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Letter to the Editor |
Maastricht University, PO Box 616, 6200 MD Maastricht, Netherlands, E-mail: p.schrauwen{at}hb.unimaas.nl
Dear Sir:
During consumption of a diet that meets energy requirements, it takes several days before fat oxidation adapts to fat intake when the diet composition is changed from low fat to high fat (1). This finding was confirmed recently by the results of a study by Smith et al (2), who showed a delay in the rise in fat oxidation when the dietary fat content was shifted from 37% to 50% of energy. Smith et al noted that there was a striking degree of variability in the rate of adaptation of fat oxidation to this increased fat intake. According to their data, postabsorptive respiratory quotient, fasting insulin, and maximal oxygen uptake (
O2max) were predictors of the capacity to adapt fat oxidation to fat intake.
Although we agree with the conclusion of Smith et al that there is high variability in the capacity to adapt fat oxidation to fat intake, we believe that these data should be interpreted with caution. It is known that there is a clear hierarchy in the maintenance of macronutrient balances, with carbohydrate and protein balance having the highest priority (3). Fat oxidation, on the other hand, is determined mainly by the difference between energy expenditure and carbohydrate and protein oxidation. Therefore, fat balance is strongly correlated with energy balance. This is where we believe caution should be taken with the interpretation of the data of Smith et al.
In Smith et al's study, the cumulative fat balance over 4 d, when subjects were consuming a high-fat diet in a respiration chamber, was considered to be a good indicator of the capacity to adapt fat oxidation to fat intake. However, because energy balance and fat balance are strongly correlated, the cumulative fat balance over 4 d might just reflect the cumulative energy balance. Although Smith et al proposed to feed the subjects in energy balance, energy balance was not reached on any of the days in the respiration chamber. More important, there was great interindividual variation in the magnitude of energy balance (SEM: 500 kJ/d; n = 6). The interindividual variation in fat balance might also reflect the interindividual variation in energy balance. In this respect, it is important to note that in the study by Smith et al, subjects' energy requirements were calculated by multiplying resting metabolic rate by a physical activity index (PAI) factor of 1.5. However, on average, a PAI factor of 1.4 was achieved in the respiration chamber, resulting in positive energy balance. It is known that physical fitness (as could be indicated by O2max) is related to the PAI, ie, the fitter subjects are more active throughout the day (4). This means that subjects with higher O2max values might have been more active in the respiration chamber (higher PAI) and consequently have showed a less positive energy balance and thus a less positive cumulative fat balance. Thus, the correlation between O2max and cumulative fat balance might be artificial. The same reasoning might hold for fasting insulin concentrations because physical fitness improves insulin sensitivity (5). It is very likely that, in this situation, energy balance determined the rate of adaptation of fat oxidation to the high fat intake. A situation of zero energy balance should be achieved before conclusions can be made about interindividual variation in the capacity to adapt to high-fat diets.
Second, we believe that the role of glycogen stores cannot be neglected in the interpretation of the data. In his 2-compartment model, Flatt (6) showed that one can adapt fat oxidation to an increased fat intake by 2 mechanisms: 1) expansion of the fat stores or 2) maintenance of the glycogen stores in a lower range. In the short term, and with subjects in (near) energy balance, the first mechanism is of no importance in the adaptation of fat oxidation to fat intake. We showed previously that glycogen stores are indeed important in the rate at which fat oxidation is adapted to a high fat intake (1, 7, 8). Because fat oxidation does not adapt rapidly to the increased fat intake with a high-fat diet, subjects will be in negative carbohydrate balance during the first days of the high-fat diet. This means that they will ultimately lower their glycogen stores, which allows, according to Flatt's model, fat oxidation to increase. Indeed, fat oxidation matched fat intake after several days of consumption of a high-fat diet (1). When glycogen stores were lowered acutely before the start of the experiment, however, both lean and obese subjects were capable of adjusting fat oxidation to a high fat intake within 1 d (7, 8). Because the subjects in the study by Smith et al were in positive energy balance, they probably did not reduce their glycogen stores sufficiently to allow fat oxidation to completely adapt to fat intake. Again, the degree of positive energy balance might have influenced the adaptation.
REFERENCES
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