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# Metabolism

me·tab·o·lism - meh-TAB-oh-liz-um

## Definition

Metabolism encompasses all of the chemical reactions occurring in the body's cells that convert food into energy, build and repair tissues, and eliminate waste products. It is the sum total of two complementary processes: catabolism (breaking down molecules to release energy) and anabolism (using energy to build complex molecules like proteins, DNA, and cell structures). Every living function - from thinking to breathing to healing - depends on metabolic processes.

Your metabolic rate - the speed at which your body burns calories - is determined by several factors. Basal metabolic rate (BMR) accounts for 60-75% of daily energy expenditure and represents the calories burned to maintain basic life functions at rest. The thermic effect of food (digesting and processing nutrients) accounts for about 10%, and physical activity accounts for the remaining 15-30%. Factors that influence metabolic rate include age, sex, body composition (muscle burns more calories than fat), [thyroid](/glossary/thyroid) function, genetics, and activity level.

The popular concept of 'fast' or 'slow' metabolism oversimplifies a complex system. While there are genuine differences in metabolic efficiency between individuals, extreme variations are usually associated with medical conditions (particularly thyroid disorders) rather than inherent metabolic 'types.' The most effective way to influence your metabolic rate is through body composition - building and maintaining lean muscle mass through resistance training.

## Also Known As

Metabolic rate Metabolic function 

## Key Facts

-   • Basal metabolic rate (BMR) - the energy needed for basic body functions at rest - accounts for 60-75% of total daily calorie burn. 
-   • Muscle tissue burns approximately 6 calories per pound per day at rest, while fat burns about 2 calories per pound. 
-   • Metabolic rate naturally declines approximately 1-2% per decade after age 20, primarily due to loss of muscle mass. 
-   • Thyroid hormones are the primary regulators of metabolic rate - hypothyroidism slows it, hyperthyroidism accelerates it. 
-   • Crash dieting can temporarily lower metabolic rate through 'adaptive thermogenesis' - the body's response to conserve energy during perceived famine. 

## How It Relates To Your Health

Metabolic health is assessed through several blood markers including fasting glucose, insulin, HbA1c, lipid panel, and thyroid function tests. [Metabolic syndrome](/glossary/metabolic-syndrome) - a cluster of conditions including [high blood pressure](/glossary/hypertension), high blood sugar, excess waist fat, and abnormal [cholesterol](/glossary/cholesterol) - affects approximately 35% of U.S. adults and significantly increases [cardiovascular](/glossary/cardiovascular) and diabetes risk.

If you're concerned about metabolic health, the most evidence-based strategies include regular exercise (both aerobic and resistance training to maintain muscle mass), adequate protein intake, quality sleep, stress management, and avoiding prolonged extreme caloric restriction. Understanding that metabolism is modifiable - primarily through body composition and activity - is more empowering than believing you're 'stuck' with a slow metabolism.

## Related Terms

[Resting Metabolic Rate](/glossary/resting-metabolic-rate)[Metabolic Syndrome](/glossary/metabolic-syndrome)[Insulin](/glossary/insulin)[Thyroid-Stimulating Hormone](/glossary/thyroid-stimulating-hormone)[Mitochondria](/glossary/mitochondria)[Macronutrients](/glossary/macronutrients)

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[](/nutrition/intermittent-fasting-science)[Nutrition & Diet](/nutrition)

### Intermittent Fasting: What the Science Actually Says

2025-09-01

## Sources

1.  [Metabolism](https://my.clevelandclinic.org/health/body/21893-metabolism) - Cleveland Clinic
2.  [The truth about metabolism](https://www.health.harvard.edu/staying-healthy/the-truth-about-metabolism) - Harvard Health Publishing
3.  [Physiology, Metabolism](https://www.ncbi.nlm.nih.gov/books/NBK546690/) - StatPearls / PubMed

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