Naturally thin people have less efficient muscle

fat vs thinFor the past eighteen months, I have worked a job that takes something close to 60 hours per week, managed a full-time school schedule and squeezed in some social relationships. This doesn’t leave me much downtime for eating—I usually end the day at about 800 calories a day, and that’s without really thinking about it. It’s not a crash-diet tactic: I legitimately forget to eat and wouldn’t have time if I remembered.

However, I am not emaciated. I’m actually still a size 8 and wish I could lose weight. I watch my delicate-boned coworkers eat greasy baskets full of fries and know that if I tried to do that, the scale would gleefully spin up a few pounds the next day. It begs the question: How did evolution result in two completely different metabolic functions when we live so geographically close and inhabit the same environment?

A new study illuminates the much-maligned “naturally thin” state. Published March 15 in the Journal of the American Physiological Society, the researchers investigated why some people can eat the same number of calories and not build up any fat content while others seem to pack on the lipids.

As it turns out, naturally thin people tend to have higher lung capacity and greater thermal output, meaning their muscles are less efficient. By way of analogy, naturally thin people are like gas guzzlers, while the more naturally heavy people are fuel-economy cars. Those who have higher lung capacity are not only more likely to engage easily in physical activity, but they also expend more energy during the activity. This burns a greater number of calories, resulting in a naturally thinner frame. Naturally heavier people have longer lung capacity and disperse less heat during exercise, essentially making them more efficient.

While naturally thin people also have a stronger tendency to build up skeletal muscle rather than bulky muscle, the main factor is lower energy efficiency.

But being fat or thin may not have an evolutionary advantage either way. In the 1960s, a geneticist named James Neel theorized that obese people have a “thrifty gene,” which is a leftover from the hunting and gathering days when food was uncertain and humans needed to save every calorie they could for survival. However, the theory falls through when one observes that not everyone is fat. Thus, JR Speakman proposed the “drifty gene,” which states that the removal of predatory threat against humans allowed genetic drift to occur, creating those with both tendencies without any evolutionary edge.

All the genetic investigation into obesity may have a tendency to create a scapegoat for rising obesity levels—if we’re fat, blame it on our genetics. Unfortunately, it’s not as easy as that. Obesity is not just a genetic condition; it is a predisposition or tendency, and lifestyle reinforces that. Another study conducted by the American Friends of Tel Aviv University found that those who sit for extended periods tend to gain weight in the rear, the result of the accumulation of lipid droplets. Fat cells exposed to extended pressure expand and have increased capacity, allowing them to store more lipids.

So while I obviously have a disadvantage compared to my coworkers who eat a burger and fries every day, I am not sentenced to my weight forever. I may have lower lung capacity, but that just means I have work harder.


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