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 [13–15]. 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 [40–42]. 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 . 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 .
The TEF encompasses the energy expended in the process of ingesting, absorbing, metabolizing, and storing nutrients from food . 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 , 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 . 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 . 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.