From nutrients to ATP
Cellular respiration
How food becomes cellular energy
Key takeaway for patientsEnergy from food is released in steps, and each step needs hormones, enzymes and vitamins to work. Persistent low energy is a symptom to assess, not a diagnosis.
Food is digested into glucose, fatty acids and amino acids, which the blood delivers to cells. Inside the cell, glucose is first split by glycolysis; its products, and those of fat breakdown, enter the mitochondria and are processed by the citric acid cycle.
The cycle hands high-energy electrons to the electron transport chain, which uses them to make ATP, the molecule that powers the cell. Insulin governs how much glucose enters cells after a meal, and several B vitamins act as cofactors along the way.
Key steps from meal to ATP
- Digestion and absorption
- Food is broken down and enters the blood.
- Uptake into cells
- Insulin signals cells to take up glucose.
- Glycolysis and the citric acid cycle
- Fuel is broken down and its electrons are captured.
- Oxidative phosphorylation
- The mitochondria make ATP.
Factors that impair energy metabolism
A fault at any step is felt as low energy:
- Insulin resistance
- Glucose enters cells less readily.
- Micronutrient deficiency
- B vitamins, iron and magnesium are cofactors.
- Thyroid dysfunction
- The thyroid sets the overall metabolic rate.
- Inactivity
- Fewer and less active mitochondria.
Clinical management and physician guidance
Low energy is assessed by checking the steps one at a time.
- Metabolic and nutritional assessment
- Glucose, insulin, thyroid function, iron and vitamin status.
- Physician-guided care
- Nutrition, activity and the correction of any deficiency, as your physician decides.
- Monitoring and follow-up
- Symptoms and markers are reviewed over time, and the plan is adjusted to them.
Research focus in energy metabolism
| Core focus area | Biological target | Clinical relevance |
|---|---|---|
| Glucose uptake | Insulin signaling and GLUT4 | Entry of glucose into muscle and fat |
| Central metabolism | The citric acid cycle | Capture of energy from fuel |
| ATP synthesis | Oxidative phosphorylation | Energy output of mitochondria |
| Fuel selection | Metabolic flexibility | Switching between glucose and fat |
Frequently asked questions
- What is ATP?
- The molecule that cells use to store and spend energy.
- Why do I feel tired after eating?
- Large meals, especially those high in refined carbohydrate, can cause swings in blood sugar. Persistent symptoms are worth discussing with a physician.
- Which tests look at energy metabolism?
- Blood glucose, thyroid function, iron and vitamin levels are common starting points.
Academic references and literature
- Rich PR (2003). The molecular machinery of Keilin's respiratory chain. Biochem Soc Trans, 31(Pt 6), 1095-105. PMID 14641005.
- Saltiel AR, Kahn CR (2001). Insulin signaling and the regulation of glucose and lipid metabolism. Nature, 414(6865), 799-806. PMID 11742412.
- Goodpaster BH, Sparks LM (2017). Metabolic Flexibility in Health and Disease. Cell Metab, 25(5), 1027-1036. PMID 28467922.
This guide belongs to the NAD+ program