise.

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