Metabolism and fat burning: How your body actually burns fat

What to expect in this article

What happens in the body when you “burn fat”?

“Burning fat” means, biochemically, that fatty acids are broken down in your cells’ mitochondria into carbon dioxide and water, and the released energy becomes available as ATP. This happens around the clock—in your sleep, while sitting, and during exercise. Metabolism encompasses more: It is the sum of all processes by which your body absorbs, transforms, stores, and releases nutrients.

Source: Wang et al., American Journal of Clinical Nutrition, 2010 · Maunder et al., Frontiers in Physiology, 2018 · LaForgia et al., Journal of Sports Sciences, 2006

Oxidizing fat is not the same as losing fat

Your body constantly oxidizes fat without you losing weight, because it is also continually storing fat again in parallel. Only the difference over a longer period becomes visible. There is also metabolic flexibility: If a lot of carbohydrate is used during one session, oxidation subsequently shifts toward fat—and vice versa. Over 24 hours, fuel selection largely balances out.

Basal metabolic rate, resting energy expenditure, and active energy expenditure

Basal metabolic rate is the amount of energy your body needs in complete rest and while fasting—for your heartbeat, breathing, brain activity, cell renewal, and body temperature. The more practical measure is resting energy expenditure.

According to the German Nutrition Society (2015), resting energy expenditure is about 10 percent higher than basal metabolic rate.

Activity energy expenditure is everything added through movement—both exercise and activities such as climbing stairs and standing; research calls the non-exercise portion NEAT. In addition, there is diet-induced thermogenesis for digestion and metabolism.

How does fat get from the cell into the mitochondria?

Fat leaves the fat cell in two steps: first, stored triglyceride is broken down (lipolysis), then the fatty acids are burned in the mitochondrion (beta-oxidation). Both steps are hormonally regulated—that is where the misunderstandings begin.

Step 1: Lipolysis in the fat cell

Fat is stored as triacylglycerol: a glycerol backbone with three fatty acids. In essence, according to the specialist textbook Endotext (Richard et al., 2020), adipose triglyceride lipase (ATGL) initiates the breakdown, hormone-sensitive lipase (HSL) continues it, and monoacylglycerol lipase (MGL) completes it. What remains are free fatty acids and glycerol.

The cascade is driven by catecholamines: adrenaline and noradrenaline bind to beta-adrenergic receptors, raise cAMP levels, and activate protein kinase A, which activates HSL. Insulin lowers cAMP via phosphodiesterase 3B. Therefore, fat release is reduced after a carbohydrate-rich meal and increased during fasting periods.

Step 2: Beta-oxidation in the mitochondrion

The fatty acids travel bound to albumin to where energy is needed—especially the skeletal muscles and liver. They enter the mitochondrion via a transport system involving carnitine. There, beta-oxidation begins: the chain is gradually shortened by two carbon atoms at a time; each cycle produces acetyl-CoA for the citric acid cycle and reducing equivalents for the respiratory chain—ultimately yielding ATP.

Why does energy balance determine fat loss?

Body fat decreases when, over a longer period, you consume less energy than you expend. This is not an opinion but an energy-balance equation. It becomes misleading because both sides can change—when people restrict their intake considerably, they often unconsciously move less. The balance remains valid, but its components are not fixed numbers.

“Among people with similar body weights, energy expenditure can differ by up to nearly 2,000 kcal per day due to differences in occupation, leisure-time activity, and unconscious movements such as fidgeting.”

This explains why blanket calorie estimates so often miss the mark—without anyone having “a broken metabolism.” Four factors determine your balance, and they can be influenced to varying degrees:

According to von Loeffelholz and Birkenfeld (Endotext, as of 2022), NEAT can vary by up to 2,000 kilocalories per day between two adults of comparable height, lean mass, age, and sex. Exercising for one hour three times a week is valuable—but it is a smaller lever than the remaining 165 hours of the week.

Where this article ends—and which one continues

This article explains the biology, not the measures.

If you feel that your metabolism itself is the problem, the article Poor metabolism—what can you do? takes you further: It explains which causes may be behind it and when medical evaluation makes sense.

The practical aspects of everyday life—nutrition, exercise, and sleep—are covered in the article How can I activate my metabolism?. This article focuses on how it works.

What’s wrong with the “fat-burning zone”?

There is more truth to the fat-burning zone than its critics claim—and less than its proponents believe. It is true that at low intensity, a relatively larger share of energy comes from fat. The mistaken conclusion is that this means you lose the most fat there. The error lies in switching between percentage and amount: A high percentage of a small number can be less than a low percentage of a large number.

Relative proportion and absolute amount are two different numbers

As intensity increases, energy supply shifts toward carbohydrates; sports science calls the point at which they replace fat as the main fuel the crossover point. This is why absolute fat oxidation in grams per minute has a bell-shaped curve: It rises from rest, reaches a maximum in the moderate range, and falls again at high intensity.

