The ketogenic diet is a very low carbohydrate diet that has shown therapeutic benefits in drug resistant epilepsy (1, 2) and is gaining increased attention in other neurological diseases (3) and in healthy aging (4). Ketones (acetoacetate [AcAc] and D-beta-hydroxybutyrate [D-BHB]) are produced by the liver when blood glucose and insulin decrease (5). Blood ketones above 0.5 mM indicates a ketosis state that reaches 3–5 mM on the ketogenic diet. One of the main benefits of ketones is their ability to act as an alternative energy source to glucose or fatty acids for production of ATP by mitochondria. Caloric restriction and intermittent fasting also produce transient mild-moderate ketosis (6, 7).

As an alternative to the ketogenic diet, exogenous ketone precursors taken orally achieve mild ketosis in the absence of dietary restriction. They can be grouped in three categories (Figure 1): First, medium chain triglycerides (MCT) composed of a mixture of 8 and 10 carbons fatty acids are efficiently digested to free fatty acids (FFA), directly absorbed, and rapidly metabolized by the liver. The acute production of excess acetyl-CoA drives the production of AcAc and then D-BHB which are both secreted into the systemic circulation (8). While a high dose of MCT can provide a moderate increase in blood ketones (+0.5–1.0 mM), gastrointestinal intolerance and high caloric load limit their use. Second, ketone esters (KE) made of a BHB ester linked to butanediol provide one molecule of D-BHB after digestion, with the butanediol being further metabolized by the liver to D-BHB (9). KE increase blood ketones above 1 mM but are also limited at high dose by their gastric tolerability and severe bitterness (10). Third, perhaps the most physiologic way to raise blood ketones is via the oral intake of D-BHB itself. Exogenous D-BHB is directly absorbed into the circulation, with some of it being converted to AcAc by the liver, and both ketones being distributed throughout the body. Until recently, only racemic mixtures of dextro (D) and levo (L) BHB (D+L-BHB) were available and oral human studies with them have been reported (9, 11–14). As L-BHB is not metabolized significantly into energy intermediates and is slowly excreted in the urine (9, 15), D+L-BHB would be anticipated to be less ketogenic than pure D-BHB.

Once ketone precursors are absorbed and metabolized, the resulting ketones are taken up by extrahepatic tissues such as brain, heart, muscle, and kidney and metabolized to acetyl-CoA for ATP production in mitochondria (5). Understanding how ketones are utilized by different organs after the intake of a ketone precursor is therefore starting to gain in importance. For instance, brain energy is derived mostly from glucose, but ketones spare brain glucose consumption when they are available (16). When blood ketone levels are increased (either by ketogenic diet or exogenous ketones), the brain utilizes ketones preferentially. Brain ketone metabolism is directly proportional to plasma ketone level over a wide concentration range. An increase in brain ketone metabolism can increase overall brain energy supply in mild cognitive impairment and Alzheimer's disease (17–19). The heart is an energy omnivore and uses both FFA and glucose as major energy substrates (20). Increased blood ketones produced by acute intravenous infusion of D+L-BHB reduces myocardial glucose utilization without affecting myocardial FFA metabolism (21). The kidney uses FFA as its main energy source (22) and, although less well-studied, ketones have been shown to be preferred over FFA, lactate and other endogenous energy substrates for the kidney (23). Despite utilizing ~20% of total body energy intake, the liver cannot use ketones as a source of energy as it lacks the enzyme succinyl-CoA:3-oxoacid-CoA transferase (SCOT) required to convert AcAc back to acetyl-CoA (3). However, the liver contributes to the interconversion of AcAc to D-BHB via mitochondrial D-BHB dehydrogenase (BDH1).

Positron-emission tomography (PET) using the ketone tracer, ¹¹C-AcAc, was developed initially to directly observe ketone metabolism in the brain of people developing MCI and AD (24, 25). It has been used to study heart energy metabolism in rodents (26) and has the potential to provide insight into whole body ketone metabolism in humans. Here, the objective was to compare the metabolism of a pure D-BHB oral supplement, i.e., the increase in blood D-BHB and AcAc after D-BHB, to that produced by the ingestion of the same amount of racemic D+L-BHB or MCT. A pilot study was also performed to assess the feasibility of using ¹¹C-AcAc PET to observe organ ketone uptake after oral ingestion of D-BHB.

