Nutrition and metabolism
What we eat provides the energy and building blocks the body needs. Metabolism is how the body uses them.
Macronutrients
Carbohydrates provide energy, about 4 kcal per gram; fats provide about 9 kcal per gram and store energy; proteins, about 4 kcal per gram, supply amino acids for building tissues and enzymes. Nine amino acids are essential: the body cannot make them, so they must come from food.
Micronutrients
Vitamins are organic molecules needed in small amounts. Fat-soluble vitamins (A, D, E, K) are stored in fat and can build up to toxic levels; water-soluble vitamins (B group and C) are mostly excreted, so need regular intake. Minerals such as calcium, iron, iodine, and sodium are inorganic. Deficiencies cause disease: vitamin C deficiency causes scurvy; vitamin D deficiency causes rickets.
Fibre and water
Dietary fibre, mainly indigestible plant carbohydrate, adds bulk, helps regular bowel movements, feeds gut bacteria, and is linked to lower risks of heart disease and bowel cancer. Water makes up about 60% of adult body mass, and losses through urine, sweat, breathing, and faeces must be replaced daily.
Metabolism
Metabolism is all the chemical reactions in the body. Catabolism breaks molecules down, releasing energy (as in cellular respiration); anabolism builds molecules, using energy (as in making proteins or storing glycogen). ATP links the two.
Absorptive and post-absorptive states
After a meal (the absorptive state), insulin promotes uptake of glucose, its storage as glycogen in liver and muscle, and conversion of excess to fat. Between meals (the post-absorptive state), glucagon promotes breaking glycogen down and making new glucose from other molecules (gluconeogenesis), while fat is broken down for fuel.
Metabolic rate
The basal metabolic rate (BMR) is the energy used at complete rest, often 60-70% of daily energy use, for keeping organs working. It depends on body size, muscle mass, age, sex, and thyroid hormones. Total energy needs add physical activity and the energy cost of digesting food.
Energy balance and body weight
Body weight is stable when energy intake equals expenditure. Excess intake is stored mainly as fat; one kilogram of body fat stores roughly 7,700 kcal. Hormones such as leptin (from fat) and ghrelin (from the stomach, signalling hunger) help regulate appetite, though many factors influence weight.
Body temperature
Metabolism produces heat. The hypothalamus keeps core temperature near 37 degrees C by balancing heat production with heat loss through radiation, conduction, convection, and evaporation of sweat. Fever is a raised set point, often triggered by chemicals released during infection.
Key takeaways
Carbohydrates and proteins give about 4 kcal/g and fats about 9 kcal/g; nine amino acids are essential. Fat-soluble vitamins are stored; water-soluble ones need regular intake. Fibre aids digestion. Catabolism releases energy; anabolism uses it. Insulin governs the absorptive state, glucagon the post-absorptive state. BMR is most daily energy use; weight reflects energy balance.
Flashcard
About 4 kcal per gram
Check your understanding
Which vitamin is most likely to reach toxic levels if taken in large supplement doses?