Metabolism

Metabolism is the sum of chemical work that keeps an organism alive: burning fuel, building tissue, clearing waste, making heat and CO2. Here it means oxidative respiration first, not a calorie spreadsheet. A slowed rate (low thyroid output, high stress hormones) shows up as low energy and weight gain even without eating more.

Metabolism is the set of chemical reactions by which cells turn food and oxygen into energy, heat, and structure.

Oxidative core

Thyroid hormones set the pace of oxygen use. Sugar and protein feed it. PUFA free fatty acids and stress hormones slow or divert it toward lactic acid and tissue breakdown. Protective CO2 rises when oxidation is real.

See Thyroid, Glucose, CO2, Mitochondria, Stress.

Path of carbohydrate oxidation

Useful oxidation of sugar is a chain, not a single switch. Rough order:

1. Glycolysis in the cytoplasm: glucose → pyruvate (small ATP; under stress or blocked mitochondria, more lactate).
2. Pyruvate dehydrogenase (PDH) at the mitochondrial gate: pyruvate → acetyl-CoA for full aerobic burning. Blocking PDH shifts cells toward fat use and lactate. Estrogen is one documented blocker of PDH (and of cytochrome oxidase further down the chain).
3. Krebs cycle (citric acid cycle): acetyl-CoA oxidized; NADH/FADH₂ generated; CO2 produced.
4. Electron transport / oxidative phosphorylation: electrons to oxygen; most of the ATP.

If a late step fails, earlier steps back up (e.g. high NADH tends to shut PDH). That is why thyroid, CO2, and intact mitochondria matter more than obsessing over one enzyme name.

Fructose can bypass a free-fatty-acid block early in glycolysis and keep pyruvate available for oxidation when glucose alone is stalled (Randle cycle context).

Rising free fatty acids suppress the oxidation of glucose (the Randle cycle); women, with higher estrogen and growth hormone, usually have more free fatty acids than men. Lactate can itself suppress cellular respiration, the Crabtree effect. Progesterone has the opposite effect of estrogen on metabolic rate, skin circulation, body temperature and heat loss, which causes the rise in temperature at ovulation.

Common brakes

These are the recurring anti-metabolic pressures in this framework (not a complete list):

• PUFA and elevated free fatty acids (Randle cycle)
• Stress hormones (cortisol, adrenaline) and low thyroid
• Endotoxin / gut leakage and inflammation
• Estrogen excess and related nitric oxide pressure on mitochondria
• Low protective CO2 (hyperventilation, poor oxidation)

Support pattern: sugar oxidation, protein with enough gelatin balance, salt, light, lower PUFA, gut care, thyroid context. See Pro-metabolic, Anti-metabolic, Roadmap.

Metabolic rate

Metabolic rate is reflected in heat production and carbon dioxide production. Body temperature and resting pulse are practical proxies for thyroid-related energy production, and the two measures should be read together. A low temperature with a high pulse can show adrenaline compensation for low thyroid, which can make a low metabolic rate appear high.

Practice

Judge metabolism by temperature, pulse, energy, sleep, and recovery, not only weight. Support food, light, salt, and thyroid context.

Left unsupported, this decline is large and continuous, not a late-life event: "From puberty to old age, oxygen consumption decreases by about 70% unless you intervene. This corresponds to the increase in mortality."

See Basal temperature, Pulse, Roadmap, Bioenergetics.

Bioenergetics

Bioenergetics is how living systems turn food into usable energy (ATP) and CO₂. Health tracks mitochondrial efficiency: high ATP and CO₂ from burning glucose, with barriers and hormones working from that energy base.

Energy and structure

Metabolic energy also helps organize cell structure. Carbon dioxide changes the behavior of proteins and the protein-water state. When mitochondrial respiration slows, glycolysis and lactate rise while ATP production falls. The change affects tissue structure as well as energy supply.

Cellular energy metabolism is the basis for maintaining the barrier functions. Energy depletion causes the endothelial cells lining blood vessels to become excessively permeable.

