- A study found that in both males and females, higher muscle mass was associated with lower mortality from cardiovascular disease (CVD).
- Higher body fat was also associated with lower CVD mortality risk in females but not in males.
- The researchers claim that in females, building muscle mass may be more important than losing weight for cardiovascular health.
The researchers behind the study note that over the past 50 years, death rates from CVD have fallen in both males and females in the United States.
However, the rate of decline has been slower among females than males, and the rate of heart attacks in females aged 35–54 years is actually increasing.
In addition, research suggests that even though females have a lower incidence of CVD than males, they have a higher mortality rate and worse prognosis after an acute cardiovascular event.
As CVD seems to affect the sexes differently, there is an urgent need to determine whether doctors should offer different advice about prevention to their male and female patients.
A new study by researchers at the University of California, Los Angeles, suggests that the focus for females should be on maintaining or increasing muscle mass rather than losing fat.
The findings appear in the Journal of the American Heart Association.
The researchers analyzed body composition data from the National Health and Nutrition Examination Survey (NHANES) 1999–2004 and CVD mortality data from NHANES 1999–2014.
The data came from a total of 5,627 females and 5,836 males, all aged over 20 years.
Based on the data, the researchers split the study participants into four groups:
- low muscle mass and low body fat
- low muscle mass and high body fat
- high muscle mass and low body fat
- high muscle mass and high body fat
In both sexes, the raw data showed that higher levels of fat were associated with higher CVD mortality, regardless of muscle mass.
However, after accounting for other factors that are known to affect CVD mortality, the relationship between body fat and the risk of dying from CVD changed completely in females.
After making these adjustments, the researchers found that females with high body fat and high muscle mass had a 42% lower risk of dying from CVD compared with females who had a low muscle mass and low body fat.
By contrast, males with high muscle mass and high body fat had a 26% reduced risk of dying from CVD compared with males with low measurements, while those with high muscle mass and low body fat had a 60% decreased risk.
The researchers believe that their study supports the need for a shift in the focus of the advice that healthcare professionals give to females. This shift is toward increasing muscle mass through physical exercise and away from weight loss.
“[I]t demonstrates the potential importance of advice to maximize muscle mass in women. This diverges from the current emphasis on weight loss in CVD prevention, and thus methods to practically achieve such body composition alteration need to be further evaluated.”
It is worth emphasizing that the apparent protective effect of fat in females only emerged after adjusting for other CVD risk factors.
Among these CVD risk factors were:
- cholesterol levels
- high blood pressure (hypertension)
- diabetes and prediabetes
- hormone replacement therapy (HRT)
There is a highly complex interplay between body fat and these other risk factors. For example, excess body weight increases the risk of diabetes and hypertension, which, in turn, raise the risk of CVD.
Nonetheless, the authors note several plausible ways in which fat might provide some protection to females.
For instance, they highlight research suggesting that fat in the thighs and buttocks has a protective effect on metabolism that offsets the harm from abdominal fat.
They also note that in premenopausal females, the body stores about 50% of its fat just beneath the skin in the thighs and buttocks, whereas in males, the body stores 98% of the total fat more deeply in the upper body.
During and after menopause in females, however, fat tissue begins to accumulate in the abdomen, where it is associated with increased CVD risk.
The authors also note that in individuals of the same age and weight, the female body tends to accumulate fat by multiplying the number of fat cells, whereas the male body typically grows larger fat cells.
Large (“hypertrophied”) fat cells are associated with negative metabolic consequences, they write.
The researchers acknowledge some limitations of their study.
Firstly, the study design did not allow them to prove cause-and-effect relationships between muscle mass or fat mass and CVD mortality, only associations.
In addition, they say that the technique that they used to measure the body composition of volunteers — called “dual energy X‐ray absorptiometry” or DXA — can overestimate muscle mass in older people and individuals who lead sedentary lives.