Weight Gain: Protection Against Hyperthyroidism?

It’s been said that if an elephant’s metabolic rate were the same as that of a mouse, the elephant would spontaneously combust. This would happen because, although an elephant burns many more calories in total than does a mouse, a mouse burns many more calories per unit mass than does an elephant.  Burning calories leads to heat production. But because an elephant cannot dissipate heat nearly as easily as a mouse can, an elephant with a mouse’s metabolism would become so hot internally that, theoretically anyway, the animal would burst into flames.

Now, keep our unfortunate elephant in mind when reading the following statement: “If you eat 5,000 calories of only fat and protein, some protein will be converted by gluconeogenesis, some will go to building muscle, the fat will go to building hormones and cellular repair and the rest will be converted to heat by various metabolic processes and wasted.”

Like me, I’m sure you’ve read some version of the above belief a few times before – that is, the idea that all excess calories from fat and protein are dispelled from the body as heat when one is on a low carbohydrate diet. But as you can see from the theoretical elephant, creating heat is not always a good thing. Heat generated internally will raise core body temperature unless it is dissipated via the body’s surface area into the external surroundings. Maintenance of core temperature is something the body must regulate within a very narrow range by some combination of thermogenesis (heat creation) and heat dissipation/conservation. Too little heat generated and/or conserved will cause hypothermia and too much heat generated and/or conserved will cause hyperthermia. Both conditions can be fatal. Because animals have a limited body surface area from which to release heat, they cannot produce heat in an unrestrained manner without risking hyperthermia. These facts should call into question the assertion that all excess calories consumed on a low carb diet will be converted to heat and wasted. Note that I said all excess calories; it has been shown experimentally that humans can increase their energy expenditure (which creates heat) when overfed 1,000 calories a day so that many individuals will not gain as much weight as expected. The most “genetically lucky” individuals were able to increase their metabolisms to such an extent as to dissipate 600 of the extra 1,000 calories. So yes, some calories can be wasted as heat, but we must remember that thermogenesis does not occur in a haphazard manner and so we cannot assume that an unlimited amount of calories can be spent in heat production without some negative consequence arising.

Jules Hirsch and Theodore Van Itallie were thinking along these lines in 1973 when they penned a letter to the editor in The American Journal of Clinical Nutrition. They were critiquing a study done by Heinrich Kasper that purported to show that a normal weight adult individual (it’s not clear if the subject was male or female) gained no weight over a 10 day period of consuming 5,950 calories per day with most of the calories (4,988) coming from fat (corn oil). Kasper and colleagues conjectured that the lack of weight gain could be evidence of “luxus consumption” (heat creation as a means of wasting energy so that it will not be stored as fat) but when Hirsch and Van Itallie crunched the numbers, they showed just how potentially metabolically damaging disposing of 2,900 calories a day could be:

“If one were to accept the authors’ contention that “ . . .under a relatively low carbohydrate and protein intake, increasing amounts of fat produce an increase in the metabolic rate that becomes particularly marked if fats high in linoleic acid are given,” then the reader appears to be drawn into the following chain of calculations. A young man requires approximately 3,000 kcal/day, assuming a modest activity level. Approximately one-half of this amount is needed to support his basal metabolism (BMR). If, by consuming large quantities of corn oil, he then increases heat production sufficient to dispose of 2,900 (5,900 minus 3,000) additional calories, this “luxus consumption” would seem to require an elevation of the subject’s BMR by nearly 200% (on the average). Because patients with severe hyperthyroidism exhibit a BMR somewhat in excess of +50%, the data of Kasper et al. would call for this normal young subject to manifest a degree of hypermetabolism (24 hr/day) three- to fourfold greater than that associated with the most toxic form of Graves’ disease. It is perhaps not out of place to mention also that, for every degree (Fahrenheit) of temperature rise above normal, there is supposed to be an associated 8% increase in metabolic rate. In view of this fact, it seems inconceivable that the sensation of “heat over the entire body” reported by the subjects on a high corn oil intake could account for the almost 200% increase in BMR implied by the observations of Kasper et al.”

Using Hirsch’s and Van Itallie’s reasoning, let’s crunch the numbers on the subjects in the overfeeding study who were able to dispose of 600 extra calories per day. Since the subjects in this study were said to be sedentary (as opposed to modestly active), let’s bump their caloric requirement down to 2,500 calories/day. Assuming that about one-half of the total caloric requirement is needed to support basal metabolism, the subjects’ BMR would be approximately 1,250 calories/day. Wasting 600 calories per day would require an increase in BMR of a little less than 50%. Confounding the calculations is the fact that the subjects in the overfeeding experiment were gaining weight where as the subject in the Kasper study was not, but it’s interesting to speculate if a 50% elevation in BMR is the maximum increase the body will allow when trying to dispose of excess calories. Thyroid hormone is the primary regulator of thermogenesis; if the body wants to burn off excess calories, it must produce and release more thyroid hormone.  Too much thyroid hormone can have deleterious effects on health including coma and death.  According to Hirsch and Van Itallie, problems with severe hyperthyroidism begin when one's BMR increases above 50% of normal.  It appears some (lucky?) individuals can rev up their metabolisms to that point before they start storing fat.  Most of us however cannot and maybe that's not such a bad thing after all.