Fat adaptation is the shift from carbohydrates as the primary fuel source to fat as the primary fuel source. Here’s a study of fat adaptation (Helge JW, Watt PW, Richter EA, Rennie MJ, Kiens B. Fat utilization during exercise: adaptation to a fat-rich diet increases utilization of plasma fatty acids and very low density lipoprotein-triacylglycerol in humans. J Physiol. 2001 Dec 15;537(Pt 3):1009-20.). The study:
was carried out to test the hypothesis that the greater fat oxidation observed during exercise after adaptation to a high-fat diet is due to an increased uptake of fat originating from the bloodstream.
The study had 13 male untrained subjects, seven consumed a fat-rich diet (62 % fat, 21 % carbohydrate) and six consumed a carbohydrate-rich diet (20 % fat, 65 % carbohydrate). Note that this was not a low carbohydrate diet (at 21%). The length was 7 weeks of training and diet.
The performance test was 60 min of bicycle exercise performed at 68 +/- 1 % of maximum oxygen uptake. This rate is the maximum rate of fat oxidation from this study (Nutrition. 2004 Jul-Aug;20(7-8):716-727. Optimizing fat oxidation through exercise and diet. Achten J, Jeukendrup AE.). The test measured the RER and found that the high fat group used significantly more fat as fuel:
During exercise, the respiratory exchange ratio was significantly lower in subjects consuming the fat-rich diet (0.86 +/- 0.01, mean +/- S.E.M.) than in those consuming the carbohydrate-rich diet (0.93 +/- 0.02).
An RER of 0.85 indicates that the subject were taking energy equally from fat and carbohydrates so the high fat group was getting about half of their energy from fat and half from carbohydrates. The high carb diet group got much less of their energy from fat.
The leg fatty acid (FA) uptake (183 +/- 37 vs. 105 +/- 28 micromol min(-1)) and very low density lipoprotein-triacylglycerol (VLDL-TG) uptake (132 +/- 26 vs. 16 +/- 21 micromol min(-1)) were both higher (each P < 0.05) in the subjects consuming the fat-rich diet.
Not only was the fatty acid uptake higher in the high fat group the fat oxidation was greater.
Whole-body plasma FA oxidation (determined by comparison of (13)CO(2) production and blood palmitate labelling) was 55-65 % of total lipid oxidation, and was higher after the fat-rich diet than after the carbohydrate-rich diet (13.5 +/- 1.2 vs. 8.9 +/- 1.1 micromol min(-1) kg(-1); P < 0.05).
Burning higher fat rates reduces the amount of muscle glycogen which is used.
Muscle glycogen breakdown was significantly lower in the subjects taking the fat-rich diet than those taking the carbohydrate-rich diet (2.6 +/- 0.5 vs. 4.8 +/- 0.5 mmol (kg dry weight)(-1) min(-1), respectively; P < 0.05), whereas leg glucose uptake was similar (1.07 +/- 0.13 vs. 1.15 +/- 0.13 mmol min(-1)). 4.
The study conclude that
…plasma VLDL-TG appears to be an important substrate source during aerobic exercise, and in combination with the higher plasma FA uptake it accounts for the increased fat oxidation observed during exercise after fat diet adaptation.
If fat is available as fuel the body will use the fat in preference to the glucose. If insufficient fat is available the body will use glucose.
The decreased carbohydrate oxidation was apparently due to muscle glycogen sparing and not to diminished plasma glucose uptake.