In the preface of the Dietary Guidelines for Chinese Residents, there is a classic three-word passage: “The people take food as the day; food to nourish, as the first; drink and food, learn a lot; the most authoritative, see the guide. In the past, the Inner Classic, called the classics; on food and drink, there are wonderful words. Five grains, appropriate for nourishment; if the loss of beans, is bad. Five animals, appropriate for the appropriate; such as excessive, the harm is very shallow.” In the United States, every five years the government also releases a new version of the Dietary Guidelines for Americans, which is a “reference book” for everyone on how to eat, and is a must-have for every family, even nutritionists. The purpose of the Dietary Guidelines is to improve the health of the entire population, as well as to guide citizens on how to eat healthy and balanced nutrition, and also to guide food companies in providing the necessary nutritional information or advice on product packaging.
In February 2015, the U.S. Dietary Guidelines Advisory Committee (DGAC) released a scientific report, and this report gives some recommendations for revising the new edition of the Dietary Guidelines to promote the prevention of diseases for the health of the nation. The U.S. Department of Agriculture (USDA) and the Department of Health and Human Services (HHS) will use the report as a basis for releasing a new edition of the Dietary Guidelines for the U.S. population in late 2015 after extensive input from all parties.
Cholesterol has long been considered an important cause of cardiovascular disease, and “do not exceed one egg yolk per day”, otherwise it is likely to cause hypercholesterolemia, which subsequently leads to cardiovascular disease. Since 1977, one of the six core points in the Dietary Guidelines for United States Residents is to control daily cholesterol intake of less than 300 mg, and the new version of the guideline opinion of the U.S. Dietary Guidelines Advisory Committee does not recommend reducing dietary cholesterol intake, which has triggered extensive discussions.
I. Cholesterol is involved in a variety of physiological functions in the human body
Cholesterol, also known as cholesterol, is a derivative of cyclopentane polyhydrophenanthrene. Cholesterol was discovered in gallstones as early as the 18th century, and in 1816, the chemist Bencher named this lipid-like substance cholesterol. Cholesterol is widespread in animals and is found in almost all tissues, but in varying amounts, especially in brain and nerve tissue, and also in kidney, spleen, skin, liver and bile. Cholesterol is similar to fat in solubility, insoluble in water and soluble in solvents such as ether and chloroform.
It not only participates in the formation of cell membranes, but is also a raw material for the synthesis of bile acids, vitamin D and steroid hormones. Cholesterol is also metabolized into bile acids, steroid hormones, 7-dehydrocholesterol, and 7-dehydrocholesterol is transformed into vitamin D3 by ultraviolet radiation, so cholesterol is not a harmful substance to humans.
Cholesterol is overwhelmingly synthesized in the liver and is involved in regulating many physiological activities in the body: it performs human cell repair, participates in the synthesis of cell membranes and nerve fibers, synthesizes vitamin D to maintain bone health, promotes fat digestion, and helps repair and keep intact the regulation of blood vessel walls. Cholesterol is a precursor substance for many hormones closely related to body functions, such as glucocorticoids, aldosterols, estrogens and androgens. Therefore cholesterol is very important for humans.
Second, the effect of dietary cholesterol on blood cholesterol concentration
Cholesterol is present in the blood in the form of lipoproteins, including HDL cholesterol, LDL cholesterol, and very low density lipoprotein cholesterol. The majority of cholesterol present in the blood is cholesterol esters bound to fatty acids, and less than 10% of cholesterol is present in the free state. A blood cholesterol level between 140-199 mg per unit is a relatively normal cholesterol level.
Elevated blood cholesterol is one of the main causes of the development of diseases such as atherosclerosis and coronary heart disease. Controlling blood cholesterol levels can effectively prevent atherosclerosis, so many people give up foods such as egg yolks, which contain many essential nutrients, for fear of high cholesterol. However, dietary cholesterol and blood cholesterol cannot be equated. A growing body of medical research has shown that blood cholesterol levels are far more complex than people think, and that increasing dietary cholesterol does not have a significant impact on blood cholesterol levels.
Many people mistakenly believe that high cholesterol is “eaten”, but in fact food intake is only one component of cholesterol production and most of the cholesterol in the body is self-synthesized. There are two main sources of cholesterol, one is produced by the body itself and the other comes from food. A normal human diet contains about 300-500 mg of cholesterol per day, mainly from animal offal, egg yolk, cream and meat. Plant foods do not contain cholesterol, but contain plant sterols such as beta gluten sterols and ergosterol, which are not easily absorbed by the body and can also inhibit the absorption of cholesterol if consumed in excess. For example, a 70Kg adult has about 140g of cholesterol in his body, which is updated by about 1g per day, most of which is metabolized in the body, and dietary intake of cholesterol only accounts for 1/3 to 1/7 of the synthetic cholesterol in the body. 200mg of cholesterol per person per day from food can meet the body’s needs. The absorption rate of cholesterol is only 30% and decreases as the cholesterol content of food increases. 200mg is equivalent to the cholesterol content of 1 egg or the cholesterol absorption of 3 to 4 eggs.
