Yoga or Fiber? Thermodynamics in the Vegetarian

To the Editor.—  We read with interest the correspondence between Dr Mann and Sacks et al entitled "Dietary Intake, the First Law of Thermodynamics, and the Properties of Yoga"¹ concerning the intriguing question of low energy requirements in vegetarians. In the original article by Sacks et al,² it is reported, as expected, that the lactovegetarian diet had a prevalence of unrefined carbohydrates, vegetables, and fruit. As a consequence, fiber intake, although it was not measured, was necessarily high.

Study.—  In a recently completed study on the effect of dietary fiber on postprandial thermogenesis, we showed that postprandial heat production is inversely related to the fiber content of the meal.^3,4 In seven healthy volunteers, we compared postprandial thermogenesis, evaluated for six hours following the ingestion of three meals with a similar energy content and composition (1200 kcal [5040 kJ]; 15% protein, 55% carbohydrates, and 30% fat) but with