Animal studies show that short-term changes to cardiac energy utilisation are reported with the administration of high fat diets. This partly results from increased availability of free fatty acids (FFA) as an energy source, and partly results from the ability of FFA to alter gene expression. For example, FFA are able to increase expression of mitochondrial uncoupling protein 3 in cardiac and skeletal muscle, and increase ketone body production in the liver, via the peroxisomal proliferator-activated receptor alpha (PPARα) transcription factor. Uncoupling protein 3 transports protons across the mitochondrial matrix without the generation of ATP. In addition, short-term alteration in metal performance are reported with high fat diets because the main source of energy for the brain under normal conditions is glucose. As glucose levels fall, activation of PPARα increases ketone body production in the liver, however, it takes time for homeostatic mechanisms to adapt to the changes.
As expected, researchers1 investigating the effects of low carbohydrate high fat diets in healthy individuals have reported short-term changes to brain and heart function. In one such study, subjects were randomly assigned to either a high fat, low carbohydrate diet containing 75% of energy from fat or to an isocaloric diet providing 23% of energy from fat. Subjects consuming the high fat diet had an increase in plasma FFA of 44% when compared to those consuming the isocaloric diet. The creatine phosphate to ATP ratio of cardiac muscle was 9% lower in the high fat group, but there was no reported change to cardiac function. In addition, cognitive tests performed on the subjects showed that those on the high fat diet had impaired attention, speed of information processing and mood were impaired.
These results suggest that increasing the fat content of the diet causes significant short-term changes in physiology. The lower creatine phosphate to ATP ratio in cardiac muscle may have been caused by increased expression of uncoupling protein 3 which allowed some of the energy contained within the FFA to be dissipated as heat. Because uncoupling protein 3 is also expressed in skeletal muscle it is inviting to speculate that similar changes may have occurred causing a further dissipation of energy as heat. This would explain the most important finding of the study (absent from the abstract) that the study participants on the high fat diet lost body fat (0.8kg) but gained lean muscle (0.9kg). The decline in cognitive performance is to be expected initially on a high fat diet as ketone body production does not sufficiently counteract the fall in blood glucose.