Third, the dangers of dietary cholesterol
Restricting dietary cholesterol intake has long been considered an important measure to prevent cardiovascular disease, so why did the U.S. Dietary Guidelines Advisory Committee relax the cholesterol restrictions? Although foods high in cholesterol (such as egg yolk, animal offal, animal brain, animal fat, crab yolk, crab paste, etc.) have been considered as one of the factors that increase the risk of cardiovascular disease, scientific studies over the years have not found a clear causal relationship between the two. Six clinical trials in recent years related to elements of dietary fat, cholesterol levels and heart disease have not found that a low-fat diet reduces death from heart disease or death from any other cause. Even the group of subjects in one clinical trial that controlled dietary saturated fat to 10% had increased mortality from heart disease or other causes. Because of the consistent reduction in clear scientific evidence and the fact that the body tends to produce much more cholesterol than is contained in the diet, the U.S. Dietary Guidelines Advisory Committee finally decided not to consider cholesterol as a “nutrient of concern for excessive intake.
The Advisory Committee noted that there is no clear correlation between dietary cholesterol and heart disease and therefore removed the restriction, but the guidelines do not deny the relationship between blood cholesterol and cardiovascular disease. This change does not mean that cholesterol is completely harmless, only that it is not harmful enough to be restricted, and that it remains a potential threat to cardiovascular health. Several large trials have shown a clear benefit of lowering cholesterol levels in reducing the risk of cardiovascular disease, and statins that lower cholesterol levels have benefited thousands of people.
The absorption of dietary cholesterol and its effect on blood lipids varies widely among individuals depending on their genetic and metabolic status. In some individuals, high cholesterol intakes inhibit the synthesis of their own cholesterol. Cholesterol intake does not directly reflect blood cholesterol levels. To date, it has not been possible to establish a tolerable maximum intake limit for cholesterol in humans.
In the case of cholesterol, there is strong evidence that hypercholesterolemia is strongly associated with the development of coronary heart disease, and dietary cholesterol intake can have a significant impact on circulating cholesterol levels. One expert noted, “Although the guideline developers considered excessive cholesterol intake to be harmful, they rescinded this recommendation because of the lack of research evidence to support an upper limit of 300 mg/d. Thus, it appears that the move should not be misinterpreted to mean that excessive cholesterol intake is harmless.” “If you don’t like taking lipid-lowering drugs, then don’t eat too much high-cholesterol food.” However, there is no unanimous conclusion as to which level of cholesterol needs to be controlled in the diet, i.e., exactly how much of a daily limit is needed to not create additional health risks.
Some experts also point out that the level of cholesterol in the blood is not simply a result of eating in the past, and that dietary cholesterol is not the main cause of elevated blood cholesterol; high blood cholesterol also depends on the amount of exercise, etc. For people who have high cholesterol themselves and lack exercise, dietary cholesterol is not to be underestimated. Since the absorption and metabolism of each person is different, some people cannot metabolize too much cholesterol and it is deposited in the blood vessels, which, over time, forms atherosclerotic plaques and brings the risk of cardiovascular diseases. Foods with high cholesterol contain more other bad things, such as high saturated fat, high trans fat, high refined sugar, etc., so there should also be intake limits for foods rich in high cholesterol.
IV. Saturated fatty acids in food
Hypercholesterolemia is one of the main causes of atherosclerosis. However, the main source of cholesterol in blood vessels is not food, but the liver. The main raw material for cholesterol synthesis in the liver is saturated fatty acids, not the cholesterol in food absorbed by the intestine. Fatty acids in most foods, especially saturated fatty acids, have a greater impact on blood cholesterol than cholesterol in food. In general, animal fats such as butter, cream and lard contain more saturated fatty acids than vegetable fats.
Although the U.S. Dietary Guidelines Advisory Committee concluded that there is no evidence of a “predictable correlation” between cholesterol intake and heart disease and no longer limits cholesterol intake, it still recommends a low intake of saturated fatty acids. Earlier reports recommended limiting saturated fatty acid intake to 10 percent of total cholesterol intake. The American Dietary Guidelines Advisory Committee, the American Heart Association, and the American College of Cardiology agree that reducing saturated fat intake can reduce the risk of cardiovascular disease in the population, and that reducing dietary saturated fat intake from 14% to 5-6% of caloric supply can significantly reduce LDL cholesterol levels and reduce the incidence of coronary heart disease.
