Fat Oxidation: How Muscle Uses Fatty Acids for Fuel During Exercise

Fat oxidation is the breakdown of fatty acids inside mitochondria to make ATP, and at low exercise intensities it can supply most of the energy working muscle uses. Before stored fat is oxidized it typically passes three lipases, a carrier protein, a membrane transporter and a gatekeeper enzyme, and each step is regulated, which makes the route a useful map for judging metabolic research claims.

This guide follows that route, then covers exercise intensity, the “fat burning zone”, measurement, and what studies of AOD-9604, HGH Fragment 176-191 and 5-amino-1MQ have shown. It is physiology, not training or dietary advice.

What fat oxidation is

Fat oxidation, also called fatty acid oxidation, is the stepwise breakdown of fatty acids inside mitochondria. The liver, heart and skeletal muscle depend heavily on it; the heart takes roughly 60–90% of its energy from fatty acids. Working muscle draws on three fat sources: fatty acids released into the blood by fat tissue, triglyceride stored inside the muscle fibres (intramyocellular triglyceride) and triglyceride carried in lipoproteins.

Lipolysis: three lipases and their hormones

Lipolysis is the hydrolysis of stored triglyceride into glycerol and fatty acids. Until 2004, hormone-sensitive lipase was thought to be the only enzyme that did this in mammalian fat tissue; that year Zimmermann and colleagues described adipose triglyceride lipase (ATGL), which catalyzes the first step.

Adrenaline binds β-adrenergic receptors (see how GPCRs work), raising cyclic AMP and activating protein kinase A, the cascade in our cyclic AMP and PKA explainer. PKA phosphorylates perilipin-1, freeing CGI-58 to activate ATGL, and phosphorylates HSL, which moves onto the lipid droplet. Together these raise triglyceride breakdown in fat cells more than 100-fold, a 2012 review by Zechner and colleagues reports.

Insulin strongly brakes lipolysis: through the kinase PKB (AKT) it activates phosphodiesterases that break down cyclic AMP. In a study summarized by Lawrence Spriet of the University of Guelph, carbohydrate eaten before exercise raised insulin, and whole-body fat oxidation fell as much as lipolysis did. In muscle’s own lipid droplets, adrenaline and contraction both activate HSL.

From blood to mitochondrion: CD36, CPT1 and beta oxidation

Fatty acids dissolve poorly in plasma, so they travel on albumin, which has about seven binding sites for them and does not appear to hinder uptake by muscle. In 2000, Bonen and colleagues showed that contraction raises fatty acid uptake into rat skeletal muscle membrane vesicles by moving the transporter CD36 from inside the cell to the surface, as it does the glucose transporter GLUT4; a CD36 inhibitor blocked the rise completely.

Inside the fibre, fatty acids are activated to fatty acyl-CoA, which cannot cross the inner mitochondrial membrane. Long-chain fatty acids use the carnitine shuttle: carnitine palmitoyltransferase 1 (CPT1) on the outer membrane makes acylcarnitine, a translocase carries it across, and CPT2 turns it back into acyl-CoA in the matrix.

In 1977, McGarry and colleagues found that malonyl-CoA, a precursor of fatty acid synthesis, blocked oxidation of long-chain oleate in rat liver at this step but spared medium-chain octanoate. Acetyl-CoA carboxylase (ACC) makes malonyl-CoA, and the energy sensor AMPK inhibits ACC when energy runs low (see our AMPK explainer). Malonyl-CoA falls in contracting rodent muscle, but in human muscle it did not fall during exercise at 35–100% of maximal oxygen uptake (VO₂max), Spriet’s 2014 review reports, so free carnitine and pH are also candidate controls.

Beta oxidation and the TCA cycle

In the matrix, beta oxidation repeats four reactions: a dehydrogenation that makes FADH₂, a hydration, a second dehydrogenation that makes NADH, and a thiolytic cleavage that releases acetyl-CoA and leaves a chain two carbons shorter. FADH₂ electrons reach the respiratory chain through electron transfer flavoprotein, NADH feeds it directly, and acetyl-CoA enters the TCA cycle or, in the liver, can form ketone bodies.

How intensity and duration shift the fuel mix

In Brooks and Mercier’s 1994 crossover concept, rising intensity increases contraction-driven glycogen breakdown, changes fibre recruitment and raises sympathetic activity, all favouring carbohydrate, while endurance training favours fat; the crossover point is the power output above which carbohydrate supplies more energy than fat.

Two stable-isotope studies show where the fat comes from: Romijn and colleagues (1993) tested five trained subjects at 25%, 65% and 85% of VO₂max, and van Loon and colleagues (2001) eight male cyclists at 40%, 55% and 75% of maximal workload (Wmax).

At high intensity several limits act together. Release from fat tissue falls, probably with its blood flow, but restoring plasma fatty acids only partly restored fat oxidation, so muscle limits it too: faster glycolysis ties up carnitine as acetylcarnitine, and in isolated mitochondria a fall from pH 7.0 to 6.8 sharply reduced CPT1 activity. Van Loon’s group named reduced CPT1 activity as the likeliest cause.

FATmax and the “fat burning zone”

Maximal fat oxidation (MFO) is the highest whole-body fat oxidation rate in a graded exercise test, and FATmax, from a 2002 test design by Achten, Gleeson and Jeukendrup, is the intensity where it occurs.

These are laboratory measurements compiled by Maunder, Plews and Kilding in 2018, not targets. Trained groups have higher MFO, while FATmax changes little with training. Endurance training increases muscle mitochondria, so at the same intensity glycogen and glucose are used more slowly and fat supplies more energy (Holloszy and Coyle, 1984); eight weeks of training roughly doubled ATGL in human muscle, Spriet reports.

