5 Nutrient Metabolism

Metabolism and cellular energy

Metabolism is the set of chemical reactions that maintain life. Catabolism breaks down nutrients and releases usable energy, while anabolism uses energy and smaller molecules to build or repair tissues and make compounds such as enzymes.

Cells transfer much of the energy released from food into ATP, which powers cellular work. In mitochondria, breakdown products of carbohydrates, fats, and some amino acids feed into shared pathways, including the citric acid cycle and oxidative phosphorylation, that produce ATP.

From digestion to cellular use

Digestion breaks food into absorbable units: carbohydrates mainly into simple sugars, proteins into amino acids and small peptides, and fats into fatty acids and related molecules. After absorption, nutrients enter cells and may be used immediately, converted into other compounds, or stored. Which route predominates depends on energy needs, nutrient availability, and hormonal signals.

Carbohydrate use and storage

Glucose can enter Glycolysis, which produces ATP and pyruvate. When oxygen is available, pyruvate can be converted to acetyl-CoA and further oxidized in mitochondria.

When glucose is plentiful, the liver and muscles store it as Glycogen through Glycogenesis. Between meals, the liver breaks down glycogen through Glycogenolysis and can make glucose from non-carbohydrate precursors through Gluconeogenesis. Liver glycogen helps maintain blood glucose, while muscle glycogen is mainly used by the muscle itself.

Fats, proteins, and other nutrients

Dietary fats and stored triglycerides supply fatty acids. Cells can break fatty acids down by Beta-oxidation, generating acetyl-CoA and energy-carrying molecules used to make ATP. Excess energy can be stored as triglycerides in adipose tissue. During prolonged fasting, the liver can convert some fatty-acid-derived acetyl-CoA into Ketone bodies, which can serve as fuel for other tissues.

Amino acids are used to build and renew body proteins and to make other nitrogen-containing compounds. The body has no dedicated storage depot for excess dietary amino acids. When amino acids are not needed for synthesis, their nitrogen is removed and ultimately excreted mainly as urea; their remaining carbon skeletons can be oxidized for energy or used to make glucose or fat.

Vitamins and minerals do not provide energy themselves, but many are needed for enzymes and other processes that allow metabolism to proceed. Water supports transport and chemical reactions.

How fed and fasting states are regulated

After a meal, rising blood glucose and other nutrient signals promote Insulin release. Insulin generally favors nutrient uptake and use, glycogen and fat synthesis, and protein synthesis, while reducing the release of stored fuels.

Between meals, lower insulin and a relative increase in Glucagon help the liver release glucose through glycogen breakdown and gluconeogenesis. As fasting continues, stored fat becomes an important energy source, and the liver also increases ketone production. These shifts overlap rather than functioning as an on/off switch, and their timing varies with factors such as activity and recent food intake.

After a carbohydrate-containing meal, some glucose is oxidized to make ATP, while some may replenish liver and muscle glycogen. Several hours later, as blood glucose falls, the liver can release glucose from its glycogen and make more glucose from suitable precursors. Meanwhile, adipose tissue can release fatty acids for use as fuel.

Energy balance and nutrient stores

Over time, Energy balance reflects the relationship between energy taken in and energy used. The body continually adjusts fuel use and storage: glycogen provides a limited carbohydrate reserve, while adipose triglycerides form a larger long-term energy store.

Energy balance describes changes in total body energy stores; it does not by itself describe diet quality or the adequacy of individual nutrients. Nutrients also have different roles: protein has essential structural and functional roles, and micronutrients support reactions without supplying calories.