According to Maunder et al. (Frontiers in Physiology, 2018), the maximum fat oxidation rate in endurance-trained men was 0.53 ± 0.16 grams per minute and was reached at 56 ± 8 percent of maximal oxygen uptake; in recreationally active men, this point was at 51 ± 8 percent.

Experts call this range Fatmax. It varies from person to person and is higher than the generic heart rate zones some fitness watches suggest.

Why the zone still hardly determines weight loss

Even at the Fatmax point, the amount remains modest: Around half a gram per minute amounts to about 30 grams in an hour—offset by a single meal. Therefore, choose the intensity based on what you can sustain regularly, not on a heart rate zone that promises high fat burning.

What does each exercise intensity do?

Each intensity has its own strengths. The overview compares three exercise intensity ranges using the same criteria. The information on fat oxidation and the afterburn effect is evidence-based; the other fields describe the general direction, not exact figures.

This leads to an unremarkable answer: The best intensity is the one you can reliably perform often enough. The afterburn effect is overestimated—according to LaForgia et al. (Journal of Sports Sciences, 2006), it accounts for only 6 to 15 percent of net oxygen costs.

Strength training is deliberately omitted from the table: Its contribution lies in preserving fat-free mass—and therefore indirectly supporting basal metabolic rate. The size of this effect is explained in the next chapter.

How much does muscle mass really increase resting metabolic rate?

Muscle mass measurably increases resting metabolic rate—but less than the rule of thumb “muscles burn calories even while you sleep” suggests.

According to Wang et al. (American Journal of Clinical Nutrition, 2010), the specific metabolic rate of skeletal muscle is around 12.6 kilocalories per kilogram per day, while that of fat tissue is around 4.4 kilocalories—whereas the liver (194), brain (233), and heart and kidneys (426) use several times as much.

One kilogram of muscle burns around 8 more kilocalories per day than one kilogram of fat tissue. One kilogram more muscle mass changes your daily energy expenditure noticeably less than most people expect. The reason: A large portion of basal metabolic rate comes from organs that weigh only a few kilograms but work continuously.

Why strength training is still worthwhile

This does not mean that strength training plays no role in weight loss. Anyone in an energy deficit loses not only fat but also fat-free mass—and that is the strongest individual factor affecting basal metabolic rate. Strength training and sufficient protein help counteract this. They also improve insulin sensitivity, increase everyday strength, and promote more stable joints and bones. Anyone who does strength training solely to increase their resting metabolic rate will be disappointed—but using it to preserve lean tissue is a well-founded decision.

How is your daily energy expenditure distributed?

Your daily energy expenditure consists of three components: basal metabolic rate, activity expenditure, and diet-induced thermogenesis. Basal metabolic rate is the largest—even in physically active people.

According to the Health Portal of the Republic of Austria (BMASGPK, as of 2025), basal metabolic rate accounts for about 60 percent of the body's total energy expenditure; thermogenesis accounts for around ten percent. The German Nutrition Society (2015) also puts the thermic effect of food at around ten percent.

What stands out is that the largest block is the one you can control the least—and the flexible middle block usually consists not of exercise, but of daily movement. According in principle to von Loeffelholz and Birkenfeld (Endotext, as of 2022), this accounts for 6 to 10 percent of total energy expenditure in a sedentary lifestyle, and 50 percent or more in very active people.

Energy expenditure from physical activity can be estimated using the PAL value, which is multiplied by basal metabolic rate: according to the German Nutrition Society (DGE, 2015), about 1.4 for sedentary work, about 1.7 for a moderately active lifestyle, and regular exercise adds around 0.3—guidelines, not measurements.

Does metabolism slow down with age?

Not as early or as significantly as long assumed. The idea that metabolism becomes noticeably slower from the mid-thirties onward is not supported by the largest data analysis to date: A research team evaluated measurements using doubly labeled water—the gold standard—from 6,421 people in 29 countries.

According to Pontzer et al. (Science, 2021), total and basal energy expenditure relative to fat-free mass remained stable between the ages of 20 and 60; the decline began only at a turning point of 63.0 years and amounted to around 0.7 percent per year thereafter.

The key point is the qualification “relative to fat-free mass.” When people gain weight between the ages of 30 and 55, their cells usually are not working more slowly—body composition and behavior change: less muscle mass, less daily movement, and more sitting.

Regardless, there are medical reasons for an altered metabolic rate, such as hypothyroidism or certain medications. Unexplained changes should be medically evaluated.

This is how you organize your investments with mybody®

Lipolysis, beta-oxidation, and the distribution of daily energy expenditure follow the same physiology in everyone. What differs are the individual characteristics—for example, satiety perception or how well someone responds to training stimuli. Some of this has a genetic component. mybody® (MYBODY Lab GmbH) offers the WeightLoss | SLIM DNA test

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Putting it into perspective: What a DNA test can—and cannot—do

A DNA test can show which gene variants you have and what research has linked them to.

