Caloric restriction but not exercise-induced reductions in fat mass decrease plasma triiodothyronine concentrations: a randomized controlled trial.

[MR. BMJ]

Caloric restriction but not exercise-induced reductions in fat mass decrease plasma triiodothyronine concentrations: a randomized controlled trial.

Weiss EP, Villareal DT, Racette SB, Steger-May K, Premachandra BN, Klein S, Fontana L.

Rejuvenation Res. 2008 Jun;11(3):605-9.

Division of Geriatrics and Nutritional Sciences, Department of Internal Medicine, Washington University School of Medicine, St Louis, MO, USA. eweiss4@slu.edu

Caloric restriction (CR) decreases circulating triiodothyronine (T(3)) concentration. However, it is not known if this effect is due to body fat mass reductions or due to CR, per se. The purpose of this study was to test the hypothesis that plasma T(3) concentration decreases with CR-induced reductions in fat mass but not in response to similar decreases in fat mass that are induced by exercise. Sedentary, nonobese 50- to 60-year-old men and women with no clinical evidence of cardiovascular or metabolic disease and not taking thyroid medications were randomly assigned to 12 months of caloric restriction (n = 18) or exercise-induced weight loss (n = 17) or to a control group (n = 9). Body weight and composition and plasma concentrations of the thyroid hormones T(3), thyrotropin (TSH), thyroxine (T(4)), and free thyroxine (FT(4)) were measured at baseline and 12 months. Fat mass changed significantly in the CR (-6.3 +/- 1.0 kg) and exercise (-5.5 +/- 1.0 kg) groups but not in the control group (-0.6 +/- 1.4 kg). The changes were not significantly different between the CR and exercise groups. Plasma T(3) concentration decreased in the CR group (-9.8 +/- 2.0 ng/dL, p < 0.0001) but not in the exercise (-3.8 +/- 2.1 ng/dL, p = 0.07) or control (-1.3 +/- 2.8 ng/dL, p = 0.65) groups. TSH, T(4), and FT(4) did not change in any of the study groups. Twelve months of CR decreased circulating T(3) concentrations in middle-aged adults. This effect does not appear to be attributable to changes in body fat mass because a comparable decrease in T(3) concentration was not observed in response to an exercise-induced fat mass reduction.

[PeteM]

Based on this it would appear that calorie restriction seems to negativly impact the performance of the enzyme responsible for converting T4 to T3. It is possible that this could be due to the absence of dietary componet in the CR group that reduced the bodies ability to manufacture the enzyme as there was no comment made as to dietary modification between the two groups.

It is also a rather small 'n' number in an older population, so it's difficult to extapolate the results to younger trained athletes. An interesting study none the less, but one that should spur further research rather then provide evidence for conclusions and influence training, nutrition and supplementation.

[MR. BMJ]

Yeah, judging by the abstract alone it is kind of hard to really determine much. Thanks for posting your thoughts on this pete. Your conclusion on it correlates with what I thought too. I pulled this from Lyle's site, but hadn't yet asked for the full study for it.

BMJ