Fats metabolism in human body

All the energy that a person has comes from the food he eats. It is important to take in the right amount of energy.  The body requires 1000-1800 calories of energy a day just to maintain itself.   Many foods are a mixture of carbohydrates, fats, proteins and other substances. If a person eats food containing more energy than needed, then the body stores the surplus as fat.
Fats are also called lipids.  Lipids are classified into three groups: simple lipids, compound lipids, or derived lipids.

Simple Lipids

Simple lipids, also called "neutral fats" are made of triglycerides. A triglyceride is a combination of three fatty acids bonded to a glycerol molecule.98% of dietary fats are triglycerides.

Fat is also stored in the body in the form of triglycerides. Simple lipids are further classified as saturated or unsaturated fatty acids.
Saturated fats come from animals and are seen in dairy products and meat. These foods contain a fat-like substance called cholesterol. People who take more saturated fats and cholesterol have a higher risk of heart problems such as blocked blood vessels.
Unsaturated fats come from plants and are seen in sunflower oil, olive oil and many cooking oils. These fats do not cause any heart problems. In fact, some of them like olive oil help to stop from getting heart diseases. Fats from oily fish such as mackerel are also good for health.
Good fats not only increase metabolism, but also immune function, and they assist in the absorption of minerals.

Compound & Derived Lipids

A compound lipid is a triglyceride combined with other chemicals. They are,
Phospholipids - One or more fatty acids combined with a phosphorus group and a nitrogen base.
Glycolipids - Fatty acids combined with glucose and nitrogen.
Lipoproteins - Lipids combined with proteins. Lipoproteins serve as the body's lipid transport system.

Derived Lipids

Derived lipids contain hydrocarbon rings instead of chains.
Cholesterol - A waxy, fat-like substance present in every cell in the body and in many foods. Some cholesterol in the blood is necessary - but a high level can lead to heart disease.

Fat metabolism

During the process of digestion, fats are broken down by hydrolysis reaction to form glycerol and fatty acids.
Fat digestion starts in the stomach. In the stomach, churning action occurs and it helps in the emulsification of fats. During emulsification of fats (to speed up the digestive process), the large lipid droplets are converted into smaller droplets, which create more surface area for the enzyme lipase to act on.
Enzyme Lipase acts on fats and converts them into fatty acids and glycerol.
In the upper part of the small intestine, the duodenum, fat is mixed with bile juice and is further emulsified. Bile juice is secreted by the liver and stored in gallbladder.
The emulsified fats are acted upon by lipase secreted by the pancreas and small intestine. Lipase catalyses the hydrolysis of fatty acids.
Larger fatty acids are reformed into triglycerides, then are bundled into lipoproteins called chylomicrons and released into the bloodstream.
Unlike amino acids and glucose, the chylomicron travels through the lymphatic system instead of the blood. The lymph eventually flows into the systemic circulation. Since lipids are not soluble in blood, fatty acids are generally transported as lipoproteins after reaction with water-soluble proteins in the blood.

Lipids in the blood are absorbed by liver cells to provide energy for cellular functions. The liver is responsible for providing the proper concentrations of lipids in the blood. Some lipids are utilized by brain cells to synthesize brain and nerve tissue.

Excess lipids in the blood are converted into adipose tissue. If lipid levels in the blood become too low, the body synthesizes lipids from other foods, such as carbohydrates, or removes lipids from storage. The body also excretes some lipids in the form of fats, soaps, or fatty acids as a normal component of faeces. Abnormally high levels of triglycerides and cholesterol are deposited on the inner walls of arteries and cause hardening of the arteries.

Fat metabolism is controlled by hormones such as insulin, growth hormone, adrenocorticotropic hormone and glucocorticoids.
The rate of fat catabolism is inversely related   to the rate of carbohydrate catabolism.
In some conditions such as diabetes, the secretion of these hormones increases to counter a decrease in carbohydrate catabolism.