Metabolism and fat burning: How your body actually burns fat
ca. 20 Minuten
Your body is constantly burning fat - even now, as you read this. But oxidizing fat and losing fat are two different things: Whether your adipose tissue gets smaller is determined by your energy balance over days and weeks, not by the fuel used in a single workout.
Metabolism is the sum of all the processes that transform food into usable energy. Basal metabolic rate accounts for the largest share of daily energy expenditure - the energy for organs, cells, and body temperature. Movement, everyday activity, and digestion add to it. Fat burning happens in two steps: lipolysis and beta-oxidation.
This article explains the biology behind it: what happens in adipose tissue, the role insulin plays, why the “fat-burning zone” is misunderstood, and what research shows about metabolism with age.
What to expect in this article
How does fat get from the cell into the mitochondria?
Why does energy balance determine fat loss?
What’s wrong with the “fat-burning zone”?
What does each exercise intensity offer?
How much does muscle mass really increase resting metabolic rate?
How is your daily energy expenditure distributed?
Does metabolism slow with age?
How to understand your predispositions with mybody®
What a DNA test can—and can’t—tell you
Conclusion
Frequently asked questions (FAQ)
Sources
What happens in the body when you “burn fat”?
“Burning fat” means, biochemically, that fatty acids are broken down into carbon dioxide and water in your cells’ mitochondria, and the energy released becomes available as ATP. This happens around the clock - while you sleep, sit, and exercise. Metabolism encompasses more: It is the sum of all the 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 is constantly oxidizing fat without you losing weight - because it is also constantly storing fat again. Only the difference over a longer period is visible. There is also metabolic flexibility: If a lot of carbohydrate is used during a workout, oxidation shifts toward fat afterward - and vice versa. Over 24 hours, fuel selection largely evens out.
Basal metabolic rate, resting energy expenditure, and activity energy expenditure
Basal metabolic rate is the amount of energy your body needs in complete rest and while fasting - for 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 by movement - training as well as climbing stairs and standing; research calls the non-exercise portion NEAT. There is also the thermic effect of food from digestion and metabolism.
Key point: Fat burning is an ongoing process, not a mode. It says nothing about whether you are losing body fat - that is determined by energy balance over days and weeks.
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 - this is where the misunderstandings arise.
Lipolysis
Three enzymes break down stored fat: ATGL, hormone-sensitive lipase, and monoacylglycerol lipase.
Journey through the blood
Free fatty acids travel bound to albumin to the muscles, while glycerol travels to the liver.
Beta-oxidation
In the mitochondrion, the fatty acid is broken down; its components enter the citric acid cycle and produce ATP.
Hormonal brake
Insulin slows lipolysis, while adrenaline and noradrenaline speed it up.
Step 1: Lipolysis in the fat cell
Fat is stored as triacylglycerol: a glycerol backbone with three fatty acids. As summarized in the Endotext textbook (Richard et al., 2020), adipose triglyceride lipase (ATGL) begins the breakdown, hormone-sensitive lipase (HSL) continues it, and monoacylglycerol lipase (MGL) completes it. Free fatty acids and glycerol remain.
The cascade is driven by catecholamines: Adrenaline and noradrenaline bind to beta-adrenergic receptors, increase cAMP, and activate protein kinase A, which activates HSL. Insulin lowers cAMP via phosphodiesterase 3B. This is why fat release is reduced after a carbohydrate-rich meal and increased during fasting.
Step 2: Beta-oxidation in the mitochondrion
Fatty acids travel bound to albumin to where energy is needed - primarily skeletal muscle and the liver. They enter the mitochondrion via a transport system involving carnitine. This is where beta-oxidation begins: The chain is gradually shortened by two carbon atoms at a time, and each round produces acetyl-CoA for the citric acid cycle and reducing equivalents for the respiratory chain - ultimately producing ATP.
Why does energy balance determine fat loss?
Body fat decreases when you consume less energy than you expend over an extended period. This is not an opinion, but a balance equation. It can be misleading because both sides can change - people who cut back significantly often unconsciously move less. The balance holds, but its values are not fixed numbers.
“Among people with similar body weight, 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 generic calorie estimates are so often off—without anyone having “a broken metabolism.” Four factors determine your energy balance, and they’re not all equally easy to influence:
The four factors that shape your energy balance
- ✓ Basal metabolic rate—the largest component, determined largely by fat-free mass, age, and sex
- ✓ NEAT (everyday movement)—the component that varies most between people
- ✓ Physical activity—can be planned, but for most people accounts for a smaller share than expected
- ✓ Thermic effect of food—energy used for digestion and metabolism, around 10 percent of total energy expenditure, according to the DGE (2015)
According to von Loeffelholz and Birkenfeld (Endotext, as of 2022), NEAT can differ by as much as 2,000 kilocalories per day between two adults of comparable height, lean mass, age, and sex. Exercising for an hour three times a week is valuable—but it’s a smaller lever than the other 165 hours of the week.
Key takeaway: Fat loss isn’t determined by which fuel you use during a workout, but by your energy balance over days and weeks—and everyday movement is the part that varies the most.