To our knowledge, this is the first report of the metabolism of D-BHB in humans and its link to the organ distribution of the ketone, AcAc, by PET imaging. This study shows that D-BHB is rapidly absorbed and metabolized (Figure 2). It also increases blood ketones above 1 mM, a level ~1.7 fold greater than the same dose of D+L-BHB or MCT, and this despite the lower caloric load of D-BHB. Moreover, the results of the pilot whole body ¹¹C-AcAc-PET experiment support the feasibility of this technique to measure organ-specific exogenous ketone utilization and they suggest that exogenous ketones are very actively utilized by the heart and kidney.

Ketone production from an exogenous dietary source has been traditionally achieved by MCT. This requires a bolus intake to saturate the liver with MCFA, producing excess acetyl-CoA which is then transformed to AcAc and BHB, which are released into systemic circulation. The C_max achieved with MCT is usually between 300 and 600 μM, with higher values being difficult to reach due to GI side effects and liver saturation. Here we show that D-BHB, a natural and biologically active ketone isomer, raises blood ketone C_max above 1 mM without noticeable side effects. In comparison, an equivalent dose of D+L-BHB or MCT only achieved half this ketone level, with similar T_max at 1 h. Thus, compared to D+L-BHB, D-BHB significantly reduces the salt intake needed to achieve the same plasma ketone response.

Results from a previous study (9) comparing KE to D+L-BHB showed that at the same dose of D-BHB equivalent, the increase blood ketone iAUC had the same magnitude, suggesting that exogenous D-BHB and KE produce similar ketosis. We note that the T_max of D+L-BHB was significantly longer (~90 min) compared to KE (~20 min) at the same dose, suggesting that D-BHB could have a longer duration of action than KE. We also found that D+L-BHB resulted in a significant increase of plasma L-BHB that eliminate slowly, consistent with a previous report (9) and indicating a difference in the metabolic utilization of D and L-BHB. While D-BHB is readily utilized as an energy source, the L-isoform is eliminated slowly in the urine and does not contribute significantly to the biological activity of ketone precursors as an energy source. Finally, D-BHB provided higher ketones per calorie consumed than MCT: the caloric density of D-BHB (4.6 kcal/g) is half that of MCT (8.2 kcal/g), making D-BHB a 2.6 fold more energetically efficient ketone precursor vs. MCT (based on iAUC over 4 h).

The present study also revealed that D-BHB conversion to AcAc provided a ~43% higher AcAc/D-BHB blood ratio (0.63) than the same dose of MCT (0.44). In this respect, KE seems to behave more like MCT, with a ratio around 0.2–0.4 (3), while on the ketogenic diet, this ratio remains unchanged around 0.5 (16). Since D-BHB needs to be converted to AcAc by BDH-1 within each organ before being metabolized to acetyl-CoA (3), a higher AcAc/D-BHB ratio could be energetically more effective. Moreover, as this conversion requires the co-factor, nicotinamide adenine dinucleotide (NAD⁺), to be reduced to NADH, a higher AcAc/D-BHB might better preserve the mitochondrial NAD⁺/NADH ratio. Interestingly, a higher NAD⁺/NADH ratio has been associated to better health (30, 31), and can be increased in the brain by nutritional ketosis (32, 33), suggesting that D-BHB might be superior to MCT or KE in this respect, a point meriting further investigation.

PET has been invaluable in measuring energy metabolism at the organ level in humans. For example, with ¹⁸F-FDG, it clearly shows that glucose is the main energy substrate of the brain and that a fatty acid such as ¹¹C-palmitate is preferentially utilized by the heart and the liver (34, 35). Our pilot ¹¹C-AcAc-PET experiment aimed simply to demonstrate the feasibility of measuring ketone uptake by the main organs when exogenous D-BHB is administered orally to humans. The key advantage of ¹¹C-AcAc as a metabolic radiotracer is that it equilibrates rapidly with D-BHB in blood and is indistinguishable from endogenous AcAc, so reflects the actual metabolism of ketones. The whole-body scan revealed for the first time that the heart and kidney are major contributors to the metabolism of exogenous AcAc after ingestion of a D-BHB supplement.

Most of the heart's energy requirement is normally provided by fatty acids and glucose (20). Our results confirm that the heart has the capacity to utilize other substrates such as ketones (21) and that ketone metabolism appears to be linked to the metabolism and clearance of the radiotracer by the myocardium. On a per gram basis, the kidney consumes a large amount of energy mostly derived from fatty acids, but our data show that ketones can clearly also be used effectively for mitochondrial respiration in the kidney as well.

