Thursday, March 19, 2009

Hypermetabolism: the basics

Many people recovering from AN experience hypermetabolism, a common phenomenon that is often overlooked. I did some more research tonight into the biology and etiology of hypermetabolism. So let's start with a basic definition from Wikipedia:

Hypermetabolism is the physiological state of increased rate of metabolic activity. The impact of the hypermetabolic state on patient nutritional requirements is often understated or overlooked. Hypermetabolism typically occurs after significant insult to the body. In hospitals and institutions, the most common causes are infections, sepsis, burns, multiple trauma, fever, long-bone fractures, hyperthyroidism, prolonged steroid therapy, surgery and bone marrow transplants.

And, yes, eating disorders. Specifically anorexia. As someone falls deeper and deeper into anorexia, their metabolic rate slows dramatically. During the Minnesota Starvation Study, resting energy expenditure (also known as basal metabolic rate- the amount of calories needed just to keep your body functioning and doesn't include ANY physical activity) fell by about one-third. The book "Introduction to Clinical Nutrition" says that

Starvation involves metabolic alterations that enhance the chance of survival by increasing the use of body fat stores, by sparing the use of glucose, by minimizing nitrogen loss, and by decreasing energy expenditure.

But when the ill person begins eating again, their metabolism kicks into high gear. Body temperature rises. A person can experience night sweats, which may also be related to hormone function returning to normal. Why? It seems a remarkably inefficient use of resources. And indeed, hypermetabolism may not be entirely adaptive from an evolutionary standpoint as the body's use of food becomes remarkably inefficient. Even so, the body needs tremendous amounts of energy to replace lost fat and muscles stores, depleted organs, bone mass, hair, nails, you name it. No organ system is spared during an eating disorder.

The amount of calories needed for people with anorexia to return to a healthy weight can vary by illness severity (a lower BMI means more calories, as well as duration of illness), and by illness subtype. One study found that those with the binge-purge subtype of anorexia needed significantly fewer calories than those with the restricting subtype; another study found that calories were about the same. Other variants include your metabolic rate before illness onset. I've known several good friends curse their fast metabolisms quire vociferously during refeeing.

When looking at the phase after weight restoration, caloric needs between people with anorexia nervosa and those with bulimia nervosa turn out to be quite different. One study found that people with AN needed more calories per kilogram of body weight than normal controls, while those with BN needed fewer. It appears that people with a current diagnoses of BN but a history of AN require more calories than those with BN alone. Further, people with the restricting type of AN needed more calories than those with the binge-purge type, both of which were greater than patients with BN (some of the studies cited above show that, for weight maintenance, caloric needs are basically the same for any patient with a current diagnosis of AN).

Some of these differences may rest in premorbid differences in metabolic rate. It makes intuitive sense that a person who finds it easy to lose weight would have a faster innate metabolism. Other reasons calorie needs may remain unusually high for a person even after weight restoration is the sheer amount of rebuilding the body needs to perform. Bulimia is violently destructive to the body, and I would never say differently. However, some of the damage done by anorexia is slightly different, and the body must rebuild and repair essentially every organ in the body. My psychiatrist told me that the nerves continue to repair themselves for up to two years after weight restoration. This can hardly be the only organ system taking a long time to recover.

How long hypermetabolism lasts will probably vary from person to person, and depends on how long you were sick, how your body responded to the damage from your eating disorder, your activity levels, among others. There's no real way to be sure. If your caloric needs are unusually high--even for hypermetabolism--you can have body composition analysis and resting energy expenditure testing done. Personally, I think this is best left to extreme cases since so many sufferers have a tendency to fixate on numbers.

I hope this helps explain hypermetabolism just a little bit. I can't answer every question, as it's been a long time since my college biochemistry days, but I can always look things up.


Kim said...

This was incredibly helpful for me to read. Before my eating disorder, I had a high metabolism (which I read in one of your other posts may actually be a "cause" of anorexia). In recovery, I seem to need to eat 3000+ calories just to be ok. Anything less seems to be "restrictive" for me. It's been hard for me to understand and rationalize, especially seeing that other "normal" people eat so much less (I still do a lot of comparing). There's a lot of comfort in science, so thank you for posting this. It makes me feel less alone and weird.

Carrie Arnold said...

Yes- Walt Kaye's study has saved my sanity more than once!

I think your high metabolism definitely made you more vulnerable to AN because you probably get into a metabolic deficit much faster than others. I didn't have a super-high metabolism before AN, but it was probably above average. That, combined with the genetic wiring, made for a quick decent into ED hell.

Kim said...

Another question for you: My nutritionist did one of those stupid metabolic tests on me...and it claimed that I only need 1200 calories/day. Of course, this totally f-ed with my mind. Obviously, it's not accurate. I don't even start to gain until I get up to around 2800-3000. Sorry for the numbers talk, but I'm trying to understand. Is it that my metabolism has decreased due to my anorexia over the years?? I'm definitely not actively restricting anymore so I expected my metabolism to be really high...If I ate 1200 calories/day, I'd lose a bunch of weight. I don't get it. Anyway, thanks for the helpful posts!

Carrie Arnold said...


It sounds like the RD measured your basal metabolic rate (BMR), which is basically how many calories your body needs to breathe and not much else. It's the measurement of how many calories you burn if you were stuck in bed all day. As a reference, they usually do about 1500 cal/day for coma patients via tube feeds.

BMR depends on (in part) your weight, so if this measurement was done when you were still underweight, it could be off. And restricting/dieting is a fantastic easy way to lower your BMR. But hypermetabolism also affects non-exercise thermogenesis. This is NOT factored into BMR but DOES affect how many calories you need to gain or maintain your weight.

The short answer? Keep doing what you're doing. :)

This was a very good question- thanks for asking.

Kim said...

Thank you for taking the time to clarify for me :) I really didn't take to heart what the RD said. She seemed confused herself. Ha. I'm going to take your advice and keep doing what I'm doing. I know my body and don't really need a measurement to tell me what's healthy :)

StephanieJoy81 said...

Oh my goodness! Thank you so much for writing this. I am in this situation right now! I've been relentlessly trying to restore my weight to a healthier place, and it seems no matter how much I eat, I can't get past the point I'm at. Do you have any suggestions for me? I'd love to chat with you about it.

firefly said...

How do you explain this? History of restrictive An. Weight restored and as I left program restriction began again. Actively have restricted for a year and recently upped exercise. I can't seem to loose and actually gained when I upped the walking. I'm so scared to eat because since I'm not loosing I obviously don't need as many cals as everyone else. Can this be explained

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