Cholesterol was blamed as the main culprit for atherosclerosis and primary hypertension. The warning of cholesterol intake restriction was started from the US Dietary Guideline published in 1961. A daily limit of 300mg of cholesterol was recommended by the American Heart Association. Since then, the warning, ‘cholesterol is bad for your health’ made its way into virtually every health or diet book. Drugs that inhibit cholesterol synthesis (e.g. Statin) were also developed. Lipid proteins, high density lipoproteins (HDL) and low density lipoproteins (LDL) contain high levels of cholesterol and were also nicknamed ‘good cholesterol’ and ‘bad cholesterol’ respectively. The ‘scientific’ warning was actually based on the results from experiments in rabbits, where an unconvincing connection between LDL and atherosclerosis was found. It was later discovered that cholesterol metabolism in rabbits is different from other animals including human beings. Therefore, those experimental results do not apply to us. For 54 years, the world was misled about cholesterol. Eventually, UAS Dietary Guidelines Committee corrected it in 2015. They stated on Page 91:
"Previously, the Dietary Guidelines for Americans recommended that cholesterol intake be limited to no more than 300 mg/day. The 2015 DGAC will not bring forward this recommendation because available evidence shows no appreciable relationship between consumption of dietary cholesterol and serum (blood) cholesterol, consistent with the AHA/ACC (American Heart Association/American College of Cardiology) report. Cholesterol is not a nutrient of concern for overconsumption." (USDA, 2015)
Although the statement “Cholesterol is not a nutrient of concern for overconsumption” marked the end of the cholesterol warning, that wrong warning still lives in many books, and still guide people to avoid cholesterol-rich foods. The cholesterol-phobia has been spread all over the world, and will continue to linger for many years to come.
What most people don’t know is that, cholesterol is actually an important nutrient, and plays more roles than any other small molecules in the body. It is an essential molecule for the syntheses of vitamin D, sterol hormones, bile salts and cell membranes. Cholesterol is mainly synthesised in the liver (85%) and partly absorbed from diet (15%). Our body needs 1.0-2.0 grams of cholesterol daily. The liver can regulate cholesterol synthesis, depending on how much cholesterol is absorbed from diet, maintaining a steady level of cholesterol in the blood stream. The bile salts are important in the transport and absorption of lipid and lipid soluble vitamins. Cholesterol is also an important component in cell membrane (Fig. 1). Most of cells in our body undergo ageing and renewing, e.g skin and lining of the gut. Every new cell needs cholesterol for its cell membrane. If the cholesterol supply is inadequate, cell growth will be affected and tissue repair or healing will be delayed, resulting in poor cell membrane and skin quality.
Fig.1 Basic components in the cell membrane.
The cholesterol for cell membrane synthesis is provided by LDL, which was previously mistakenly labelled as ‘bad cholesterol’. When doctors discover high levels of LDL in patients, they tend to prescribe statins (an inhibitor of cholesterol synthesis) to lower it. However, high blood LDL may be caused by oxidation and the oxidized LDL cannot deliver cholesterol to cells. The normal half-life of LDL in the blood is only 2-5 days. If a cholesterol-free diet is consumed at the same time as taking a statin, where would the body get the cholesterol from to maintain its essential functions?
For the past 50 years, cholesterol has been treated as an enemy in CVD prevention, and its good qualities have been ignored. We now know that this is wrong. However, it is never too late to correct our mistakes. In the words of USDA, “Cholesterol is not a nutrient of concern for overconsumption”.
Our knowledge and understanding of the human body, health and disease has dramatically increased over the past few decades, but we still have much more to learn. What we know to be true now may not be so in 10 years when new evidence comes to light. We need to face the future with an open mind, and be led by evidence rather than the comfort from our existing knowledge. Just as the guideline regarding cholesterol intake has been changed, the management of hypertension could be the next biggest revolution in drug-based medicine.
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