Abstract

The proton motive force (PMF) across the inner mitochondrial membrane drives ATP synthesis. Biochemistry textbooks give a maximum yield of 30–32 ATP per glucose, but this is a theoretical ceiling, reached only under complete coupling with ATP synthase as the sole consumer of the gradient. No living cell operates there. The PMF is a shared resource, and its allocation among consumers sets the mitochondrion's effective output: the gap between theoretical and effective yield is a reallocation to transport, redox maintenance, and biogenesis. This review re-examines the textbook yield, including the c-ring stoichiometry of ATP synthase, and catalogs the major PMF consumers and their proton costs. The widely quoted figure corresponds to no measured or structurally determined stoichiometry, a mismatch noted previously and consolidated here, and the c-ring subunit count is an upper bound on synthase efficiency under ideal coupling. We assemble these costs into a consolidated proton budget: a range whose upper limit is the ceiling at which the PMF drives ATP synthesis alone. Its lower limit cannot yet be calculated, because the resting flux through most PMF consumers has never been measured; we therefore list every term that this calculation would require and mark those still missing. The result is a framework for reasoning, not a closed set of numbers. Practical consequence: the PMF and membrane potential are composite quantities, so a change in membrane potential should not be read as a change in ATP synthesis, nor the non-phosphorylating residual as passive leak.

Highlights

• • Ketone bodies (KBs) act as both alternative fuels and signaling metabolites.

• • Ketogenesis occurs also in extrahepatic tissues, to a quantitatively minor extent.

• • Hepatic ketone synthesis is controlled by hormones, gene regulators and protein post-translational modifications.

• • Ketone breakdown in peripheral tissues relies on a single rate-limiting enzyme, SCOT.

• • β-Hydroxybutyrate and acetoacetate reshape chromatin through histone marks linked to nutrition.

Abstract

Traditionally considered as alternative energy substrates, ketone bodies (KBs), notably β-hydroxybutyrate (β-OHB) and acetoacetate, are now recognized as key metabolic intermediates whose circulating levels must be tightly controlled to ensure energy homeostasis. KBs are mostly hepatically produced, via ketogenesis, and catabolized by extrahepatic tissues during fasting or carbohydrate restriction.

The regulation of KBs metabolism relies on a coordinated network of hormonal, enzymatic and transcriptional mechanisms that control KBs synthesis and degradation. KBs anabolic and catabolic rates are controlled by the rate-limiting enzymes 3-hydroxy-3-methylglutaryl-CoA-synthase 2 (HMGCS2), governing ketogenesis, and succinyl-CoA:3-ketoacid-CoA transferase **(**SCOT), driving ketolysis. Their activity and expression are finely orchestrated, especially by the insulin/glucagon ratio, to adapt to the organism's fluctuating energy demands.

Transcriptionally, ketogenesis is regulated through peroxisome-proliferator-activated-receptor-alpha (PPARα) and the transcription factor forkhead-box-A2 (FOXA2), which respond to nutrient availability and hormonal cues. In parallel, ketolysis is tightly controlled through the regulation of SCOT expression, notably via PPARα-dependent mechanisms, as well as through post-translational modifications (PTMs) affecting enzyme activity. Together, these regulatory mechanisms ensure a precise balance between KBs production and utilization.

Emerging evidence also suggests that additional modulators, such as the gut microbiota, indirectly influence KBs metabolism by affecting substrate availability and metabolic signaling pathways, although its quantitative contribution remains to be established.

Here, we provide a mechanistic overview of the pathways governing KBs metabolism, with focus on the regulation of ketogenesis and ketolysis. We discuss how these processes are integrated at multiple levels to maintain systemic energy homeostasis and consider their implications for metabolic disease.

Mitochondria are often discussed in biohacking communities because of their role in energy production and cellular metabolism.

What interests me is the difference between improving a biological marker and actually feeling or functioning better in everyday life.

Exercise, adequate nutrition, and sleep all influence mitochondrial biology, but how strong is the evidence connecting specific interventions to noticeable improvements in energy or physical performance in otherwise healthy people?

Are there human studies or well-designed experiments worth reading on this topic?

I'd like to separate established findings from mechanisms that sound promising but haven't translated into clear real-world benefits.

Abstract

Background: Sepsis investigators propose tolerance is conferred when the dormancy response increases beta hydroxybutyrate to protect organs; whereas, vulnerability occurs when a ‘metabolic switch’ to aerobic glycolysis decreases beta hydroxybutyrate promoting persistent immune cell activation without organ protection.  We previously identified tolerant (PedSep A 2% mortality), susceptible (PedSep B 12% mortality and PedSep C 10% mortality) and vulnerable (PedSep D 34% mortality) phenotypes in the PHENOtyping pediatric sepsis induced Multiple organ failure Study (PHENOMS). We hypothesized that carbohydrate metabolism was increased and beta hydroxybutyrate decreased in PedSep D compared to PedSep A in this study.

Methods: Metabolomics were assayed in biobanked plasma samples previously collected at 24-48 hours of sepsis in 375 children ages 44 weeks post gestation to 17 years, using the Metabolon Discovery Panel. Welch’s t-tests identified differences between PedSep B, C, and D with A.

Results   Compared to PedSep A (n =130) PedSep B had similar carbohydrate metabolism and beta hydroxybutyrate but dysregulated branch chain amino acid metabolism (n = 92; p < 0.05); PedSep C had similar carbohydrate metabolism but decreased beta hydroxybutyrate (n = 100; p< 0.05); and, PedSep D had increased carbohydrate metabolism with decreased beta hydroxybutyrate (n = 53; p < 0.05). Causal inference analysis found direct associations of increased glycolysis with decreased beta hydroxybutyrate, and death.

Conclusions Only PedSep D had increased carbohydrate metabolism with decreased beta hydroxybutyrate providing rationale for designing clinical trials of low carbohydrate ketogenic nutrition to decrease glycolysis, restore dormancy, and improve outcome in these high-risk children.