I’m getting an increasing number of questions about exogenous ketones. Are they good? Do they work for performance? Is there a dose-response curve? If I’m fasting, can I consume them without “breaking” the fast? Am I in ketosis if my liver isn’t producing ketones, but my BOHB is 1.5 mmol/L after ingesting ketones? Can they “ramp-up” ketogenesis? Are they a “smart drug?” What happens if someone has high levels of both glucose and ketones? Are some products better than others? Salts vs esters? BHB vs AcAc? Can taking exogenous ketones reduce endogenous production on a ketogenic diet? What’s the difference between racemic mixtures, D-form, and L-form? What’s your experience with MCTs and C8?
The major determinant of whether the liver will produce ketone bodies is the amount of liver glycogen present (8). The primary role of liver glycogen is to maintain normal blood glucose levels. When dietary carbohydrates are removed from the diet and blood glucose falls, glucagon signals the liver to break down its glycogen stores to glucose which is released into the bloodstream. After approximately 12-16 hours, depending on activity, liver glycogen is almost completely depleted. At this time, ketogenesis increases rapidly. In fact, after liver glycogen is depleted, the availability of FFA will determine the rate of ketone production. (12)
If you’ve done any reading about ketosis, you no doubt read at some point that ketosis is a “natural” state. You may have read on a bit more and learned what is meant by that statement or you may have simply skipped ahead to the keto success stories and decided to give it a try. But we’d like to direct your attention back to that little tidbit of information about keto being “natural” for a moment.
And now, you can take ketone supplements (salts and esters), known as exogenous ketones, without actually restricting anything. According to those promoting this nasty-tasting supplement, that means you can have a brain and body fuelled by ketones, along with all of the supposed health benefits that come with running on fat. Well, don't fall for it.
I simply use this to attempt to reduce the symptoms of the "keto-flu" when I'm entering ketosis after blowing my carbs out. The holidays are particularly bad for falling off the keto band-wagon. I've used this three times now to transition back into ketosis and I can report that it does seem to reduce the effects of the keto flu (headache, weakness) that I'd normally experience transitioning back into a low-carbohydrate diet. I typically take it for 3 days and then stop because by that time I'm in ketosis again, but I'd imagine you could take it longer.

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.

Taking exogenous ketones not only eliminates the need to follow a strict ketogenic diet to achieve ketosis (so you can have your high carb cake and eat it too), it can also help users get there faster. “They can expedite the process of getting into ketosis and becoming fat adapted,” Davis explains. “They can also help people push past the keto flu and potentially experience more mental energy and clarity than from diet alone.”
All of the data I’ll present below were from an experiment I did with the help of Dominic D’Agostino and Pat Jak (who did the indirect calorimetry) in the summer of 2013. (I wrote this up immediately, but I’ve only got around to blogging about it now.) Dom is, far and away, the most knowledgeable person on the topic of exogenous ketones. Others have been at it longer, but none have the vast experiences with all possible modalities (i.e., esters versus salts, BHB versus AcAc) and the concurrent understanding of how nutritional ketosis works. If people call me keto-man (some do, as silly as it sounds), they should call Dom keto-king.
I carried out a survey among Diet Doctor users as background research to the experiment (a big thank you to the 638 people who responded!). In the survey, 28% of the respondents reported that they do take ketone supplements. The top four benefits that these respondents reported experiencing were increased energy, improved focus/cognition, reduced hunger and weight loss.
Comments are welcomed and encouraged. The purpose of comments on our site is to expand knowledge, engage in thoughtful discussion, and learn more from readers. Criticism and skepticism can be far more useful than praise and unflinching belief. There’s an art and science to critical thinking and how to conduct yourself. There’s a multitude of fallacious appeals we could spell out, but a good rule of thumb is not to attack the person, attack the ideas. Don’t look for the flaws in the person, look for the flaws in the hypothesis. Let’s keep the brawling to movies depicting minor league hockey teams and political “news” shows. Thank you for adding to the discussion.
MCT oil has recently been used to induce nutritional ketosis although it produces dose-dependent gastrointestinal (GI) side effects in humans that limit the potential for its use to significantly elevate ketones (>0.5 mM). Despite these limitations, Azzam and colleagues published a case report in which a 43-year-old-man had a significant decrease in seizure frequency after supplementing his diet with 4 tablespoons of MCT oil twice daily [96]. An attempt to increase his dosage to 5 tablespoons twice daily was halted by severe GI intolerance. Henderson et al. observed that 20 % of patients reported GI side effects with a 20 g dose of ketogenic agent AC-1202 in a double blind trial in mild to moderate Alzheimer’s patients [24]. We visually observed similar gastrointestinal side effects (loose stools) in the rats treated with MCT oil in our study. Rats were closely monitored to avoid dehydration, and gastric motility returned to normal between 12–24 h. Interestingly, the BMS + MCT supplement elevated βHB similarly to MCT oil alone, without causing the adverse gastrointestinal effects seen in MCT-supplemented rats. However, this could be due to the fact in a 10 g/kg dose of BMS + MCT, only 5 g/kg is MCT alone, which is less than the 10 g/kg dose that elicits the GI side effects. This suggests that this novel combination may provide a more useful therapeutic option than MCT oil alone, which is limited in its ability to elevate ketones in humans.

The liver is always producing ketones to some small degree and they are always present in the bloodstream. Under normal dietary conditions, ketone concentrations are simply too low to be of any significant benefit. A ketogenic diet and exogenous ketone supplements will increase the amount of ketone in your body. The idea that  ketones are “toxic” is ridiculous. Ketones are a normal physiological substance that play many important roles in the human body.
Other studies have found that fasting was as effective as chemotherapeutic agents in delaying progression of different tumors and increased the effectiveness of chemotherapeutic drugs against melanoma, glioma, and breast cancer cells. Although this research may not apply to your life, it does suggest that intermittent fasting can help support your body in times of toxic stress.
It's important to listen to your body when going through the ketogenic process. This means that you should only eat when you're hungry and not every single time you get a craving. It's our obsession with food that causes us to stuff ourselves whenever we feel like it, and you should know by now that it's not healthy to do that. When you make it a point to eat only when you're hungry, you're diminishing any food intake that your body doesn't really need. 
Humans in the hunter-gatherer era survived thanks to metabolic flexibility — the body’s ability to use different fuels for energy depending on the nutrients available. This adaptation was vital during a time when the source, quantity, and frequency of food was uncertain[*]. Most of the time, people were fasting, so their bodies ran on ketones, not glucose.

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