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May 2017
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METABOLISM OF LIPIDS (FATS)

In the human body, lipids are structural components of cell membranes, they are energy stores and they are precursors of many important molecules.

Lipids (fats) are made up of long chains of hydrocarbons which are greasy to touch and insoluble in water. They can be classified into three main groups:

  • SIMPLE LIPIDS Triglycerides (glycerol and fatty acids)
  • COMPOUND LIPIDS Phospholipids and lipoproteins
  • DERIVED LIPIDS Cholesterol

SIMPLE LIPIDS

These are mainly made up of triglycerides, the most common fat in the body (95% of body fats are triglycerides). Triglyceride molecules are glycerols and fatty acids connected together.

Fatty acids are saturated when their chemical structure does not contain double bonds. Fatty acids are unsaturated when there is more than 1 double bond within the chemical structure. Whether there are double bonds, or not, is important because there is evidence that saturated fatty acids increase the level of cholesterol and LDL lipoproteins in the blood, advancing the atherosclerotic process. Unsaturated fatty acids, on the other hand, tend to reduce these levels. Some polyunsaturated fatty acids are essential nutrients because they cannot be synthesised by mammals. Unsaturated fats are contained in plant based foods, particularly those that are liquid at room temperature (oils) and fish, whilst saturated fatty acids are found in animal based products and in fats in condiments that are solid at room temperature (butter, lard, margarine, etcetera). 

Fatty acids are precursors of certain substances, for example prostaglandins, which are a series of compounds that are spread out throughout the body and whose job is to stimulate the contraction of muscular cells and modulate cells’ responses of certain stimuli.

COMPOUND LIPIDS

Compound lipids are made up of triglycerides connected to other compounds.

Phospholipids
These contain molecules of fatty acids that are connected to a phosphate group and a nitrogenous base. They are synthesised inside the body, particularly in the liver, and due to their greater solubility, they facilitate the transport of other fats, however, their main job is to create cell membranes, in particular the membrane of red blood cells and myelin, which is the membrane of nerve cells.

Lipoproteins
Since lipids are usually insoluble in aqueous environments, they need molecules to act as carriers to transport then around the body, therefore, in plasma, the liquid part of blood, insoluble fats are present as compounds; lipoproteins 

Lipoproteins are made of triglycerides, cholesterol, phospholipids and apoproteins. They are usually classified by electrophoresis according to their chemical properties:

  • Chylomicrons (triglycerides and, in small quantities, also cholesterol and phospholipids). They form following the emulsion of lipid droplets that leave the intestine and travel to the lymphatic ducts. Chylomicrons are taken on by the liver where they are metabolised and transformed into stored fat deposits. They also transport liposoluble vitamins; vitamins A, D, E and K.
  • HDL (high density lipoproteins). These are produced in the liver and small intestine and are also called “good cholesterol” because they remove cholesterol from the walls of arteries and take it, via the bloodstream, to the liver, where it is used to form bile.
  • LDL (low density lipoproteins) and VLDL (very low density lipoproteins) contain a large percentage of lipids and cholesterol and a small quantity of protein.

LDLs create a ‘relationship’ with the artery’s endothelial cells, freeing cholesterol from the artery walls where, if it remains there, it can be partially oxidised or play a part in the process of cell proliferation, which causes a functional alteration of the arterial walls and a narrowing of the vessel lumen.

DERIVED LIPIDS

These contains substances that are derived from simple and complex lipids. The most well known is cholesterol, a sterol that is found exclusively in animal tissues.

Cholesterol was first found in 1784 in a gallstone. Unlike what is commonly believed, cholesterol is not necessarily dangerous and can be very useful for the body as it plays a part in essential functions:

  • it is structural element of cell membranes
  • it is necessary for the synthesis of steroid hormones
  • it is a precursor of vitamin D
  • it is a starting material for the biosynthesis of bile acids.

Cholesterol can be of endogenous origin (synthesised directly by cells) or exogenous origin (it comes from food). Endogenous synthesis increases when the amount of saturated fatty acids increases in the diet. The amount of cholesterol produced by endogenous synthesis normally satisfies our organs’ needs and therefore, even when there is no cholesterol provided in the diet, there is no risk of endangering our health (expect in the case of infants).

The major sources of cholesterol in the diet are shown in the table below.

Cholesterol in foods (given in mg per 100g of edible part of the product)

Oils and fats
Butter 250
Lard 95
Margarine (vegetable fats) 0
Vegetable oils (olive, corn...) 0
Egg (from chicken)
Whole 411
Yolk 1337
White 0
Cheeses
Parmesan 109
Brie 98
Mozzarella 46
Meat
Chicken 119
Beef 60/70
Pork  60
Fish
Prawns 150
Eel, fillet 97
Sea bream, fillet 64

The function of lipids in the body

The main functions of lipids in the body are:

  • energy reserves
  • mechanical protection for certain organs
  • thermal insulation.

Energy Reserves
From an energy point of view, these energy reserves are an ideal base layer for cells because they can release a large amount of calories per unit of mass. The amount of calories they release is more than double that of sugars and proteins.

Mechanical protection
Approximately 4% of body fat is used as mechanical protection for important organs, such as the heart, liver, kidneys, spleen, brain and the spinal cord.

Thermal insulation
Fat that is deposited under the skin carries out an important role as thermal insulation, especially from the cold, however excess fat can cause problems for thermoregulation.

Amount of lipids in our diets
The percentage of calories we consume daily, provided by lipids, should be 30% for children and 25% for adolescents and adults. These percentages, if applied to a normal food regime, are very important for our health. These guidelines become more complex and particular according to the quality of the lipids we consume, however, as well as the amount of saturated, unsaturated and polyunsaturated fatty acids we eat, along with essential fatty acids. 
The amount of saturated fatty acids we consume must not be more than 10% of our total, daily calorie intake, however, the amount of cis-monounsaturates can be more (approximately 12%). Amongst monounsaturates, oleic acid should be favoured because it is readily oxidised or stored by the body and then easily disposed of when energy is required. Regarding polyunsaturates, the recommended quantity is less than 10% of our daily calorie intake because of their susceptibility to oxidisation. These oxidative changes actually encourage the production of derivatives (peroxides) that are potentially toxic and that can cause atherosclerotic processes and aging. An excess of polyunsaturates could also bring about the production of lithogenic bile and intestinal carcinogenesis. As far as our daily requirements of essential fatty acids goes, the recommended daily amount is 4.5-6g for adult males and 1-1.5g for adult females. 
 
The recommended daily amount of cholesterol that we should consume, and that will not cause any problems for healthy bodies, should be no more than 300mg a day.

METABOLISM OF LIPIDS (FATS)

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