Against metabolism

the pound mouse, a leptin-insensitive mouse used to study obesity in the lab

the pound mouse, a leptin-insensitive mouse used to study obesity in the lab

Note: My internal definition of metabolism, in the context of “I have a slow metabolism”, translates to “basal metabolic rate”, scientifically speaking. This is also how I’ve been perceiving most people to mean it, when speaking colloquially. “Metabolic rate” is broader and includes total daily energy expenditure–in addition to basal metabolic rate, it includes energy burned by movement and in digesting food. When I was talking with my husband about this blog post after it was mostly finished, he expressed surprise that “how hungry you are” isn’t part of metabolism, so in some cases the reason people are attributing so much importance to metabolism may be that their understanding of the meaning of “metabolism” is different than mine.

I think that metabolism is overemphasized in online fat activism.

People like to point out that individual fat people don’t necessarily eat more than the average thin person–and the converse, that individual thin people sometimes eat much more than the average fat person. (This is especially likely to be true if you compare a fat person who is short, female, and older to a thin person who is tall, male, and a teenager.) People also like to point out that for any given weight (even if fat-free mass is well-preserved), a person who is at that weight after a diet will have a slower metabolism than a person who’s at that weight who hasn’t dieted. This is known in the scientific literature as adaptive thermogenesis*. (Conversely, metabolism disproportionately increases for people who are above their normal weight.) (When it comes to long-term weight-loss maintainers, the research is mixed, but it seems to be quite well established that those with relatively-recent weight loss burn fewer calories–20-25% fewer–than would be predicted by their new body weight.)

And this is true as far as it goes. But it seems to me that metabolism is one of the least important factors in why people weigh what they do (which is not to say that’s it’s completely unimportant), and it is increasingly unimportant the more people that people let their hunger and fullness signals guide what they eat, rather than eating mindlessly or eating what they believe they’re “supposed to” eat, whether “supposed to”  means “what Mark Bittman says” or “what Weight Watchers says” or “manly men are supposed to order big steaks” or “this is the amount of food the restaurant served me/my spouse served me/what’s in the package, so I guess that’s what I’m supposed to eat”.

The more important metabolism is in affecting people’s BMI, the more we would expect to see that short people have higher BMIs than tall people. And in fact, we do see that to some degree–though apparently the relationship has been getting weaker, and tall people’s BMIs have been catching up to short people’s. (How this may compare to people who are genetically predisposed to become fat in the current food environment is left as an exercise to the reader.) Additionally, some argue that one of the flaws of the BMI system is that while it works well for people of about average height, it gives short people a lower BMI than they “should” have, and vice versa for tall people. Still, it’s not as though there are no thin short people and no fat tall people. Anecdotally, I remember a short, thin friend talking about how a normal-looking-to-me pastry was SO BIG and made her so full, she couldn’t eat it all.

The folk explanation of hunger seems to go something like this: everyone (sometimes even regardless of age, height, activity level, and sex) is equally hungry. Fat people don’t actually require more food to sate their hunger, they just overeat all the time. (See JK Rowling’s description of how Dudley Dursley eats. She seems to be thinking, “Well, this is how I would feel if I ate that much–overstuffed and uncomfortable–so if other people eat that much, that must also be how they feel.”) If lifting weights allows you to eat more food and not gain fat, then that’s great, and we won’t even bother to ask whether you’ll have an increase in hunger that matches or even exceeds the greater amount of food you’ll “be allowed” to eat. Thin people (often teenagers) who eat a lot aren’t eating a lot because they’re that much more hungry than the other thin people; they just can “get away with it”, so they do. If you look at hunger and fullness hormones like ghrelin and leptin, this, like many folk understandings of natural phenomena, does not seem to be how it actually works.

In Traci Mann’s recent Washington Post interview, she talks about, among other things, the Cliff’s Notes version of why people usually regain weight after dieting. The change in metabolism is one of three factors that she lists; the other two are neurological (“Basically your brain becomes overly responsive to food, and especially to tasty looking food. But you don’t just notice it — it actually begins to look more appetizing and tempting.”) and hormonal (“As you lose body fat, the amount of different hormones in your body changes. And the hormones that help you feel full, or the level of those rather, decreases. The hormones that make you feel hungry, meanwhile, increases. So you become more likely to feel hungry, and less likely to feel full given the same amount of food.”).

I suspect that the hormonal changes are the ones that have the most powerful effect on weight regain, and that hormones have the most powerful effect on weight in general. (Do the neurological changes have something to do with the “eat impulses” that DebraSY describes?) While studies have found that things like how much food is presented does affect how much people eat (at least at that particular meal–I suspect that those bottomless soup bowls, different sized plates, etc are often balanced out by less food at a later meal, or a missed snack) my sense is that the strongest influence on how much people eat is how hungry they are, and that ultimately is mostly controlled by your homeostatic mechanisms, the stuff that tries to keep you in a given set point range, which is where the hormones come in.