Where this article ends—and what to read next
This article explains the biology, not what to do about it.
If you feel that your metabolism itself is the problem, the article Slow metabolism—what can you do? is a good next step: It explains the possible causes and when it makes sense to seek medical advice.
You can find the practical application—nutrition, exercise, sleep—in the article How do 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 admit—and less than its supporters believe. What’s true: At low intensity, a relatively larger proportion of energy comes from fat. What’s false is the conclusion that this is where you burn the most fat. The mistake lies in switching between percentages and quantities: A high percentage of a small number can be less than a low percentage of a large one.
Relative proportion and absolute quantity are two different numbers
As intensity increases, energy supply shifts toward carbohydrates; sports scientists call the point at which they replace fat as the main fuel the crossover point. That is why absolute fat oxidation in grams per minute follows a bell-shaped curve: It rises from rest, reaches a peak 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 maximum 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 used by some fitness watches might suggest.
Why the zone still barely matters for weight loss
Even at the Fatmax point, the amount remains modest: Around half a gram per minute adds up to about 30 grams in an hour—offset by a single meal. So choose an intensity based on what you can sustain regularly, not a heart rate zone that promises high fat burning.
In short
At low intensity, the percentage of energy from fat is high, but energy expenditure per minute is low.
The highest absolute fat oxidation occurs at about 51 to 56 percent of maximum oxygen uptake (Maunder et al., 2018)—in the moderate, not very easy, range.
For fat loss, the day’s energy balance matters, not the heart rate zone of a workout.
What does each exercise intensity offer?
Each intensity has its own strengths. The overview compares three exertion ranges using the same criteria. The figures for fat oxidation and the afterburn effect are evidence-based; the other fields describe the general trend, not exact numbers.
| Criterion | Rest & very gentle movement | Moderate endurance pace | High intensity & intervals |
|---|---|---|---|
| Relative share of fat in energy supply | Highest | Moderate to high | Low, carbohydrates dominate |
| Absolute fat oxidation per minute | Low | Highest: approx. 51–56% VO2max (Maunder et al., 2018) | Falls again |
| Energy expenditure per minute | Very low | Moderate | Highest |
| Afterburn effect (EPOC) | Virtually none | Low | Greatest, but only 6–15% of net oxygen cost (LaForgia et al., 2006) |
| Ability to sustain it in everyday life | Very high, possible for hours | High, typically 30–90 minutes | Limited, just a few minutes of net exertion |
| What it is especially suited for | Everyday movement, NEAT, recovery | Endurance base, easy to control | Time efficiency, improved endurance performance |
That leads to an unremarkable answer: The best intensity is the one you reliably do 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 cost.
Strength training is deliberately omitted from the table: its contribution lies in preserving fat-free mass—and thus indirectly supporting basal metabolic rate. The next chapter explains the size of the effect.
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, compared with around 4.4 kilocalories for fat tissue—while the liver (194), brain (233), and heart and kidneys (426) burn several times as much.
Compared with a kilogram of fat tissue, a kilogram of muscle burns around 8 extra kilocalories per day. An extra kilogram of muscle mass changes your daily energy expenditure far less than most people expect. The reason: a large share of basal metabolic rate comes from organs that weigh only a few kilograms but work nonstop.
An extra kilogram of muscle mass changes your daily energy expenditure far less than most people expect.
Why strength training is still worthwhile
This doesn’t mean strength training has no role in weight loss. When you’re in an energy deficit, you lose not only fat but also fat-free mass—and that’s 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 support stronger joints and bones. If you strength-train only to raise your resting metabolic rate, you’ll be disappointed—but if you do it to preserve muscle, you’re making a well-founded choice.
How is your daily energy expenditure distributed?
Your daily energy expenditure consists of three components: basal metabolic rate, energy expenditure from physical activity, 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.
Shares of daily total energy expenditure
Source: Health Portal of the Republic of Austria (BMASGPK), as of 2025 (basal metabolic rate approx. 60%, thermogenesis approx. 10%); German Nutrition Society, 2015 (thermogenesis approx. 10%). Energy expenditure from physical activity makes up the remainder. Reference values for adults.
What stands out is that the largest portion is the one you have the least control over—and the variable middle portion usually comes not from exercise but from daily movement. According to von Loeffelholz and Birkenfeld (Endotext, as of 2022), this accounts for 6 to 10 percent of total energy expenditure for a sedentary lifestyle and 50 percent or more for very active people.
Energy expenditure can be estimated using the PAL value, which is multiplied by basal metabolic rate: according to the DGE (2015), around 1.4 for sedentary work, around 1.7 for a moderately active lifestyle, and regular exercise adds around 0.3—these are estimates, not measured values.
Does metabolism slow with age?
Not as early or as much as long believed. The idea that metabolism noticeably slows from the mid-thirties onward is not supported by the largest data analysis to date: a research team analyzed 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 ages 20 and 60; the decline began only at a turning point of 63.0 years and was around 0.7 percent per year thereafter.
