What dietary source has the lowest thermic effect on metabolism?

The correct choice is fat, which has the lowest thermic effect on metabolism. The thermic effect of food (TEF) refers to the energy expenditure associated with the digestion, absorption, and metabolism of nutrients. Different macronutrients have varying impacts on this effect.

Fat, being more calorie-dense and less metabolically demanding to process compared to carbohydrates and protein, results in a lower thermic effect. It requires less energy for digestion and conversion into usable energy forms. In contrast, protein has a significantly higher thermic effect because it takes more energy to break down amino acids and process them for metabolic use. Carbohydrates also have a higher thermic effect than fat, as they require energy for digestion and conversion.

Alcohol, while also generally considered to have a lower thermic effect than protein and carbohydrates, has unique metabolic pathways that can influence how it is processed, contributing to its energy expenditure profile. However, when directly comparing them based on TEF, fat consistently ranks as the macronutrient with the least metabolic cost in terms of energy expenditure.