— Ray Peat

Albert Szent-Györgyi and Hans Selye sit behind the frame: cell energy and state set how hormones and nutrients act. Fixed lock-and-key receptors alone miss that field.

Efficient and stress oxidation

Efficient: glucose → CO₂ + water + abundant ATP. Supports: thyroid, low PUFA, low endotoxin.

Stress: glycolysis → lactic acid; high free fatty acids. Drivers: cortisol, estrogen, endotoxin, serotonin. See Randle cycle, Free fatty acids, Mitochondria.

Sugar oxidation makes more CO₂ than fat oxidation and protects proteins.

Oxidation of sugar is metabolically efficient in many ways, including sparing oxygen consumption. It produces more carbon dioxide than oxidizing fat does, and carbon dioxide has many protective functions, including increasing Krebs cycle activity and inhibiting toxic damage to proteins.

— Ray Peat

Uncoupling and heat

In mitochondria, fuel oxidation is normally coupled to ATP. Uncoupling lets oxidation run with less ATP. Pathological uncoupling from PUFA damage wastes fuel and suppresses respiration enzymes.

Thyroid (T3) and aspirin raise uncoupling in a directed way: sugar that would become lactate is burned toward CO₂ instead. Small mammals with highly uncoupled mitochondria show long life for their size in that literature. A study tracking individual outbred mice found the most metabolically intense quartile had 17% higher resting oxygen consumption and greater mitochondrial proton leak, yet lived 36% longer than the least intense quartile. Heat production is a feature of that high-throughput oxidative state, tied to thyroid and CO₂.

Hormones from energy

Steroid hormones (pregnenolone, progesterone, cortisol, androgens) come from cholesterol. Conversion needs thyroid, vitamin A, and oxidative capacity. Low energy and low thyroid leave cholesterol high as conversion fails. See Cholesterol, Progesterone, Thyroid.

Central organs and signals: thyroid sets metabolic rate; progesterone and pregnenolone protect against catabolic stress; estrogen and serotonin push stress metabolism; CO₂ and sugar stabilize the efficient side.

Randle link

Free fatty acids block full glucose oxidation. Polyunsaturated free fats amplify stress. Saturated free fats end stress signals. Carbohydrate availability stops lipolysis. See Randle cycle, Saturated fat, Linoleic acid.

Related pages

Start with Mitochondria and CO2. Then Thyroid, Glucose, Stress. Fats: PUFA, Free fatty acids, Randle cycle. Barriers: Tissue leakiness. Soviet systems view: Soviet medicine, Pyotr Kuzmich Anokhin.

Cold exposure

Cold exposure includes cold showers, ice baths, cold plunges, and winter outdoor stress. It raises adrenaline and can increase metabolic demand short-term. Ice baths specifically are popularized for recovery and dopamine spikes, often chasing biohacking status rather than an actual metabolic need.

White cells and cold

Cold, like hypothyroidism, tends to suppress or delay white blood cells' phagocytic clean-up activity, since the rate white cells move through tissue depends strongly on temperature. For hypothyroid, underfed, or high-stress people, aggressive cold is a stressor first, with high stress-hormone cost. Warmth supports thyroid-friendly physiology; prefer sleep, food, and warmth if temperatures and energy are already low. Mild cool air is not the same as ice-bath branding. A study following regular winter swimmers over a four-month season found improved mood and energy (significantly higher vigour-activity scores than non-swimming controls) and reported pain relief among swimmers with rheumatism, fibromyalgia, or asthma; the benefit tracked the duration of regular, adapted exposure rather than showing up immediately.

Practice (Cold exposure)

Ice-cold shower and cold-plunge use depends on metabolic state going in.

