Nutritional status, exercise, and insulin sensitivity : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Public Health, Massey University, Manawatu, New Zealand
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Date
2011
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Massey University
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Abstract
The insulin-glucose system in lean-healthy people adapts its normal function in the face
of challenging metabolic conditions. To improve understanding of these adaptations, I
exposed subjects to periods of starvation, high-protein-low-carbohydrate diet (HPLC) and
overfeeding.
In six lean-healthy men, dietary carbohydrate was eliminated but gluconeogenic substrate
supply was maintained by three-day HPLC diet, compared with three-day starvation and
three-day mixed-carbohydrate diet. Insulin sensitivity, vastus lateralis intramyocellular
lipid (IMCL) and fasting glucose were unaffected by HPLC diet, although they were
significantly altered after starvation. These results indicate that dietary carbohydrate
restriction does not trigger metabolic adaptations, although total metabolic carbohydrate
supply remains likely to be important.
Six lean-healthy men underwent two three-day periods of starvation with either no
exercise or daily endurance exercise (80 min.day−1 at 50% VO2Max) and a three-day mixed
diet without exercise. Compared to mixed diet, starvation elevated fasting FFA and
IMCL and decreased insulin sensitivity and fasting glucose. Exercise during starvation
prevented the elevation of IMCL but did not prevent other metabolic disruption, in
contrast with exercise during lipid infusion.
Maintaining high physical-activity may prevent the metabolic disruption associated with
overfeeding, while insulin sensitivity may predict partitioning of fuel between tissues
during overfeeding. Nine endurance-trained athletes maintained their normal physical activity while consuming a diet providing 90 kJ.(kg body mass)-1.day-1 above their normal
dietary intake for four weeks. Subjects’ body-mass, fat-mass and body fat% increased
while insulin sensitivity tended to decrease (14.5 ± 5.9 to 9.5 ± 4.1 min-1.mU.l-1, p =
0.08). Change in insulin sensitivity was correlated with change in body fat % (r = -0.77,
p < 0.023). Initial insulin sensitivity was correlated with change in body fat% (r = 0.90, p
< 0.009) and the proportion of mass gained as lean tissue (r = 0.86, p < 0.024).
Maintenance of already high physical-activity cannot prevent metabolic disruption
associated with overfeeding. These results also suggest that insulin sensitivity influences
energy partitioning between tissues.
The results in this thesis suggest important interaction effects between exercise and diet.
I propose that carbohydrate availability is a key modulator of the effects of exercise on
metabolism in lean-healthy men.
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Keywords
Insulin-glucose system, Dietary effects, Insulin sensitivity, Metabolic adaptations, Metabolism, Exercise, Exercise and diet, Nutrition