On Leanness

Although many acknowledge that some people are inherently (perhaps genetically) leaner than others, it remains unclear what the biological basis for the body's "set-point" might be. A study appearing in the open-access journal PLoS One suggests one possible factor¹.


from reference 1

Both rat and human data were collected in this work, which concludes that "the lean phenotype is characterized by high endurance capacity and high activity and may stem from altered skeletal muscle energetics." These researchers gathered data from a population of people who they categorized as non-exercisers (less than 1 hour per week of activity exceeding 4 METS). They subjected these individuals to a treadmill test to determine their endurance (as assessed by oxygen consumption during exercise) and kept track of their average daily activity over a period of 10 days; finding that there was a significant relationship between endurance and leanness, as well as average daily activity and leanness. Furthermore, they found that there was no significant difference in the amount of food consumed by lean versus non-lean rats, and fascinatingly, that the skeletal muscle tissue of lean rats has significantly higher levels of the enzyme PEPCK-C.

Of course, it is not surprising that those individuals with higher daily activity are leaner in general, rather, this study is suggesting that there may be a fundamental, biological reason why certain individuals are more active: they simply have a greater capacity for activity. Indeed, if one fatigues more easily, it wouldn't be surprising if they were less active; it is also conceivable that reduced activity could feed-back on behavior in the sense that an individual with lower endurance might progressively reduce the amount of physical activity they engage in so as to reserve energy for other tasks. This is especially true in contemporary society, where mental activity is often the basis for work and play.

It is unclear as yet, however, whether the differential muscle-properties found in rats extend to humans; further work will be required to clarify what the molecular-biological basis for increased human endurance might be.

References
1. Novak CM, Escande C, Gerber SM, Chini EN, Zhang M, et al. (2009) Endurance Capacity, Not Body Size, Determines Physical Activity Levels: Role of Skeletal Muscle PEPCK. PLoS ONE 4(6): e5869. doi:10.1371/journal.pone.0005869