Practical read: check Basal temperature and pulse before treating cold plunges/ice showers as a routine practice, not after. If temperature and pulse are already low, deliberate cold exposure is more likely to push further into a stressed, low-thyroid state than to "train" the thyroid to respond faster. If temperature and pulse are solidly normal, an occasional brief cold exposure is more likely to be tolerated the way Peat describes in a healthy animal. Either way, check temperature again a few hours after exposure. A drop rather than a quick rebound is a sign to stop, not push through.

Weather cold and endocrine glands

Cold exposure intensifies thyroid activity: capillary congestion in the gland and loss of colloid from the follicles, a pattern that can progress toward hyperthyroidism; heat stress does the reverse, with colloid accumulation and gland inactivity.

Cold stress lowers serum calcium and was framed as a parathyroid deficiency state; the same weather pattern (below about 5.5°C with scarce sunshine, high rainfall, and strong winds) was linked to magnesium-poor grass and grass-tetany in cows.

Cold fronts and polar air raise 24-hour urinary 17-ketosteroid excretion by up to 100%, with higher winter averages than summer, and enlarge the adrenal cortex; large injected vitamin C doses blocked that cold-driven cortical hypertrophy in animal work.

Cold stress also raises capillary resistance. The adrenal cortex and pituitary control capillary tone and membrane permeability.

Post-exercise cold water immersion and muscle growth

A systematic review and meta-analysis of 8 studies on post-exercise cold water immersion combined with resistance training found small-to-negligible hypertrophy gains from training alone plus cold water immersion, compared to strong hypertrophic adaptations from resistance training alone; in the authors' conclusion, immersing in cold water immediately after lifting may blunt the muscle-growth response. This fits the broader pattern above: the inflammation and stress response to training is part of the adaptive signal, and blunting it immediately afterward with aggressive cold can blunt the adaptation itself, not just the soreness.

Glycolysis

Glucose molecular structure

Glycolysis breaks glucose down to pyruvate (and lactate under some conditions), yielding a small amount of ATP without oxygen: the reaction nets only 2 ATP directly per glucose molecule, versus about 6 more ATP once the 2 NADH it generates pass through oxidative phosphorylation, a small fraction of what full aerobic oxidation ultimately yields.

Aerobic and anaerobic glycolysis

Aerobic glycolysis is the conversion of glucose to lactic acid even when oxygen is present; oxygen normally restrains this so glucose is converted to CO2 instead. Anaerobic glycolysis is the increased conversion of glucose to lactic acid when oxygen supply is insufficient, a normal event during intense muscle action.

Glycolysis itself is not the problem. Trouble starts when pyruvate cannot enter efficient oxidation and lactate dominates under stress, low thyroid, or damaged mitochondria. Cutting sugar intake to "avoid glycolysis" misreads the mechanism.

Crabtree effect

The Crabtree effect, observed originally in yeast, is the inhibition of respiration by the presence of glucose; it shows up in cancers and in rapidly proliferating normal cells. Context matters: healthy tissue oxidizes glucose normally when thyroid and mitochondria are working; reliance on glycolysis is the pathological state, not the presence of sugar in the diet of someone recovering.

Gate to full oxidation

Pyruvate must pass pyruvate dehydrogenase (PDH) into the Krebs cycle for high-yield aerobic burning. When PDH is inhibited (e.g. by estrogen in Peat's mitochondrial notes), metabolism shifts toward fat use and lactate. See Mitochondria.

Free fatty acids can stall the use of glucose at a step in glycolysis where the sugar is converted toward fructose. Dietary fructose can bypass that barrier and keep oxidative carbohydrate metabolism going when pure glucose is blocked. See Fructose, Randle cycle, Sugar.

Heat

Heat is ambient or body temperature elevation. Humans need heat to run enzymes at full rate. Warmth as therapy means using ambient heat, warm food, clothing, and baths to support metabolism instead of chronic cold exposure.

Heat and the thyroid

Heat stress reduces thyroid activity, with colloid accumulation in the follicles, the opposite histologic pattern from cold-driven capillary congestion and colloid loss.

