Fats are an important constituent of diet and are found in both animal and plant foods. They are the main energy store in the body and are one of the three main classes of food essential to the body. All fats, solid and liquid, are classified as lipids and are made up of carbon, hydrogen and oxygen.
Most fats are made up of one molecule of glycerol, which is combined with three molecules of fatty acids. Fatty acids are chains of carbon atoms with hydrogen atoms attached to it. These chains can be short or long, depending on the number of carbon atoms in the chain. The carbon atoms in these fatty acid chains are joined together by single or double bonds.
Saturated fatty acids: These do not have any double bonds linking the carbon atoms in the entire chain and are predominantly found in animal foods.
Unsaturated fatty acids: These have at least one double bond joining the carbon atoms in the fatty acid chain. A fatty acid with only one double bond is termed as monounsaturated, while the presence of two or more double bonds makes the fatty acid polyunsaturated.
Fats have many important functions in the body, one of the foremost being to supply energy. Fats can produce about nine calories of energy per gram, a value more than double that produced by carbohydrates and proteins. Fats are ideal as the storage fuel for the body, and carbohydrates and proteins are also converted into fatty (adipose) tissue to be used later in time of need.
Apart from providing satiety and palatability to a diet, fats are important for absorption of fat soluble vitamins like Vitamin A and Vitamin D in the body. The internal fatty tissues provide insulation by surrounding important organs and protecting them from injury and trauma.
Certain fatty acids, known as essential fatty acids (EFA) cannot be made in the body, and hence have to be provided in the diet. These EFA are necessary for the structure and functioning of cells, as also for growth and maintenance. They control inflammation, are building blocks for membranes and are important for brain development.
Fats in the diet can be of two kinds, visible and invisible fats. Visible fats are obtained from animal fats, such as fat in butter and meat, and those derived from vegetable fats like mustard, coconut etc. These fats obtained from animal sources are saturated and a risk for heart disease, as they seem to increase cholesterol in the blood. Unsaturated fats like polyunsaturated, are believed to help in lowering cholesterol levels. Most liquid vegetable oils are unsaturated, exceptions being coconut and palm oils.
Invisible fats are widely present in foodstuffs, such as milk, eggs, cereals and pulses. Many of these are an especially good source of EFA. Margarines and shortenings are produced by hydrogenation of vegetable oils like corn and soybean oils etc.
Hydrogenation is the process of hardening fats by artificially adding hydrogen at the double bonds, thus making the fatty acid less unsaturated. This process leads to formation of trans fatty acids, which again raise blood cholesterol and should be avoided. Trans-fatty acids are found in fried foods, processed foods, margarines and commercial foods like cookies and crackers.
The quantity of fat which should be included in the diet is not known with certainty. However, it should be such as to meet the EFA requirement, to promote absorption of fat soluble vitamins, provide palatability and yet not have undesirable effects due to excessive intake. Though there is no specific Recommended Daily Allowance for dietary fats, al least 15-25 grams visible fat from foods is advisable. This includes vegetable sources like cereals to meet the EFA requirement. However, the upper limit of fat intake should not exceed 25%-30% of the total calories on a daily basis.
Deficiency and Excess
Fat deficiency is not very common and the bigger problem is fat excess. Since EFA play a role in several metabolic reactions, a deficiency of these can lead to a skin condition characterized by dryness, roughness or eczema of the skin. Also, the absorption of fat soluble vitamins like vitamin A, D, E and K will be affected with fat deficiency.
Saturated fats have been associated with an increase in blood cholesterol levels. Cholesterol causes narrowing of the blood vessels, which is a precursor for heart disease. Polyunsaturated oils on the other hand, seem to lower cholesterol levels. However, these should be consumed in moderation, as not only the type of fat, but also the total amount of fat in the diet is important for health.