Cholesterol is an essential substance for all living organisms. It is synthesized within the body and a certain amount of it is obtained from food. Lipoproteins carry cholesterol in the blood since fat is not soluble in water. There are two major types of lipoproteins which carry cholesterol to and from cells. They are low density lipoprotein (LDL) and High density lipoprotein (HDL). Cholesterol makes a significant contribution to our body. However, the increase in blood cholesterol level is not good for health. Saturated fats in food can be converted in to cholesterol and it may lead to excessive cholesterol in the blood.

Function of cholesterol
Cholesterol forms the components for cell structures and is vital for synthesis of hormones, vitamin D and other substances.

• Cell membrane synthesis – Cholesterol helps to regulate membrane fluidity over the range of physiological temperatures. It has a hydroxyl group that interacts with the polar head groups of the membrane phospholipids and sphingolipids. They exist with a nonpolar fatty acid chain of the other lipids. Cholesterol also prevents the passage of positive hydrogen ions and sodium ions across the plasma membranes.
• Cell transporters and signaling molecules – The cholesterol molecules exist as transporters and signaling molecules along the membrane. Cholesterol also helps in nerve conduction. It forms the invaginated caveolae and clathrin-coated pits, including caveolae-dependent and clathrin-dependent endocytosis. Endocytosis means engulfing of foreign molecules by the cell. Cholesterol helps in cell signaling by assisting in the formation of lipid rafts in the plasma membrane.
• Provide protection as a cover – The nerve cells are covered with a protective layer or myelin sheath. The myelin sheath is rich in cholesterol. It helps in providing protection, insulation and allows more efficient conduction of nerve impulses.
• As a precursor – Within the cells, cholesterol is the precursor molecule in several biochemical pathways. For example, in the liver, cholesterol is converted into bile, which is then stored in the gallbladder. Bile is made up of bile salts. This helps in making the fats more soluble and helps in their absorption. Bile salts also aid in the absorption of fat soluble vitamins like Vitamins A, D, E and K. Moreover, Cholesterol is an important precursor molecule for the synthesis of Vitamin D and the steroid hormones like Corticosteroids, Sex-steroids (Sex hormones like Estrogen, Progesterone and Testosterone etc.)

Cholesterol synthesis within the human body
The liver is the primary organ that is involved the synthesizing of cholesterol and approximately 20–25% of total daily cholesterol production occurs in the liver. Cholesterol is also synthesized to smaller extents in the adrenal glands, intestines, reproductive organs etc.

The synthesis of cholesterol begins with a molecule of acetyl CoA and one molecule of acetoacetyl-CoA, which are dehydrated to form 3-hydroxy-3-methylglutaryl CoA (HMG-CoA). This molecule is then reduced to mevalonate by the enzyme HMG-CoA reductase. This step is an irreversible step in 
cholesterol synthesis. 

Then Mevalonte converts into 3-isopentenyl pyrophosphate and it is decarboxylated to isopentenyl pyrophosphate. Three molecules of isopentenyl pyrophosphate condense to form farnesyl pyrophosphate through the action of geranyltransferase. Two molecules of farnesyl pyrophosphate then condense to form squalene. This requires squalene synthase in the endoplasmic reticulum. Oxidosqualenecyclase then cyclizes squalene to form lanosterol. Eventually, Lanoststerol is converted to cholesterol.

Risk factors
There are several factors which can increase the risk of bad cholesterol. 