Warmth is pro-metabolic relative to chronic cold-exposure fashion; safe heating differs from fever or burns. Hypothyroid people already fail to generate heat, so ice baths as status symbols work against the goal. Safe sauna, infrared, and warm rooms are closer allies.

A single session of whole-body hyperthermia (raising core body temperature to 38.5°C via infrared heat) produced a significant reduction in depression symptoms that persisted for six weeks in a randomized, sham-controlled trial of adults with major depressive disorder, an effect notably fast compared to the weeks-long lag typical of SSRIs.

Post-exercise hot bathing may enhance strength gains from resistance training: in a short-term high-intensity leg-training program, participants who immersed their lower body in 40C (104F) water for about 10 minutes after each session gained roughly twice the leg strength improvement of a hot-shower control group, without adverse cardiovascular effects.

Krebs cycle

The Krebs cycle (citric acid cycle) oxidizes acetyl-CoA in mitochondria, feeding the electron transport chain and producing CO2 as a byproduct. Acetyl-CoA mainly arrives from pyruvate (via pyruvate dehydrogenase after glycolysis) or from fat oxidation; amino acids can also feed intermediate pools.

CO2 feeds back into the cycle

CO2 itself increases Krebs cycle activity and inhibits toxic protein damage; the glycation of proteins happens under stress, when CO2 production drops. Thyroid hormone and adequate cofactors keep the cycle turning over; PUFA and inflammatory stress slow it down.

Amino acids drain into the cycle

Glutamate and aspartate are normally drained off into the cycle, glutamate as alpha-ketoglutarate and aspartate as oxaloacetate (aspartate is also consumed in urea synthesis). When energy production is deficient, that drain fails: the backed-up glutamate and aspartate act as excitotoxic amino acids instead, promoting nerve excitation rather than being burned. Progesterone and thyroid hormone support the energy production that keeps this drain open; their deficiency tends toward instability of nerves instead. See Progesterone.

Chain dependence

The cycle is not independent of the rest of respiration. Product and cofactor balance (including NAD⁺/NADH) couples it to the electron transport chain and to PDH upstream. When late respiration fails, earlier steps back up and cells dump more lactate. A controlled human trial demonstrated this backup directly: raising plasma free fatty acids into the diabetic range (about 500-800 μmol/L) for 4 days cut first-phase insulin secretion 60% in people genetically predisposed to type 2 diabetes. Free fatty acids inhibit the PDH gate into the cycle.

Under stress, the cycle is also limited directly by the unavailability of CO2 itself, which wastes glucose as lactate instead of letting it finish burning to CO2 and energy. When Peat did name supplements for this kind of intracellular fatigue, they were niacinamide, aspirin, and supplemental CO2, alongside normalizing thyroid and steroid metabolism and a diet of fruit juice, milk, eggs or liver, and gelatin. Supporting whole-body oxidation (thyroid, sugar, lower PUFA, less endotoxin) matters more than isolated "Krebs supplements."

Pasteur effect

The Pasteur effect is the normal response of cells to restrain glycolysis in the presence of adequate oxygen, shifting toward efficient oxidation.

A working Pasteur effect means mitochondria are functioning normally. Loss of that respiratory control pushes cells toward lactate production and stress chemistry; thyroid hormone and membrane health support the oxygen-using path.

Thermogenesis

Thermogenesis is heat production from metabolism. Thyroid status, uncoupling, and muscle activity all contribute.

Body temperature

Warm body and warm extremities track useful oxidation better than cold-challenge fashion. Low morning temperature is a practical signal.

Brown fat

Brown adipose tissue burns fuel to make heat via uncoupling-protein pathways; it's more studied in infants and in cold exposure research than in typical adults. Cold-activated brown fat is not a prescription to ice-bath a hypothyroid adult; whole-body thyroid status and nutrition dominate adult heat-production stories, brown fat notwithstanding.

See also

• Harm reduction
• Lactic acid
• ATP
• CO2
• Warburg effect
• Oxidative phosphorylation
• Basal temperature
• Oxidative stress
• Ammonia