The key qualifier is “relative to fat-free mass.” When people gain weight between ages 30 and 55, their cells usually aren’t working more slowly—their body composition and behavior are changing: less muscle mass, less daily movement, more sitting.
Key takeaway: According to Pontzer et al. (2021), energy expenditure per kilogram of fat-free mass remains stable between ages 20 and 60. What changes in middle age is body composition and daily movement—not the metabolic rate of cells.
There can also be medical reasons for a change in energy expenditure, such as an underactive thyroid or certain medications. Unexplained changes should be discussed with a doctor.
How to understand your predispositions with mybody®
Lipolysis, beta-oxidation, and the distribution of daily energy expenditure follow the same physiology in everyone. What differs is how these processes manifest—for example, feelings of fullness or how well someone responds to exercise. Some of this has a genetic component. mybody® (MYBODY Lab GmbH) offers the WeightLoss | SLIM DNA test.
WeightLoss | SLIM DNA test
The test analyzes over 80 genetic variations from a saliva sample and summarizes them in 24 assessments across five chapters in approximately 140 pages—as guidance, not a diagnosis.
Price: €169.00 · Sample type: Saliva sample (at home) · Processing time: approx. 20–25 business days · Laboratory: ISO-27001-certified analysis
WeightLoss | View SLIMSee the DNA Metabolism Tests collection for more analyses. The next chapter explains the limitations of this kind of test—and they are significant.
Details about price, sample type, processing time, and scope are from the mybody® product page (as of July 2026) and may change.
What a DNA test can—and can’t—tell you
A DNA test can show which gene variants you have and what research has linked them to.
What a DNA test can’t do: It cannot diagnose anything or predict weight loss. Gene variants are statistical associations at the group level, not individual predictions. The evidence is particularly clear for genotype-tailored diets. The DIETFITS study tested this: 609 adults with overweight followed a low-fat or low-carbohydrate diet for twelve months.
According to Gardner et al. (JAMA, 2018), weight loss after twelve months averaged 5.3 kilograms in the low-fat group and 6.0 kilograms in the low-carbohydrate group, a difference that was not significant; no significant interaction between diet and genotype pattern was found.
Honesty about this is important: there is no measurable evidence that a genotype-tailored diet is superior. Differences within the groups were far greater than the differences between them.
A DNA test is therefore a source of information, not a tool for controlling your body. If you have significant symptoms—persistent fatigue, unexplained weight changes, or sensitivity to cold—the first step is a medical examination, not a self-test.
Conclusion
Fat oxidation is an ongoing process: lipolysis releases fatty acids from fat cells, and beta-oxidation burns them in the mitochondria; insulin slows the process, while catecholamines speed it up.
Whether your body fat decreases is determined not by this process, but by your energy balance over days and weeks. Basal metabolic rate is the largest component, at around 60 percent, while thermogenesis is the smallest, at around 10 percent. Everyday movement varies the most.
The “fat-burning zone” is half right: a Fatmax range exists at around 51 to 56 percent of maximal oxygen uptake, but the absolute amounts are so small that heart rate zones barely matter. The afterburn effect and increase in resting metabolic rate from muscle mass are real too, but less significant than they’re often made out to be.
And metabolism doesn’t slow down in your mid-thirties: relative to fat-free mass, it remains stable between the ages of 20 and 60. To understand changes in body weight, look at body composition, everyday movement, and eating habits.
Frequently asked questions (FAQ)
Get to know your predispositions
The WeightLoss | The SLIM DNA test analyzes over 80 genetic variants across 24 assessments from a saliva sample. Intended as a guide – not a diagnosis or a prediction of results.
WeightLoss | View SLIM All DNA metabolism testsYou might also be interested in
→ Slow metabolism – what can I do?
Possible causes and appropriate evaluation.
→ How do I activate my metabolism?
Putting it into practice in everyday life – nutrition, exercise, sleep.
→ DNA metabolism test: lose weight healthily
How the analysis works.
Sources
- 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 on energy intake)
- 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 activity energy expenditure.” gesundheit.gv.at
- Pontzer, H. et al. (2021): “Daily energy expenditure through the human life course,” Science 373(6556):808–812. science.org
- 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
- Endotext (NCBI Bookshelf): von Loeffelholz, C. & Birkenfeld, 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
- Maunder, E., Plews, D. J., Kilding, A. E. (2018): “Contextualising Maximal Fat Oxidation During Exercise”, Frontiers in Physiology 9:599. frontiersin.org
- 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
- 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 lifespan on [3]; specific metabolic rates on [4]; lipolysis, beta-oxidation, and NEAT on [5]; maximal fat oxidation on [6]; the afterburn effect on [7]; genotype-tailored diets on [8]. All sources were reviewed on July 28, 2026; [3] is access-restricted through the publisher, and the information is from the freely accessible full-text version. Product information comes from mybody-x.com and may change.
mybody® Editorial & Expert Team
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 authors page.
Published on July 28, 2026 · Last updated on July 28, 2026
Medical disclaimer: This article is for general informational purposes and does not replace medical advice, diagnosis, or treatment. If your symptoms persist, please consult a doctor.
