The title should make the point of this post clear.
1) Its just simple thermodynamics
The point of this rather stupid statement is that people get fat in direct proportion to the difference between energy ingested as food, and energy expended as activity, and all this talk of gut microbes and environmental toxicity- or certain forms of caloric food being “better” than others- is unnecessary. Anyone using this claim in an argument, however, understands neither thermodynamics nor obesity. Here’s why.
There are two reasons why the energy in = energy out + fat idea is stupid. First, humans are inefficient. Remember that a “calorie” in food is actually a kilocalorie in physics, or 4184 Joules. The 125 calories burned by running a mile are equivalent to 523kJ, or enough energy to lift your center of mass 37cm inches with every step. For comparison, hurdlers lift their center of mass about 13cm when clearing a hurdle (and burn a lot more calories per mile, too!) Two people with two different running styles will burn more or less energy per mile based solely on differences in efficiency alone, and that’s not even touching muscle fiber types, or weight, or track condition, or whatever. The other reason is that for most people, the majority of calories burned in a day are expended on basic bodily housekeeping- bone remodeling, manufacturing immunoglobulins, heating and cooling, new red blood cells etc. You can estimate this with a BMR calculator but once again, any two people’s estimates are bound to be different from their actual values.
Now I know its trendy to pick on people who claim thyroid conditions as a root cause of obesity, but consider that it is, in fact, possible, for someone to have a condition that prioritizes fat storage over BMR. Bodies can economize- it takes a lot of energy to create a pH gradient in the stomach, or under an osteoclast, and slowing that down to save energy is perfectly possible. This is why starving people are more prone to disease- in order to keep moving and maybe get to safety/food, the body reduces its energy investment in new B cells and immunoglobulin synthesis.
Finally, how energy is absorbed from food can vary, although this is not really not well understood. The actual thermodynamic equation would be energy ingested (X a factor responding to efficiency of gut digestion) = energy expended through activity (adjusted for varying efficiency) plus BMR (adjusted for all sorts of toxicological, inflammatory, or just plain idiosyncratic reasons, person-by-person or day-by-day) plus fat storage. Everybody’s fond of calculating how much weight you gain in a year from eating one M&M more than you burn, every day. You think the variable coefficients in efficiency and BMR don’t have a more significant effect?
These coefficients are why people care about fructose vs. glucose, or gut microbiota, or oxidative damage to mitochondrial DNA. Or whatever the next theory is.
2) BMI doesn’t mean anything, look at Lebron James
Yes, folks, there are people out there, like Mr. James (BMI: 27.4 per wikipedia) who appear overweight numerically but are clearly in better shape than the average bear. Get on a bus, though, and tell me how many Lebron James physiologies you see. BMI is a public health statistic useful for calculating risk: if I take a population of people, and put everybody with a BMI>30 in one room, and everybody with a BMI between 19 and 22 in another, there will be more heart attacks in the first room over the next year, per person, than in the second. Will I accidentally include a few very muscular people like Shaquille O’Neal (BMI 31.5) in the first room? Yes, and also there will be a few cachectic people in the second room whose death risk I’ve underestimated. Still, it wouldn’t even take a very large sample before you’d be a fool to bet against me. So that’s BMI.
What disturbs me about this irritating thing people say, is that the guys who say it (and its almost always guys) are usually fairly stacked with big arms and a visible gut. There is still plenty of debate about why, exactly, obesity is associated with disease risk, but a pretty good guess is that visceral fat (more on which later) is pro-inflammatory, and screws up the body’s ability to recover from minor vascular and cellular problems. So when people say they have “only 5% body fat” it almost misses the point- the goal isn’t to have this much fat overall, or this ratio of fat to muscle, but to have as little visceral fat specifically as possible.
What is visceral fat? Visceral fat is the fat blobs attached to the intestines, liver, and the membranes connecting the two- the mesentery and the omenta. Anatomists will understand if I say “fat deposits in tissue drained by the portal venous system.” For some reason, this is just dangerous stuff, in a way that fat on your hips, or arms, or whatever isn’t. It also (here’s the kicker) lies under the abdominal muscles. So when Mr. BMI-doesn’t-matter asserts that his gut is all muscle- and flexes his abs rock-hard to prove it- what he’s actually saying is that he has no subcutaneous fat over his muscle, and that all the bulk is visceral. That is not a good sign.
The problem with percent body fat is that it obscures this connection between visceral fat and risk. Lets say we have Kelvin and Melvin, both 1.75m tall. Kelvin is a bookish fellow, weighs 67kg, has a BMI of 21, and a pbf of 13%. Melvin hits the gym, packs on the muscle, and weighs 92kg, with a lean mass of 84kg (more than Kelvin’s whole body!) and a pbf of 9%. Melvin has a BMI of 30 (“obese”) but nobody would dispute that he is an elite athlete. However, both of these guys are carrying more or less the same amount of fat, which means the same amount of inflammatory risk. You can’t “dilute” it out with muscle, which is the impression one gets from studying the pbf literature.
3) People didn’t evolve to run long distances because runners don’t lose weight.
I’m not going to wade into the literature of persistence predation, or specific weight loss practices, I’m going to focus on one stupid word: “because.” Look, I know everybody’s all into the paleo thing, with this idea that pursuing an evolutionarily adapted life will save us. I get that too. The problem is that evolution selects for efficiency: if humans had, indeed, evolved to run long distances, one strong piece of supporting evidence (indeed, one definition of “evolved to run long distances”) would be that we do it while burning as little energy as possible. No animal evolves to lose weight needlessly- body fat is the accessible residue of successful foraging (which term includes predation) and keeping it around while going through a normal day means that an abnormal day tomorrow won’t necessarily kill you. Like it or not, if you want to lose weight by changing your calorie balance, you’re going to have to pursue activities (or dietary restrictions) in excess of those for which the human organism is evolutionarily adapted. You’re going to have a bunch of those abnormal days, all in a row.
But that’s assuming there’s no underlying toxicity (in food, groundwater, the air) or microbial change underlying the obesity epidemic.
4) Obesity isn’t associated with mortality, once you control for blood pressure, diabetes, etc.
The Cato institute has weighed in with this one (not linking- they don’t need the hits.) I don’t see why the Koch brothers even need to have an opinion on the epidemiology of obesity, but whatever. The point is, obesity is one aspect of the metabolic syndrome, which links blood pressure, diabetes, high cholesterol, high triglycerides, and a few other common diseases (gout, depression) into a single devastating package. Obesity may be the “first cause” trigger, or it might be a symptom of some other metabolic trigger (you get pre-diabetes, it makes you fat, then later it causes diabetes, for instance.) Either way, this is as stupid as saying that poor balance isn’t associated with hip fractures, once you control for falls. There is a high degree of collinearity in all these factors, and anyone with even a first-year statistics background would laugh at this statement. Still, you hear it.