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I posted earlier this month on “why weight loss isn’t just calories in minus calories out,” which led to an interesting discussion on the ways in which the body tries to prevent itself from gaining or losing weight. On that topic, I came across an interesting study published earlier this year in the American Journal of Physiology – Regulatory, Integrative and Comparative Physiology, from researchers at Columbia University. The gist: it’s the efficiency of our muscles, on a cellular level, that conspires to hold our weight steady.
It’s well-known that, if you lose weight, your metabolism slows down to burn fewer calories. (That’s why, as a recent JAMA paper pointed out, eating one less 60-calorie chocolate-chip cookie per day won’t allow you to keep losing weight forever. Instead, if exercise and other factors are kept constant, you’ll plateau after a few years at a weight six pounds lighter.) Part of that is because, as Phil Koop pointed out, “adipose tissue has a metabolic cost.” Or as the JAMA paper puts it:
A person who consumes an extra cookie every day will initially experience weight gain, but over time an increasing proportion of the cookie’s calories will go into repairing, replacing, and carrying the extra body tissue.
But the Columbia paper makes it clear that the extra tissue, on its own, isn’t enough to explain the changes in how many calories you burn:
Maintenance of a body weight 10% below “usual” for a lean or obese individual is accompanied by a reduction in systemic energy metabolism (~300–400 kcal/day less than that predicted solely on the basis of changes in body weight or composition), neuroendocrine function (decreased circulating concentrations of leptin and of bioactive thyroid hormones), autonomic nervous system physiology (decreased sympathetic nervous system tone and increased parasympathetic nervous system tone), and behavior (decreased satiety) that act coordinately to return body weight to its initial level.
To figure out where these missing calories go, the Columbia group has been doing extremely careful experiments that involve checking volunteers into an inpatient research clinic for several months at a time, and feeding them only a liquid diet (40% corn oil, 45% glucose, 15% casein protein, by calories) so they can precisely monitor energy intake. They control the amount of feeding to make the subjects gain or lose 10% of their body mass, then study the changes to their metabolism. Pretty neat stuff.
Anyway, the major finding of this new study is that “skeletal muscle work efficiency” changes dramatically when you change your body weight. It’s not just because your muscles have less weight to carry around — it seems to have more to do with changes in the ratio of enzymes that determine whether the muscle burns carbohydrate or fat. When you lose weight, your muscles get more efficient — which seems like a good thing, except that it means you burn significantly fewer calories when you move around, which pushes your weight back up. The opposite happens when you gain weight: your muscles get less efficient.
So what’s the take-home (other than “Try not to gain weight, because it’s really hard to lose once you do.”)? Because the ratio of carbohydrate-to-fat utilization depends on the intensity of physical activity, there’s reason to believe that the efficiency changes are only relevant at very low intensities — corresponding to the activities of day-to-day life as opposed to “exercise.”
From a therapeutic standpoint, exercise post-weight reduction might be more effective at higher workloads, i.e., that the weight-reduced individual might “escape” this increased efficiency by altering the intensity of exercise even without necessarily increasing the work performed.
In other words, exercise harder rather than longer might be the most effective strategy for keeping weight off.