The cerebral metabolic rate for ketones (CMR_ketones) obtained here with D-BHB (3.0 μmol/100 g/min) exceeded that obtained after MCT supplementation in mild cognitive impairment [2.49 μmol/100 g/min; (17)] and Alzheimer's disease [1.9–2.0 μmol/100 g/min; (18)]. While the CMR values obtained here for D-BHB are preliminary, our data suggest that a D-BHB supplement represents a promising intervention to provide ketones that can rescue the brain energy glucose deficit in these conditions (17, 25).

¹¹C-AcAc uptake by the liver (Figures 4, 5) could reflect both the conversion of ¹¹C-AcAc to ¹¹C-D-BHB by BHD-1 and the high blood reservoir capacity of the liver, but do not reflect ketone metabolism by the liver itself. We show here that the blood radioactivity values of ¹¹C-AcAc were similar to those of the liver (see Figure 5) supporting the view that the liver produces but does not metabolize ketones (5). This is consistent with the liver radioactivity after ¹¹C-AcAc infusion being due to blood and not to the conversion of AcAc into AcAc-CoA and other energy intermediates (5).

Recent studies have shown the potential importance of ketones in cardio-metabolic health. Infusion of D+L-BHB has beneficial hemodynamic effects in adult patients with heart failure and lower ejection fraction (36). In children with fatty acid oxidation defects such as multiple acyl-CoA dehydrogenase deficiency, D+L-BHB improved heart function and cognitive performance, which resulted in better walking ability and correction of neurological symptoms (37). Moreover, clinical trials in diabetes with sodium/glucose co-transporter-2 (SGLT-2) inhibitors, which increase plasma ketones (38), show markedly reduced risk of cardiovascular events and kidney failure (39). Given the significant dysregulation of energy metabolism in type 2 diabetes, it is plausible that part of the effect of SGLT2 inhibitors on heart and kidney is mediated via improved ATP production using ketones as an alternative energy fuel to fatty acid and glucose (38, 40). This opens additional opportunities for therapeutic exploration of D-BHB supplements beyond energy rescue of the brain.

This study has several limitations. First, the pharmacokinetic comparison of D-BHB, D+L-BHB and MCT was an acute, single 4 h study; the extent to which it reflects long-term differences in their metabolism remains to be determined. Second, the whole-body PET scan was done on a single person and (except for brain) was a semi-quantitative comparison across organs. The brain was chosen for the quantitative scan (CMR_AcAc). Dynamic PET scanning is required in order to quantify the tracer uptake and cannot be done simultaneously on the brain as well as other organs. Cardiac gated image quality could be improved if the image was acquired sooner after tracer injection. Third, ketone production and metabolism vary depending on post-prandial metabolic status, which possibly influences organ ketone distribution. Hence, follow-up of this single observation under different feeding conditions will be needed to verify the relative differences across organs.

D-BHB appears to be a promising supplement to produce significantly higher blood ketones than D+L-BHB or MCT, and at a lower calorie intake for an equivalent dose. Moreover, exogenous D-BHB does not appear to lower the blood AcAc/D-BHB ratio, which, altogether, might make it a more effective “super fuel” compared to other ketone precursors such as MCT, D+L-BHB or KE.

The pilot ¹¹C-AcAc-PET study clearly identifies the heart and kidney as significant consumers of exogenous ketones, in fact, more than the brain. Therefore, D-BHB supplementation could be tested in conditions such as heart failure and diabetic cardiomyopathy to improve cardiac energy efficiency and function, and in chronic kidney disease.

All datasets generated for this study are included in the article/Supplementary Material.

The studies involving human participants were reviewed and approved by the Ethics Committee of Canton de Vaud (Switzerland) under the generic protocol reference 2018-00503, Trial registration number NCT03603782 (pharmacokinetic study), and by the CIUSSS de l'Estrie, CHUS Research Ethics Committee, Sherbrooke (Canada; PET study). The patients/participants provided their written informed consent to participate in this study. Written informed consent was obtained from the individual(s) for the publication of any potentially identifiable images or data included in this article.

MH, EC, C-AC, SC, and BC designed the studies. Data collection was performed by MH, EC, C-AC, J-PG, and MH. C-AC and EC analyzed data. The manuscript was drafted by BC and SC and all authors discussed the results and revised critically the manuscript.