What a DNA test cannot do: It does not diagnose conditions and cannot predict weight-loss success. Gene variants are statistical associations at the group level, not individual predictions. The evidence is particularly clear for genotype-tailored diets. The DIETFITS study examined this: 609 overweight adults, twelve months, and either a low-fat or low-carbohydrate diet.

According to Gardner et al. (JAMA, 2018), weight loss after twelve months—an average of 5.3 kilograms in the low-fat group and 6.0 kilograms in the low-carbohydrate group—did not differ significantly; no significant interaction between diet and genotype pattern was found.

This honesty is part of the picture: There is no measurable evidence that a genotype-tailored diet is superior. Variation within the groups was far greater than the difference between them.

A DNA test is therefore a source of information, not a control instrument. If you have pronounced symptoms—persistent fatigue, unexplained changes in weight, or sensitivity to cold—the first step is a medical examination, not a self-test.

Conclusion

Oxidizing fat is an ongoing process: lipolysis releases fatty acids from the fat cell, and beta-oxidation burns them in the mitochondrion; insulin slows the process, while catecholamines accelerate it.

Whether your body fat decreases is determined not by this process, but by your energy balance over days and weeks. Basal metabolic rate, at around 60 percent, is the largest component; thermogenesis, at around ten percent, is the smallest—and daily movement is the component with the greatest variation.

The “fat-burning zone” is half right: A Fatmax range exists at around 51 to 56 percent of maximum oxygen uptake—but the absolute amounts are so small that the heart-rate zone barely matters. The afterburn effect and increase in resting metabolic rate from muscle mass are real too, but smaller than their reputation suggests.

And metabolism does not slow down in your mid-thirties: Relative to lean mass, it remains stable between ages 20 and 60. To understand weight changes, look at body composition, daily movement, and eating habits.

Frequently asked questions (FAQ)

Sources

1. German Nutrition Society (DGE) (2015): Press release “How much energy does a person need?” and FAQ “Energy intake” – basal metabolic rate, resting energy expenditure, PAL values, and the thermic effect of food. dge.de (press release) · dge.de (FAQ Energy intake)
2. Health portal of the Republic of Austria (Federal Ministry of Labour, Social Affairs, Health, Care and Consumer Protection), as of 2025: “Basal metabolic rate and active metabolic rate.” gesundheit.gv.at
3. Pontzer, H. et al. (2021): “Daily energy expenditure through the human life course”, Science 373(6556):808–812. science.org
4. Wang, Z. et al. (2010): “Specific metabolic rates of major organs and tissues across adulthood”, American Journal of Clinical Nutrition 92(6):1369–1377. pubmed.ncbi.nlm.nih.gov
5. Endotext (NCBI Bookshelf): von Loeffelholz, C. & Birkenfeld, A. L. (as of 2022), “Non-Exercise Activity Thermogenesis in Human Energy Homeostasis” and Richard, A. J. et al. (2020), “Adipose Tissue: Physiology to Metabolic Dysfunction”. NEAT chapter · Adipose tissue chapter
6. Maunder, E., Plews, D. J., Kilding, A. E. (2018): “Contextualising Maximal Fat Oxidation During Exercise”, Frontiers in Physiology 9:599. frontiersin.org
7. LaForgia, J., Withers, R. T., Gore, C. J. (2006): “Effects of exercise intensity and duration on the excess post-exercise oxygen consumption”, Journal of Sports Sciences 24(12):1247–1264. pubmed.ncbi.nlm.nih.gov
8. Gardner, C. D. et al. (2018): “Effect of Low-Fat vs Low-Carbohydrate Diet on 12-Month Weight Loss in Overweight Adults and the Association With Genotype Pattern or Insulin Secretion” (DIETFITS), JAMA 319(7):667–679. jamanetwork.com

Basal metabolic rate, resting energy expenditure, PAL values, and thermogenesis are based on [1] and [2], energy expenditure across the life span on [3], specific metabolic rates on [4], lipolysis, beta-oxidation, and NEAT on [5], maximal fat oxidation on [6], the afterburn effect on [7], and genotype-adapted diets on [8]. All sources were checked on July 28, 2026; [3] is access-restricted by the publisher, and the information comes from the freely accessible full-text version. Product information is sourced from mybody-x.com and may change.

mybody® Editorial & Expert Team

Energy metabolism Nutritional science Exercise physiology Nutrigenetics

This article was created by the mybody® editorial and expert team, which brings together expertise in laboratory diagnostics, nutritional science, exercise physiology, and nutrigenetics. Learn more about us on the editorial and author page.

Published on July 28, 2026 · Last updated on July 28, 2026

Medical notice: This article is intended for general informational purposes and is not a substitute for medical advice, diagnosis, or treatment. If your symptoms persist, please consult a doctor.