Fat is one of the three macronutrients (the others being carbohydrate and protein) that make up the majority of the food that we eat. It is an essential macronutrient in that the body requires certain elements that are derived from dietary fats for normal body processes. Secondly four essential vitamins namely A, D, E and K, are fat soluble and the absorption of these vitamins is dependent upon the presence of fat in the gut.
Significant proportions of the fatty acids in the body are obtained from the diet in the form of triglycerides of either animal or plant origin. The fatty acids obtained from the fat in land animals tend to be saturated, whilst those in the triglycerides of fish and plants are usually polyunsaturated and therefore present as oil.
Triglycerides cannot be absorbed from the intestine. They are broken down into mono and diglycerides plus free fatty acids by an enzyme produced by the pancreas called lipase. The fats are then emulsified by the bile salts, produced by the liver allowing the lipase to work optimally at a water-fat interface.
The products of this digestion consist of tri-, di- and monoglycerides and free fatty acids. Together with the other fat soluble contents of the diet and bile salts, they form mixed micelles, in the fluid contents of the duodenum. The micelles enter the epithelial cells lining the small intestine where they are resized into triglycerides and then are released into the lymphatic system of the small intestine. This then drains into the venous circulation and in so doing it does not pass through the liver first, unlike the other products of digestion.
Lipoprotein lipase in the capillaries, especially in adipose tissue, partially digests the chylomicrons into free fatty acids and glycerol. The fatty acids are absorbed by the fat cells (adipocytes) whilst the glycerol component remains in the blood plasma and is ultimately removed from the circulation by the liver. The free fatty acids released in this manner are absorbed by the adipocytes where they are resynthesised into triglycerides using glycerol derived from glucose via the glycolytic pathway. These glycerides are stored by the adipocytes until needed for fuel requirements by other tissues.
Glucose that is absorbed from the small intestine travels to the liver via the portal vain. The liver absorbs some of this glucose and replenishes its glycogen stores. (Only about a 100g of glycogen when full). Most of the rest is converted into fatty acids. These fatty acids are combined with glycerol to form triglycerides in the form of droplets, similar to chylomicrons, but known as very low density lipoproteins (VLDL). VLDL droplets are handled in exactly the same way as chylomicrons.
Glucose from the blood is also taken up by adipose tissue for conversion to triglycerides. However, these tissues do not release the triglycerides as VLDL into the blood. Instead it stores them in their fat droplets, ultimately to release them again as free fatty acids and glycerol in the blood when conditions demand, i.e. when blood glucose is at its base line level, and therefore when insulin is low and both glucagon and adrenaline are high.
The cells of the body are required to manufacture and maintain certain cellular functions in particular the integrity of the cell wall and membranes. It is not known wherever they rely for this entirely on free fatty acids that are absorbed from the blood or if they are able to make their own fatty acids from blood glucose. None of the cells in the body can manufacture essential fatty acids which have to be obtained from the diet. Because of the blood brain barrier, the cells of the central nervous system have the capability of manufacturing their own fatty acids.