(Complicating this, metabolism is affected by these hormones–or at least one of them. While looking up citations for this post, I came across a study that found that injecting leptin into people who’d recently lost 10% of their body weight nullified the slowdown of metabolism that caused them to need disproportionately (20-25%) fewer calories. So in a way it is misleading to talk about them as separate factors.)

There is a difference between “genes that affect weight” and “particular genes that we’ve identified that affect weight”, but with that caveat in mind: so far only one of the genes involved in body weight that’s been discovered, MRAP2, works by decreasing metabolism rather than other mechanisms such as increasing hunger.

I suspect that I am not one of those people with an abnormally-slow metabolism. My subjective assessment is that I eat about the same amount as an average man my age–I’m more active than a lot of people, but still, I think I do eat more than the average woman my age. But it’s because I am hungry.

My theory about why people put so much emphasis on metabolism: eating the same amount of food but weighing more feels a lot more concrete and objective as a defense of being fat than saying that you get hungry more easily. And people can easily assert that fat people probably aren’t really more hungry, they’re just deluded or not as tough in the face of hunger or whatever. And yet, we have the pound mouse, which gets so large because of the lack of a functioning leptin receptor, not because of how it extracts energy from food–and certainly not because of some genetically-encoded tendency for it to lie to itself about how hungry because it’s looking for a justification for being fat. And yet, if we lose 10% of our body weight, we have the same alterations to our leptin and ghrelin levels (and the same changes in energy expenditure relative to body weight) as a thin person who loses 10% of their body weight. And yet, plenty of anti-fat people will say “if you need to eat less than other people to be thin, just eat less”–but it’s not so simple to do that when you have neurological and hormonal changes trying to get you to do exactly the opposite. And yet, people will assume that fat people are deluded or lying when they say that they don’t eat more than other people. (They never seem to consider that they’re deluding themselves with the idea that they’re thinner because they work harder at being thin.) But we shouldn’t have to justify ourselves to these people in the first place. We’ve been chided all our lives to second-guess whether we’re really hungry, and we don’t need their help.

*How adaptive thermogenesis works, from “Metabolic adaptation to weight loss: implications for the athlete”:

Metabolic rate is dynamic in nature, and previous literature has shown that energy restriction and weight loss affect numerous components of energy expenditure. In weight loss, TDEE has been consistently shown to decrease [38,39]. Weight loss results in a loss of metabolically active tissue, and therefore decreases BMR [38,39]. Interestingly, the decline in TDEE often exceeds the magnitude predicted by the loss of body mass. Previous literature refers to this excessive drop in TDEE as adaptive thermogenesis, and suggests that it functions to promote the restoration of baseline body weight [1315]. Adaptive thermogenesis may help to partially explain the increasing difficulty experienced when weight loss plateaus despite low caloric intake, and the common propensity to regain weight after weight loss.

Exercise activity thermogenesis also drops in response to weight loss [4042]. In activity that involves locomotion, it is clear that reduced body mass will reduce the energy needed to complete a given amount of activity. Interestingly, when external weight is added to match the subject’s baseline weight, energy expenditure to complete a given workload remains below baseline [41]. It has been speculated that this increase in skeletal muscle efficiency may be related to the persistent hypothyroidism and hypoleptinemia that accompany weight loss, resulting in a lower respiratory quotient and greater reliance on lipid metabolism [43].

The TEF encompasses the energy expended in the process of ingesting, absorbing, metabolizing, and storing nutrients from food [8]. Roughly 10% of TDEE is attributed to TEF [44,45], with values varying based on the macronutrient composition of the diet. While the relative magnitude of TEF does not appear to change with energy restriction [46], such dietary restriction involves the consumption of fewer total calories, and therefore decreases the absolute magnitude of TEF [41,46]. NEAT, or energy expended during “non-exercise” movement such as fidgeting or normal daily activities, also decreases with an energy deficit [47]. There is evidence to suggest that spontaneous physical activity, a component of NEAT, is decreased in energy restricted subjects, and may remain suppressed for some time after subjects return to ad libitum feeding [29]. Persistent suppression of NEAT may contribute to weight regain in the post-diet period.

In order to manipulate an individual’s body mass, energy intake must be adjusted based on the individual’s energy expenditure. In the context of weight loss or maintaining a reduced body weight, this process is complicated by the dynamic nature of energy expenditure. In response to weight loss, reductions in TDEE, BMR, EAT, NEAT, and TEF are observed. Due to adaptive thermogenesis, TDEE is lowered to an extent that exceeds the magnitude predicted by losses in body mass. Further, research indicates that adaptive thermogenesis and decreased energy expenditure persist after the active weight loss period, even in subjects who have maintained a reduced body weight for over a year [14,48]. These changes serve to minimize the energy deficit, attenuate further loss of body mass, and promote weight regain in weight-reduced subjects.

