Carbohydrate, fat, and protein metabolism in muscle and in the whole body after mixed meal ingestion.
Level 4 - case-series / case-control
Single-arm pre-post physiological study of substrate metabolism in humans
PubMed 3374320 · doi:10.1016/0026-0495(88)90169-2
What was done
Participants ingested a 3,275 kJ mixed meal (47.3% carbohydrate, 39.4% fat, 13.2% protein). Whole-body substrate oxidation was evaluated using indirect calorimetry, urinary nitrogen excretion, and changes in plasma urea concentration over four hours postprandial. Concurrently, forearm muscle substrate kinetics were assessed via forearm blood flow measurements and arterialized-deep venous metabolite gradients.
What was found
Over the first four hours postprandial, resting energy expenditure rose by 15% (P < .01), equivalent to 5.3% of the meal's energy content. Whole-body carbohydrate oxidation increased by 111% (P < .01) and protein oxidation increased by 40% (P < .05), while fat oxidation decreased by 21% (P < .05). Muscle uptake of glucose increased up to sevenfold alongside increased amino acid uptake, while muscle nonesterified fatty acid (NEFA) uptake was markedly suppressed. Muscle accounted for an estimated 20% to 25% of the total ingested carbohydrate. Fructose was not taken up by muscle in significant amounts. Muscle oxygen uptake showed only a small, non-significant change.
Why it matters
This study provides physiological quantification of postprandial energy partition, indicating that skeletal muscle is not the primary site of diet-induced thermogenesis and accounts for only a minority of early carbohydrate disposal.
Limits
The abstract does not state the sample size (n), participant sex, age, health status, or baseline physical characteristics. The study lacked a non-meal control arm to isolate postprandial changes from diurnal metabolic fluctuations.
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