In the early 1970s, researchers from the University of Vermont published the results of the landmark Vermont experimental obesity studies, in which young men were deliberately overfed for seven months. That these men gained weight, ending up an average of twenty-five percent above their ideal bodyweight, was hardly an earth-shattering finding. What was surprising was that the men required fifty percent more calories to maintain this new heavier weight than they did at their usual lean weights. The researchers later discovered that short-term overfeeding was associated with increased thermogenesis (energy expenditure). Speculating that changes in thyroid function could be responsible, they proceeded to examine the effect of altering calorie and carbohydrate intake on thyroid hormone levels.
The first of these experiments involved closely supervised volunteer inmates from Vermont State Prison. During the overfeeding experiment, one group consumed a hypercaloric mixed (high-carb) diet for 7 months, while another group ate a hypercaloric high-fat diet for 3 months from primarily fat.
Again, that both groups gained weight should come as no surprise. However, the group overfed the mixed diet required more calories (2,625 kcal/m2 per day) to maintain their new heavier weights than did the group overfed fat (1,840 kcal/m2 per day). Baseline differences in metabolism between the two groups were ruled out, as there was no difference in total calories required to maintain initial lean weights.
Before and after the mixed diet overfeeding phase, the volunteers spent four weeks consuming maintenance level high-carbohydrate or low-carbohydrate diets. Thyroid hormone levels were measured at the end of each maintenance period, and after the overeating phase. For an average adult, the amount of carbohydrate consumed during the high- and low-carbohydrate weight-maintenance phases would translate to around 600 and 200 grams daily, respectively.
During maintenance eating, levels of T3 (triiodothyronine) were higher on the high-carb diet. When subjects on the low-carb diet began eating the higher-carb mixed weight gain diet, their T3 levels rose. T3 levels among those who went from the high-carb maintenance diet to the mixed diet remained unchanged. In contrast to T3, serum concentrations of T4 were unchanged by overeating or changes in dietary composition.
In the men overfed fat for 3 months, there was no change in mean T3 levels before and after the diet.