• Poor diet- Eating saturated fat, mainly in animal products, and trans fats, found in some commercially baked cookies and crackers and microwaved popcorn, can raise cholesterol level. Foods that are high in cholesterol, such as red meat and full-fat dairy products, will also increase cholesterol.
• Obesity- Having a body mass index (BMI) of 30 or greater exposes the individual to the risk of having high cholesterol.
• Lack of exercise- Exercise helps boost our body’s HDL (high density lipoprotein) which is good cholesterol for our health while increasing the size of the particles that make up LDL (low density lipoprotein) or bad cholesterol, whereas large particle size of LDL makes it less harmful.
• Smoking- Cigarette smoking damages the walls of blood vessels, making them more susceptible to accumulate fatty deposits. Smoking might also lower the level of HDL, or “good,” cholesterol.
• nAge-Normally, our body’s chemistry changes with age. Therefore, the risk of high cholesterol due to liver function decreases with age and as a result of that the liver becomes less able to remove LDL cholesterol.
• nDiabetes- High blood sugar contributes to higher levels of a dangerous cholesterol called very-low-density lipoprotein (VLDL) and lowers HDL cholesterol. High blood sugar also damages the lining of arteries.

Hypertension
Having a family history of premature heart diseases:

• Pre-existing heart diseases
• Heart attack experience
• Liver or kidney disease
• Polycystic ovary syndrome
• Pregnancy and other conditions that increase levels of female hormones
• Insufficient hormones produce by thyroid gland
• Drugs that increase LDL cholesterol and decrease HDL cholesterol, such as progestins, anabolic steroids, and corticosteroids

Diagnosis
The increase in blood cholesterol level is measured by a lipid profile analysis and needs a pre-preparation of 12-14 hours fasting status before undergoing a lipid profile test. There are 5 main components including;

• Total cholesterol- Measure total amount of cholesterol 
• Triglycerides- Triglycerides are a type of fat in our body. If we take more calories than our body required, the extra calories, alcohol or sugar are converted into triglycerides. These triglycerides are stored in fat cells for later use.
• High density Lipoprotein (HDL) - Good cholesterol which picks up excess cholesterol and takes it back to liver.
• Low density Lipoprotein (LDL) - Bad cholesterol which transports cholesterol particles throughout the body and builds up in the walls of arteries make them hard and narrow.
• Very Low density Lipoprotein (VLDL)- VLDL cholesterol is produced in the liver and released into the bloodstream to supply body tissues. It is bad cholesterol since it is associated with the development of plaque deposits on artery walls, which narrows the passage and restrict blood flow. 

Complications
High cholesterol can cause a dangerous accumulation of cholesterol and other deposits on the walls of arteries. These deposits can reduce blood flow through arteries, which can cause complications such as chest pain, heart attack and stroke. 

Lifestyle changes to improve lipid profile
Eat Healthy foods
Reduce saturated fats- Saturated fats, mainly in red meat and full-fat dairy products, raise your total cholesterol. Decrease consumption of saturated fats can reduce low-density lipoprotein (LDL) cholesterol.

Eliminate trans fats-  Hydrogenated vegetable oil  often uses in margarines and store-bought cookies, crackers and cakes. Trans fats raise overall cholesterol levels. 

Consume omega-3 fatty acids- Omega-3 fatty acids don’t affect LDL cholesterol. But they have other heart-healthy benefits, including reducing blood pressure. Foods with omega-3 fatty acids include salmon, mackerel, herring, walnuts and flaxseeds.

Increase soluble fiber - Soluble fiber can reduce the absorption of cholesterol into bloodstream. Soluble fiber is found in such foods as oat meal, kidney beans, Brussels sprouts, apples and pears.

Add whey protein- Whey protein, which is found in dairy products, may account for many of the health benefits attributed to dairy. Research studies have shown that whey protein given as a supplement lowers both LDL cholesterol and total cholesterol as well as blood pressure.

Increase physical activities
Exercise can improve cholesterol. Moderate physical activity can help raise high-density lipoprotein (HDL) cholesterol.
Stop smoking
Quitting smoking improves your HDL cholesterol level. The benefits occur quickly:
 Lose weight

Conclusion
Increase level of cholesterol lead more complication in our health including heart attaches and stroke condition. Therefore, we have to control our blood cholesterol level by changing our bad life style pattern as mention above.

The writer is a medical laboratory technologist at a private hospital and holds a MSc. Degree in Industrial and Environmental Chemistry from the University of Kelaniya and a BSc. In Food Production and Technology Management degree from the Wayamba University 
of Sri Lanka.