TL:DR version: total energy expenditure includes the energy you burn just by existing, the energy you expend during exercise, the energy you expend by moving but not exercising, and the energy you use to digest food. When you lose weight, not only are all of these reduced because of your reduced body size, but all of them are also reduced because your body works more efficiently (beyond what would be expected just by your smaller size) in using energy for just existing and for exercising, because you move around and fidget less, and because all these other adaptations mean you need less food to maintain your weight and so you use less energy digesting food.

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3 Responses to Against metabolism

  1. Michelle says:

    As you can tell I am so excited you’re writing about this stuff! And I completely agree that “metabolism” as a concept is misused a lot in HAES and fat acceptance communities, especially online. I find that annoying. I can understand WHY, because people are trying to defend themselves against dehumanizing stereotypes and stigma.

    Fat people actually only need a modest amount more energy than a thinner person would need in order to reach and maintain a significantly higher weight. Calories are funny that way. So I can understand WHY people feel like they’re not eating more than thinner people (because they may not be overtly binge eating, which is usually the stereotype), but in reality an extra 100 calories daily over time can create significant weight differences. I know that I eat somewhat more than my thinner peers (usually — there are always outliers), but I also don’t eat massive amounts more.

    Whatever amount we eat, ultimately I don’t believe we CHOOSE that amount consciously. I believe our underlying biology, which contributes to the weight diversity we see in the population as a whole, drives us to eat that amount and to maintain weight in a certain range. The environment can certainly influence this as well, but “environment” /= 100% voluntary behaviour.

    • ““environment” /= 100% voluntary behaviour”
      I really wish more people understood this.

      I was trying to remember an article that questioned the “100 calories can lead to big changes” idea… I found it, but it is mostly talking about cutting 100 calories to lose weight, and part of the reason why the change is less dramatic than we would think with the magazine math/bunsen burner view is related to set point theory and the body’s resistance to weight loss. If one’s set point/homeostatic mechanisms are both causing the extra 100 calories to “stick” better and leading one to be more likely to consume the 100 calories in the first place… once again metabolism and eating behavior are not so neatly separated.

      Anyway, yeah, more than once, people outside of HAES have pointed out that most people’s weight stays within a pound of the previous year’s each year, and if they were trying to calculate that based on nutrition labels there’s no way they’d come that close, so I agree that the evidence is strong that it’s generally not determined by conscious behavior.

      This is one article saying that–I was looking at while looking up citations for this post:

      “If energy intake and output were not regulated by interlocking control mechanisms that work concordantly to maintain energy stores, then a very small persistent change in input relative to output would, over time, lead to substantial gain or loss of stored calories. Yet, the average U.S. adult gains only 500–1000 g of weight (approximately 2000–2500 kcal of stored energy) per year… despite ingestion of approximately 900,000–1,000,000 kcal/year. The remarkable constancy of body weight in this context, presumably without conscious constant calculation of how many calories are being consumed and/or expended by most individuals, suggests that energy intake and expenditure vary directly to maintain relatively stable energy stores.”

      • Michelle says:

        Yeah, even though I recognize that a small increase in calories can result in significant weight differences (over time), I’ve always been skeptical that the reverse is true. I mean, it feels intuitive to think that, if you can gain a lot of weight over a decade by eating an extra 100 calories above your TEE, then the reverse should also be equally true. But I’m not sure the body’s energy balance mechanisms work that way. In biochemistry we see that sometimes it is energetically impossible, or very very difficult, to reverse a chemical reaction, and I think that can be analogous to this situation. I am fully willing to believe that the body more closely defends against weight loss than against weight gain, and there has been at least one article suggesting that this may be true –

        I’ve always found it interesting that, if a person were going to attempt the project of eating 100 fewer calories per day, the level of tight monitoring and precision required to monitor such a small deficit might be beyond the level of precision currently available in existing nutrient databases, and possibly even TEE equations, that people use to track their intake and set caloric goals. In some ways, it would be MORE difficult to attempt to eat precisely 100 fewer calories than to cut down your eating by a much larger amount, like 500-1000 calories per day, purely because calorie tracking is not actually as precise and accurate as it seems. And it would take a long time to verify (through weight loss) whether your calculations were correct within the margin of error. It’s easier to do something drastic, because your short-term weight loss will quickly verify that you are eating at a deficit, even though you will be hungrier.

        And if you were not tracking at all while attempting to eat 100 fewer calories per day, it would be impossible to know whether you were unconsciously making up that deficit elsewhere in your day, either through less spontaneous movement, or by eating a little bit extra of something else. 100 calories is such a small amount that it might be insensible either way.

        Overall, I’m pretty convinced by the evidence I’ve read (probably in various textbooks about metabolism) that the body attempts to match energy intake to expenditure, and that we are not in completely conscious control of the drive to eat.

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