Interestingly, the effects of exogenous ketones on blood substrate concentrations were preserved with the metabolic stimulus of a mixed meal. Following KE drinks, FFA and glucose fell and remained low in both fed and fasted subjects, despite higher insulin throughout the fed arm, suggesting that there was no synergistic effect of insulin and βHB to further lower blood glucose or FFA. In agreement with previous work, the threshold for the effects of βHB on glucose and lipids appears to be low (<1 mM), as there was no significant dose-response relationship between increasing blood βHB and the small changes in plasma FFA, TG or glucose across all of the study drinks (Mikkelsen et al., 2015).
That said, there also remains the question of the relative benefits of AcAc versus BOHB, both as independent signaling molecules and as redox modulators in peripheral (aka non-hepatic) tissues. Seen from this perspective, AcAc generated in the liver acts as a NAD+ donor for the periphery, whereas pure BOHB taken orally, to the extent that it is retro-converted to AcAc (Sherwin 1975), potentially deprives the periphery of NAD+.
Glucose and BHB went down slightly throughout the effort and RQ fell, implying a high rate of fat oxidation. We can calculate fat oxidation from these data. Energy expenditure (EE), in kcal/min, can be derived from the VO2 and VCO2 data and the Weir equation. For this effort, EE was 14.66 kcal/min; RQ gives us a good representation of how much of the energy used during the exercise bout was derived from FFA vs. glucose—in this case about 87% FFA and 13% glucose. So fat oxidation was approximately 12.7 kcal/min or 1.41 g/min. It’s worth pointing out that “traditional” sports physiology preaches that fat oxidation peaks in a well-trained athlete at about 1 g/min. Clearly this is context limited (i.e., only true, if true at all, in athletes on high carb diets with high RQ). I’ve done several tests on myself to see how high I could push fat oxidation rate. So far my max is about 1.6 g/min. This suggests to me that very elite athletes (which I am not) who are highly fat adapted could approach 2 g/min of fat oxidation. Jeff Volek has done testing on elites and by personal communication he has recorded levels at 1.81 g/min. A very close friend of mine is contemplating a run at the 24 hour world record (cycling). I think it’s likely we’ll be able to get him to 2 g/min of fat oxidation on the correct diet.
BHB isn’t just an energy source for the brain–it has other effects which promote brain health. BHB can trigger the release of chemicals called neurotrophins, which support neuron function and synapse formation. One of these neurotrophins is called BDNF (brain-derived neurotrophic factor), which is a protein in the brain associated with cognitive enhancement, alleviation of depression and reduction of anxiety.10
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