Should postmenopausal women eat more protein? Or do the elite just exercise and eat better?
In this issue, the Silva and Spritzer4 paper titled, “Skeletal muscle mass is associated with higher dietary protein intake and lower body fat in postmenopausal women: a cross-sectional study,” asks a relatively simple exploratory question: do postmenopausal women who consume more protein benefit in terms of common health markers including body composition (percent body fat [%fat], SMI), metabolism, and cardiovascular biomarkers? Healthy postmenopausal women were recruited to participate in this observational cross-sectional study. The investigators collected nutrient data using a validated food frequency questionnaire, body composition using dual-energy X-ray absorptiometry (DXA), and a battery of other measures including anthropometry, pedometry, indirect calorimetry, and blood biomarkers. Some of the key findings with statistically significant trends by tertiles of protein consumed are summarized in Figure 1. They conclude, “SMI was positively associated with protein intake and negatively associated with % body fat in the present sample of healthy postmenopausal women … the emerging evidence suggests that healthy postmenopausal Brazilian women with low protein intake and no medical contraindications should be encouraged to increase their protein intake to reduce the risk of low skeletal muscle mass.” But let's look closer at the characteristics of the three groups. First, the high protein group stayed in school longer (11 years) versus the mid and lowest tertiles (8 and 5 years). Secondly, they consumed more daily calories, 30% more than the mid group and 50% more than the low protein consumers, without consuming more fats or carbohydrates. Lastly, the protein levels are only marginally higher when viewed as percentage of the total diet, only 1% to 2% differences between groups. So, the highest tertile of protein consumers are also better educated, exercise more, and eat larger meals with slightly more protein content than their counterparts. Does consuming the slightly higher fraction of protein cause one to stay in school longer, exercise more, and eat a higher volume of food? Maybe. But an alternative causal pathway is suggested: Women with more education may exercise more and this would lower their %fat and increase their SMI. Increased lean mass requires more energy to maintain body weight so they eat more. The higher protein content of their food may also be a consequence of their higher education since one can presume this would translate to a higher income and the ability to afford a slightly higher protein content.
Although there is an association of protein intake, and SMI and % fat in these postmenopausal women, it may be premature to conclude that Brazilian women with low protein intake would see marked gains in their muscle mass after increasing their protein intake with all other confounders held constant. Short-term studies, especially observational studies like this, cannot conclude this.