EFFECT OF A LOW CARBOHYDRATE DIET ON FUEL UTILISATION AND EXERCISE PERFORMANCE IN MILDLY OBESE ADULTS

Abstract

In light of the obesity epidemic and health risks associated with excessive body fat content, a growing number of people try to lose weight by switching to low carbohydrate and high protein and/or high fat diets. These diets are known to induce increased fat oxidation and utilisation of ketone bodies. Limited and conflicting research findings regarding such diets have prompted us to study the effects of a glycogen depleting diet on metabolism and duration of exercise known to depend mainly on carbohydrate stores. PURPOSE: To examine fuel utilisation and exercise performance at 85% VO2max in response to 14 days on a low carbohydrate (< 20grams/day), high fat and high protein diet in mildly obese individuals. Healthy adults, 7 females and 4 males, age 24 ± 0.85 (mean ± SE) with body fat 21.8 ± 3% in males and 30.1 ± 1.2% in females (hydrostatic weighing), took part in the study. Their VO2max estimated on cycle ergometer, was 29.6 ± 0.8 (ml/kg/min) and 31.8 ± 3.9 (ml/kg/min) in females and males, respectively. RESULTS: 14 days on a low carbohydrate, high fat and high protein diet led to a significant (p < 0.05) body fat mass loss (2.53 ± 0.35 kg) and a marked (p < 0.05) shift in the respiratory exchange ratio (RER). RER, (reflecting energy source utilised) decreased from 0.81 ± 0.02 to 0.71 ± 0.006 at rest and from 1.05 ± 0.002 to 0.93 ± 0.02 (mean ± SE) during exercise. Resting serum urea concentration (reflecting protein degradation) increased significantly (p < 0.05) in response to the diet (from 4.8 ± 1 to 6.1 ± 1 mmol/L), but was not affected by exercise and not associated with any lean body mass changes. Pre- and post-diet blood glucose and lactic acid levels did not differ at rest, during exercise and the recovery period. Also exercise time until exhaustion did not change in response to the diet (8.18 ± 1.80 vs 8.00 ± 1.25). CONCLUSIONS: Low carbohydrate, high fat and high protein diet induces metabolic shift towards increased fat and protein oxidation. Lack of any changes in blood glucose and lactic acid levels at rest, during exercise and the recovery period and maintenance of exercise duration in the absence of dietary carbohydrates, indicate increased gluconeogenesis and glycogen sparing effect of enhanced fat utilisation in response to the diet. Increased post-diet serum urea concentration and lack of change in lean body mass imply that increased protein degradation is due to its increased dietary intake and not muscle-wasting. Study supported by the University of Ottawa Faculty Development Fund.