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Skip to 0 minutes and 5 secondsThe food we eat contains nutrients that we all need to survive, function and grow. Carbohydrates, proteins and fats are called macronutrients, as these are needed in large quantities. They occur in foods as complex forms that are not readily absorbed by the body. Complex carbohydrates contain chains of sugar molecules. Proteins contain chains of amino acids. And fats occur mainly as triacylglycerol, a complex of three fatty acid molecules attached to a sugar-like molecule called glycerol. Our digestive system helps us to break down these nutrients into small, soluble substances that can be absorbed into the blood and converted into energy. This process begins in the mouth, where chewing breaks food into pieces that are more easily digested.

Skip to 0 minutes and 51 secondsSaliva provides lubrication to help with processing and swallowing, and contains important molecules called enzymes that break large molecules into smaller ones. On swallowing, food passes from the mouth through the oesophagus and into the stomach. In the stomach, muscle contractions continue to mechanically break down food, churning it and mixing it with digestive enzymes and hydrochloric acid. In the small intestine, nutrients are absorbed through the intestinal wall and into the blood. Only small, soluble substances can pass across the wall. So just like the stomach, the small intestine has its own special blend of enzymes that continue to break the food down into smaller molecules. Specialised cells then help absorbed materials cross the intestinal lining into the bloodstream.

Skip to 1 minute and 43 secondsOnce in the bloodstream, the digested food molecules are carried around the body. The nutrient-rich blood first enters the liver, which absorbs a large amount of the sugars and amino acids. When the blood enters the pancreas, the rising level of glucose triggers the release of the hormone insulin. This signals to the body to start using glucose as the main source of energy, and to store any excess glucose for future use as glycogen in the liver and muscles. Fats are stored for energy in adipose tissues, and amino acids are used to make new proteins, replenishing those that have been lost through natural turnover. The absorption of sugars and amino acids reaches a peak about 30 to 60 minutes after eating.

Skip to 2 minutes and 26 secondsFat absorption peaks around three hours, and all the nutrients in a meal are normally fully digested and absorbed within four to five hours. After a meal, as the levels of absorbed nutrients in the blood go down, the level of insulin decreases while the level of another hormone, glucagon, rises. This switch in hormone levels signals to the body to stop storing glucose and fats, and start releasing them to maintain energy production. The body's stores of glycogen are converted back into glucose and can provide energy for around 12 to 24 hours.

Skip to 3 minutes and 2 secondsStores of fat in adipose tissue can provide energy through the release of free fatty acids for several weeks, meaning that fat acts as a long term or emergency source when foods are scarce. Protein can also be used to provide energy during starvation. But as there is no dedicated store of protein, especially for energy production, the body will break down proteins from muscles and other organs for energy.

What happens to the food we eat?

The nutrients that we need to strengthen muscle, maintain stable joints and keep bones strong are all contained within the food we eat. But these nutrients often occur in complex forms which are not readily absorbed by the body.

In this animation, Peter Grabowski explains how the body breaks down the food we eat so that it can be used for energy or as the raw materials to repair and build new tissue.

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This video is from the free online course:

The Musculoskeletal System: The Science of Staying Active into Old Age

The University of Sheffield