Heavier people are no stronger than thin people?

Gretchen Reynolds reports in the New York Times that carrying weight in the form of fat and carrying weight in the form of lead-weighted vests affect rats’ muscles differently:

Recently scientists at Penn State sewed tiny weighted vests and slipped them around the middles of healthy laboratory rats, hoping to discover how animals’ muscles respond to changes in body size. The vests increased the animals’ weight by as much as 36 percent. After five days, the scientists found that the rats’ muscles contained increased amounts of certain proteins involved in the generation of muscle force. The muscles were redesigning themselves to be stronger.

In a separate group of obese rats, however, no such changes were evident. The rats were heavy, generally exceeding the weight of the animals wearing vests, and they continued to pack on ounces during the experiment. But their muscles did not show the same increases in the proteins that improve muscle power. The obese animals were not getting stronger as they became heavier. They were in danger of becoming too fat to move.

I’m really wondering about that last sentence. If the rats’ muscle does not increase proportionately with their fat, it doesn’t necessarily mean that they could easily continue to get fatter without adding muscle until they were too fat to move. The media sometimes makes statements like this without making it clear that they’re based on assumptions that a trend will continue indefinitely without plateauing, oftentimes in situations where that’s unlikely to actually be true.

That “redesigning themselves to be stronger” sentence is also a little weird. The design of the muscles wasn’t fundamentally changing, I don’t think; the muscles were mainly just getting bigger (enough to increase the animals’ entire body weight by 36%).

But the main thing I wanted to talk about with this article is that it just doesn’t fit with my experience at all. I know, I know, the plural of anecdote is not data. But in my experience with human beings, heavier individuals are more likely to be stronger. My experiences in this respect are mostly with people with BMIs on the low end of obese or less–which is where most obese people in the US are, anyway. When they talk here about people complaining that moving is difficult, though, they talk about people who are “extremely overweight”, which I’m guessing corresponds to the “morbidly obese” BMI category.

Still, this sentence may explain why their study doesn’t match up with my experience:

When they biopsied the leg muscles of rats bred to be fat, they found that the chubby animals’ troponin T gene seemed to malfunction.

Bred to be fat? So how do they know that the muscular differences aren’t due to genetic differences that cause both the tendency to be fat and the tendency to gain little or no muscle with increased weight? If they’re really using different strains of rat for wearing weighted vests than for being fat, that’s a major problem in controlling their variables in my opinion.

Now, anecdotally, I do see some people who really do have trouble moving, and a lot of them are unusually heavy. So I wonder if there is a genetic difference here that affects a minority of fat people. But based on a single rodent study, I’m not yet ready to believe that this a common problem affecting overweight and obese people, when it directly contradicts my experience. All things being equal, I wouldn’t be surprised if the average 200-lb person has less muscle than a 150-lb person wearing a 50-lb vest all the time would, but I would still expect them to have more muscle than a 150-lb person who didn’t carry around a vest. In other words, I could believe that obese people have less muscle than expected based on their weight, but not that there would be no increase in muscle with an increase in weight.

Just after writing the above paragraph, I realized that I hadn’t looked at the paper that the NYT article links to, and probably should. Full text is behind a paywall, but I did read the abstract. I’m not sure I fully understood it, but it sounds like the lack of increase in muscle with an increase in weight may indeed be confined to only certain strains of rats:

Using rats, we examined how variation in body weight affected alternative splicing of fast skeletal muscle troponin T (Tnnt3)… The response depended on weight per se, as externally attached loads had the same effect as an equal change in actual body weight…. For a subset of the Tnnt3 splice forms, obese Zucker rats failed to make the same adjustments; that is, they did not show the same relationship between body weight and the relative abundance of five Tnnt3 β splice forms…. Heavier obese Zucker rats displayed certain splice form relative abundances (e.g. Tnnt3 β3) characteristic of much lighter, lean animals, resulting in a mismatch between body weight and muscle molecular composition. Consequently, we suggest that body weight-inappropriate skeletal muscle Tnnt3 expression in obesity is a candidate mechanism for muscle weakness and reduced mobility.

If I’m interpreting it correctly, though, and only some of the obese rats showed the mismatch between body weight and muscle, why would one of the researchers say this?

It also is not known what mechanisms cause the troponin T gene to malfunction in fat animals, although fat itself is an obvious suspect. “Fat is a very physiologically active tissue,” Dr. Schilder said.

Maybe things will become clearer with more research. What we don’t need more of, though, is what’s seen at the end of the article: worrying by scientists that by reporting results like this, they will be seen as trying to “rationalize” obesity.

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