A balanced diet will contain many different fats and oils. The commonest type of molecule in these lipids is called a triglyceride. They are made from a small molecule of glycerol attached to three fatty acids.
These triglycerides are too large to be absorbed in the small intestine (ileum) and so need to be broken down into their constituent parts. In the digestive system there are enzymes called lipases that can catalyse the digestion of lipids into fatty acids and glycerol.
Most digestion of lipids happens in the duodenum. The pancreas produces a lipase enzyme that mixes with the food in the duodenum. Although bile does not contain any digestive enzymes, it does have bile salts that play an important role in the digestion of lipids. Bile salts cause large fat droplets in the duodenum to change into many smaller lipid droplets – a process called emulsification. (Read my post on bile if you want more details here….)
Fatty acids and glycerol molecules are small enough to cross the epithelium in the villus in the ileum. This absorption of fatty acids and glycerol is slightly different to the other products of digestion as they do not pass immediately into the blood. They are assembled immediately into structures called chylomicrons and these move into the single, blind-ended tube in the villus called a lacteal. The lacteals merge together into lymph vessels that eventually empty into the blood in the neck. (Please read my post on absorption in the small intestine to read more about this)
All animals are heterotrophic. This means that they cannot make their own food molecules but need to get them from some external source. Humans get a variety of different food molecules from what they eat. Diet is a term for what an animal eats (and in a biological context has no associations with any attempt to lose weight or change body shape). A balanced diet is a combination of foods that provides the correct proportions of all the various food molecules for any particular individual at any particular stage of their life.
Components of a Balanced Diet
Carbohydrates are a family of molecules that includes sugars, starch and other polysaccharides. They contain C,H and O atoms only and their main function in the diet is to provide molecules that can be respired to release energy for cells. Carbohydrates are thus one of the main respiratory substrates in our diet. All sweet foods will contain sugars of course and starch-rich foods are vegetables like potatoes, pasta and rice. Starch is a polymer of glucose and so needs to be digested to glucose because it is too large a molecule to be absorbed in the small intestine.
Proteins are a family of macromolecules needed to build new cells and thus for growth. Like starch, Proteins are also polymers and thus get digested into their constituent monomers, in this case amino acids in the digestive system. Protein-rich foods include all meat and some pulses and beans. Proteins in the diet are needed to build muscle tissue, to form some components of cell membranes and to make all the enzymes that catalyse all the metabolic reactions in cells.
Lipid is a general term for all fats and oils. Despite the popular misconception that fat is “bad” in our diet, in fact lipids are essential molecules in the diet. We need lipids as a respiratory substrate, for long term energy storage in adipose tissue under the skin and for the electrical insulation of nerve cells. Foods rich in lipids are red meat, many processed foods, and food containing olive oil or other vegetable oils.
Humans need a wide variety of mineral ions in very low concentrations in our diet. The most important mineral in our diet is Calcium which is needed for making healthy teeth and bones. Iron is also needed in relatively high amounts as it is required to make the protein haemoglobin found in red blood cells. Mineral ions come from eating a wide variety of foods, but the main source of calcium is from milk and other dairy products. Iron is found in high concentrations in red meat.
Rather like minerals, vitamins are needed in very small amounts in a diet but are absolutely crucial for the healthy functioning of the body. The diseases associated with a lack of a particular vitamin in the diet are called deficiency diseases. You need to know about three vitamins – A. C and D Vitamin A is a molecule called retinal found in carrots, red peppers and swede. It is needed for healthy growth and a functioning immune system. Vitamin A is also essential for normal vision since it is used to make the pigment found in rod cells in the retina. Vitamin A deficiency in the diet often causes poor vision, especially at night. Vitamin C is needed for the enzyme that produces the protein collagen in the body. It is found in all fruit especially citrus fruits. A lack of vitamin C causes the deficiency disease scurvy. Vitamin D is an unusual vitamin since it can be made in the skin using UV light. Vitamin D is needed in the small intestine to absorb mineral ions such as calcium, magnesium, etc. into the blood. A lack of vitamin D often results in a deficiency disease called rickets in which the bones malform.
Dietary Fibre is actually made up from the molecule cellulose. No mammal including humans possesses a cellulase enzyme and so when plant material passes through the intestines, dietary fibre is never digested. This means it passes into the large intestine where it helps prevent constipation. Foods rich in fibre included wholegrain bread, vegetables and some breakfast cereals.
Water is the final component of a balanced diet. It is needed to replace water lost by sweating and in urine and acts as a solvent of course for all the metabolic reactions that happen in every cell.