Five, red meat and processed meat products
Red meat refers to meat that shows red color before cooking, such as pork, lamb, beef, rabbit and other mammalian meat are red. The color of red meat originates from the myoglobin contained in mammalian meat. In contrast, the meat of birds (chickens, ducks, etc.), fish, reptiles, amphibians, crustaceans (shrimps, crabs, etc.) or shellfish (oysters, clams, etc.) non-mammalian animals is not red meat and can be counted as white meat.
Red meat is characterized by thick and hard muscle fibers and higher fat content, especially saturated fatty acid content than white meat. The fat content of pork is the highest, followed by lamb, and beef is the lowest. Even in lean meats, the fat content is still quite high, for example, lean pork has 6.2% fat, lean lamb 3.9% and lean beef 2.3%. In fact, saturated and unsaturated fatty acid content are relative, and almost all natural foods contain both. The World Health Organization recommends that the proportion of energy provided by saturated fat to total energy should not exceed 10%, so moderate intake of red meat is allowed. However, the current meat intake of our residents is still dominated by pork, which has a high fat content and more saturated fatty acids, which is not conducive to the prevention of cardiovascular, overweight, obesity and other diseases, so the proportion of pork intake should be reduced and lean meat, especially lean beef, should be promoted.
However, there is no association between unprocessed red meat and heart disease and type 2 diabetes. Red meat is rich in minerals, especially iron and zinc, and is easily absorbed and utilized by the body, as well as rich in protein, vitamins (B1, B2, A, D), trace elements, high-quality protein, healthy fats and other nutrients essential for human and brain health. But processed meat is indeed not healthy, such as high-temperature frying and other overcooking, and has nothing to do with the meat itself. The U.S. Dietary Guidelines Advisory Committee recommends reducing the intake of red meat and processed meat products, while noting in a footnote that lean meat is part of a healthy diet. The North American Meat Institute criticized the report, saying that the benefits of lean meat should be prominently advertised, not just featured in a footnote.
VI. The “time bomb on the table” —- trans fatty acids
The main source of trans fatty acids (TFA), also known as trans fatty acids, is partially hydrogenated vegetable oils. Partially hydrogenated oils have the advantages of high temperature resistance, low deterioration and long storage, and are commonly used in margarine, baked goods, fried foods, cocoa butter, vegetable cream, margarine and other foods. The intake of trans fatty acids in the diet is closely related to the occurrence of ischemic cardiovascular diseases. Excessive intake of TFA can increase blood cholesterol, thus increasing the risk of cardiovascular disease.
The former Ministry of Health issued a national standard numbered GB28050-2011 on October 12, 2011 to restrict the use of TFA, and on June 16, 2015, the U.S. FDA announced that it would completely ban the use of artificial TFA in food within three years to help reduce the incidence of cardiovascular diseases. Trans fatty acid intake should be strictly limited in daily life, and it is best to do without.
VII. How to face cholesterol in food
The expert panel of the U.S. Dietary Guidelines Advisory Committee did not give a recommendation on the upper limit of cholesterol intake per day or how many eggs can be eaten per day. This conclusion has left the people confused as to how we should eat cholesterol in the face of it.
The US Dietary Guidelines Advisory Committee only recommends that there is no need to deliberately limit cholesterol, and does not encourage excessive intake of foods containing cholesterol. The national dietary guidelines are part of health policy, and their primary purpose is to gradually correct the most prominent dietary problems. Some of the recommendations in the dietary guidelines are not as stringent as clinical guidelines, and there is some variation or compromise on the so-called “ideal way to eat”.
The U.S. Dietary Guidelines Advisory Committee is only a report on healthy eating for the public, not a guide for the treatment of disease. For the majority of patients with cardiovascular disease, especially those with hypercholesterolemia, lowering cholesterol to prevent cardiovascular disease is still an important preventive tool. The American Heart Association still recommends a low-fat diet: total fat should be 25% or 35% of total daily dietary caloric intake, and saturated fat should be even less than 7%.
There is no conclusive evidence on the effect of food sources of cholesterol on cardiovascular disease. Perhaps until the results of the study become clearer, the public should follow the “Mediterranean diet” as much as possible as an option. The “Mediterranean diet” is rich in a variety of fruits and vegetables, healthy fats such as olive oil, fish, grains, nuts and legumes. The term “Mediterranean diet” is now also used to refer to a simple, light and nutritious diet that is beneficial to health. Current studies show that people at risk for heart disease who choose the Mediterranean diet are less likely to develop heart disease than those on other diets.