lipid

Fat are often referred to as lipid . lipid are described as substances that are poorly soluble or poorly soluble in organic solvents. Over 90% of dietary fats are triglycerides, other types of fat include cholesterol , phospholipids , sterols , and carotenoids. Regardless its source, is usually divided into three major categories:
Fatty Acids: fats consist of fatty acids that have carbon chains containing up to 22 carbon molecules in the chain.  Hydrogen is added to fatty acids to make them more solid when manufacturing some food products such as vegetable spreads; this process is known as hydrogenation. A fatty acids with hydrogen atoms on every arm is said to be saturated. Unsaturated fatty acids contain double carbon bonds where there is no hydrogen. If there is only one double bond the fatty acids is monounsaturated . when more than one double bond is present the fatty acid will be polyunsaturated.
Saturated fatty acids (SFA): It contain  carbon atoms linked by single bonds & hydrogen on all available arms. They are relatively high melting point and tend to be solid at room temperature .
 SFA are obtained from animal storage fats & their products e.g. meat fat, lard, milk, butter, cheese, & cream. Fats from plant origin tend to be unsaturated with the exception of coconut oil & palm oil.   Plasma low-density lipoprotein (LDL) cholesterol & total plasma cholesterol tends to be raised by SFA .  high intake of SFA are associated with atherogenesis & cardiovascular disease.
Monounsaturated fatty acids (MUFA): it contain only one double bond & are usuallu liquid. Olive oil & rapeseed oil are the most concentrated dietary sources of MUFA.  MUFA are Present in many foods including meat fat & lard. Dietry MUFA dose not raise plasma cholesterol and lowers LDL lipoprotein without the detrimental affect on high density lipoprotein (HDL)  .
Polyunsaturated fatty acids (PUFA): it contains 2 or more double bonds &  are liquid at room temp. they are easily oxidized in foods & in the body.
PUFA are involved in the metabolism of cholesterol, are component of phospholipids in cell membranes & are precursors of prostaglandins, interleukins, & thromboxanes, so play important role in the immune response, blood clotting & inflammation.
PUFA are derived from the essential fatty acid , linolenic acid ( omega 3) &  linolic acid (omega6) .they are two useful oils that can not be produced in our body & have to be included in all diets because they are essential for health.
Omega 3 are found in fish & fish oil & their health benefits are being more fully explored, increase consumption of oily fish oil include improved cardiovascular risk factors.
Omega6  are found in vegetable oils and evening primrose are rich in omega 6 fatty acids.
These essential fatty acids regulate complex processes , such as inflammation, platelet aggregation & oxidative damage in cells.
Trans fatty acids: are rare in naturally occurring fats. Some is made in the rumen of cows & sheep & is found in lamb, beef, milk & cheese. Trans fatty acids have been associated with adverse effects on lipoproteins by elevating  LDL & depressing  HDL .
Sterol: are relatively simple molecules; the most common sterol is the wax-like cholesterol , are only found in animal foods. Phytosterols are found in plant foods. Cholesterol has structural role in lipoproteins and membranes and is a precursor for bile acids , steroid hormones , and vitamin D . reduction intake of saturated fat results in lower plasma cholesterol level .  
2- Biological functions:
a- Energy source: Triacylglycerols, stored in adipose tissue, are a major form of energy storage The complete oxidation of fatty acids provides high caloric content, about 9 kcal/g, compared with 4 kcal/g for the breakdown of carbohydrates and proteins.
b-  fat is a carrier for The "fat-soluble" vitamins (A, D, E and K) – which are essential nutrients, with a diverse range of functions.
c- fat provides essential fatty acids.
d- increase palatability by improving taste perception & appearance of food.
e- cholesterol is converted to bile acids, which are important in digestion.
f-some fats are important constituents of cell membranes and can be converted to biologically active compounds such as steroid , hormones , interleukins , thormboxanes , and prostaglandins .
Metabolism: once we eat a fatty meal, the pancreatic enzyme ( pancreatic lipase) breake down all ingested fats to smaller molecules . these are then absorded in the small intestine& are  repackage together with protein to form a large particles called chylomicrons, which circulate & are mainly removed in adipose tissue by lipoprotien lipase. Eventually, they bind to the membranes of hepatocytes, adipocytes or muscle fibers, where they are either stored or oxidized for energy. The liver acts as a major organ for fatty acid treatment, processing chylomicron remnants and liposomes into the various lipoprotein forms . there are five classes of lipoprotein which vary in density :
• Chylomicrons :  carry diet-derived lipids to body cells
•  Very low density lipoprotein (VLDL)  :  carry lipids synthesized by the liver to body cells
• low density lipoprotein  (LDL ) : transport  cholesterol to peripheral tissue and liver 
• High density lipoprotein (HDL) :  carry cholesterol from the body back to the liver for breakdown and excretion.
• Lipoprotein ( a)     ( LP(a) ) :albumin transports  free fatty acids from adipose tissue to peripheral tissue.
Dietery sources of cholesterol:
1- high: liver, eggs, mayonnaise, shellfish.
2- medium: meat fat, full fat milk, dairy products
3- low: skinless poultry, skimmed milk.
4- cholesterol free: friuts & vegtebales, cereals, pasta ,  rice, egge white, sugar
Requirment: metabolic requirment of fat is 10-20 gm daily.
Lipid  and health
High fat intake contributes to increased risk of obesity, atherosclerosis, IHD, diabetes, cancer (colorectal), gall bladder disease and arthritis, alzhiemer , CVA., Brain, nerves, and other tissues are dependent on omega-6 and omega 3