What the “fat burning zone” gets right, and what it misses

The “fat burning zone” is the popular name for a low-to-moderate intensity at which fat supplies much of the fuel. It is right that fat’s share is highest at low intensity: after endurance training, lipids supply most of the energy up to about 45% of VO₂max. It misses three things.

• Share is not rate. Low intensities give fat a large share of a small total; the absolute rate keeps rising into moderate intensity.
• The peak is individual. Heart rate, power and estimated VO₂max have not predicted MFO or FATmax, the 2018 review reports.
• Conditions change the answer. Reported day-to-day variation runs from about 3% to over 15%, possibly reflecting how well diet and prior exercise were controlled.

How fat oxidation is measured

The MFO values above come from indirect calorimetry. Analyzers measure oxygen uptake and carbon dioxide output; their ratio, the respiratory exchange ratio (RER), falls as fat supplies more energy, and stoichiometric equations convert the gases into grams of fat and carbohydrate oxidized per minute.

Protein oxidation is assumed negligible, the standard equations assume glucose rather than glycogen as the carbohydrate, and at high intensities changes in the bicarbonate pool distort the carbon dioxide signal; a 2022 analysis states RER should not be used as an index of substrate use above 75% of VO₂max. Stable isotopes add the source: van Loon’s group used carbon-13 palmitate for plasma fatty acids and deuterium-labelled glucose, with biopsies for glycogen, attributing the remaining fat oxidation to muscle and lipoprotein triglyceride.

Much individual variation remains unexplained: the 2022 analysis pooled 434 studies, and its multivariable models, using data from 1,221 participants, explained up to 59% of the variation in RER. Our guide to reading a peptide study covers how to judge methods like these.

Growth hormone, its fragments and 5-amino-1MQ

Each finding below is labelled by where it was made: people, rodents or cells.

Growth hormone: the physiology behind the fragments

In people, growth hormone (GH) is strongly lipolytic: Møller and Jørgensen’s 2009 review names marked stimulation of lipolysis and higher free fatty acids as its most prominent metabolic effect. During fasting it mainly promotes fatty acid release and oxidation, sparing glucose and protein, and it acts partly through IGF-I (see what IGF-1 is). GH appears here only as physiology.

AOD-9604: rodent data, human safety trials

AOD9604 is Tyr-hGH 177–191 (YLRIVQCRSVEGSCGF), from a Monash University group in Victoria, Australia.

• Obese mice: over 14 days, GH and AOD9604 both reduced body-weight gain and raised fat oxidation and plasma glycerol in ob/ob mice; unlike GH, AOD9604 did not raise blood glucose, and in cells it did not compete for binding to the human GH receptor.
• Obese Zucker rats: over 19 days, weight gain was less than half that of controls, adipose lipolytic activity rose and insulin sensitivity was unimpaired.
• People: a 2013 paper by authors with ties to the sponsor summarized six placebo-controlled trials with 893 participants, including two phase IIb studies designed to assess weight reduction, and reported safety but no efficacy results. A February 2007 BioWorld headline reported that development in obesity was terminated after phase IIb results.

HGH Fragment 176-191: the native sequence

HGH Fragment 176-191 is growth hormone’s last 16 residues (FLRIVQCRSVEGSCGF, UniProt P01241), differing from AOD9604 only at residue 1. The lipid work used AOD9604 or the 15-residue hGH 177–191 (AOD9401), which in rat adipose tissue stimulated hormone-sensitive lipase and inhibited acetyl-CoA carboxylase, as GH did. We found no fat-oxidation study and no human study of the native sequence; our HGH Fragment 176-191 guide untangles the names, and the AOD-9604 guide covers the trials.

5-amino-1MQ: an NNMT inhibitor

5-Amino-1MQ is a small molecule that inhibits nicotinamide N-methyltransferase (NNMT), which methylates the NAD+ precursor nicotinamide (see NAD+, NMN and nicotinamide riboside). Knocking NNMT down in fat tissue and liver protected mice from diet-induced obesity by raising energy expenditure, with more NAD+ in fat tissue (Kraus, 2014); our sirtuins explainer covers the SIRT1 link. In 3T3-L1 fat cells, 5-amino-1MQ raised NAD+ and SAM and reduced lipid accumulation; in diet-induced obese mice, 11 days of it left body weight about 5% below baseline against a 1.4% gain on vehicle, with about 35% less epididymal fat and unchanged food intake (Neelakantan, 2018). That study did not measure fat oxidation or energy expenditure. See our 5-Amino-1MQ guide.

What’s established and what isn’t

Sourcing metabolic research reagents in Canada

For Canadian labs, the documentation questions are those for any research peptide, plus two specific ones.

• Check the first residue on growth hormone fragments. AOD9604 starts with tyrosine and the native 176–191 fragment with phenylalanine; the sequence and measured mass should match the molecule ordered.
• Expect small-molecule data for 5-amino-1MQ, a methylquinolinium compound rather than a peptide.
• Match the certificate’s batch number to the vial. Our COA guide and our overview of research peptides in Canada cover what to check.
• Keep lyophilized material cold, dry and dark; see the peptide storage guide.

AOD-9604Tyr-hGH 177–191, the analogue used in the rodent lipolysis studies aboveView product HGH Fragment 176-191The last 16 residues of growth hormone; differs from AOD-9604 at residue 1View product 5-Amino-1MQA small-molecule NNMT inhibitor studied in cultured fat cells and miceView product

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