Dietary guidelines are reference books that guide people to eat reasonably and promote health. Many countries have formulated and published dietary guidelines for residents in order to promote national health and prevent diseases, and they are constantly updated based on comprehensive evaluation of new findings and evidence from scientific research. For many years one of the core points of the Dietary Guidelines for United States Residents was the recommendation to control dietary cholesterol intake to less than 300 mg per day. However, the 2015-2020 Dietary Guidelines for United States Residents, released on January 7, 2016, no longer continues to set a limit on dietary cholesterol intake.
In 1989, China first published the Dietary Guidelines for Chinese Residents, and then revised and published the guidelines in 1997 and 2007, with this year being the third revision of the guidelines. In recent years, the relationship between food, nutrition and health has received more and more attention, and this revision of the dietary guidelines has undoubtedly become the focus of people’s attention. The removal of the 300 mg daily cholesterol limit from the dietary guidelines was a surprise.
What does the removal of this limit in both the US and China mean? Is cholesterol beneficial or harmful? Does it mean that we don’t have to worry about high cholesterol anymore?
The Basics of Cholesterol Metabolism
Let’s start by reacquainting ourselves with cholesterol. Cholesterol is an essential substance in our bodies and is contained and synthesized in almost all human tissues, but in varying amounts. Not only is cholesterol a component of the body, but it also plays a very important role. If the cholesterol level in the blood is low, the walls of blood vessels become weak and easily broken, potentially causing brain hemorrhage. Low cholesterol also has the potential to induce easy nerve impulses. However, high cholesterol levels in the blood increase the chances of cardiovascular disease. The mechanism is that cholesterol in the blood forms plaques in the walls of blood vessels, causing relative narrowing of the vessels. On the one hand, the pressure on the vessel walls rises leading to increased blood pressure; on the other hand, it contributes to the development of atherosclerosis.
There are two sources of cholesterol in the circulation.
First, it is biosynthesized by the liver and peripheral tissues in the body, called endogenous cholesterol, which is the main source of cholesterol in the human body, accounting for about 70% to 80%;
The second is through dietary intake, called exogenous cholesterol, which accounts for about 20-30%.
Controlling and reducing circulating cholesterol levels in the human body has long been considered an important measure to prevent cardiovascular diseases. Therefore, in order to control circulating cholesterol levels, it is logical to start with the 2 main sources of cholesterol, i.e., reducing biosynthesis in the body (i.e., the statin mechanism of action) and controlling dietary intake.
Food cholesterol intake still needs to be moderately limited
The cholesterol synthesis pathway shows that while the main determinant of cholesterol levels in the body is internal biosynthesis, dietary cholesterol intake also has an impact on this. So why do the new dietary guidelines in China and the United States relax the restrictions on cholesterol intake?
First of all, due to genetic and metabolic factors, there is a great deal of individual variation in the absorption of dietary cholesterol and the effect of cholesterol on blood lipids, and over a short period of time, cholesterol intake does not directly reflect blood cholesterol levels;
Secondly, the effect of controlled dietary cholesterol intake on blood cholesterol levels is not very obvious in a short period of time. Moreover, changing dietary habits, such as changing from meat to vegetarian, does not necessarily lower cholesterol levels. What’s more, some people can have abnormal cholesterol even if they are vegetarians for life.
Again, more and more studies show that there is no direct causal relationship between cholesterol levels in food and cholesterol levels in circulation, which suggests that we should not overly restrict or lower cholesterol intake. In fact, the 2013 edition of the Dietary Reference Intakes for Chinese Residents no longer sets upper limits for cholesterol intake. For most people, cholesterol in food is far less influential than the composition of fat in food on fat in the blood.
However, we cannot deny that dietary cholesterol content has an impact on circulating cholesterol levels. The liver is the primary site of cholesterol synthesis, and exogenous cholesterol intake plays an important role in regulating the rate of cholesterol synthesis and catabolic capacity of the liver.
Cholesterol levels are significantly higher in the Chinese population
Before China’s reform and opening up, the Chinese diet was low in cholesterol and other lipid components, and although the liver was synthesizing endogenous cholesterol, the number of patients with coronary heart disease was extremely small, and even pathological specimens of coronary heart disease for medical teaching were difficult to find. It was only after more than 30 years of reform and opening up and the great material abundance of life that a vast, growing and younger coronary heart disease cohort was ushered in due to the significant increase in cholesterol and saturated fat intake in the Chinese diet. From 1984 to 1999, total cholesterol levels in the Beijing population increased by 24%, and coronary heart disease mortality increased exponentially, with elevated cholesterol levels contributing to 77% of the increase in coronary heart disease deaths. In a short period of 15 years, it is unlikely that the genetic make-up of the Chinese population has changed significantly, so it is clear that the increase in population cholesterol levels can only be due to dietary factors.
The Dietary Guidelines for the Population should not be misrepresented
The 2015-2020 Dietary Guidelines for Americans do not deny that excessive cholesterol intake in the diet is harmful. The reason why the guidelines eliminate the upper intake limit is that in recent years, the development of various guidelines in the United States has placed special emphasis on the results of randomized controlled trials (RCTs), with fewer recommendations as long as they are not confirmed by RCTs. Although the guideline developers believed that excessive cholesterol intake could be harmful, the lack of evidence from RCT studies to support an upper limit of 300 mg per day led to the removal of this recommendation. Without a careful reading of the guidelines, many people have been misinformed that there is no need to restrict dietary cholesterol intake, and some even mistakenly believe that cholesterol is not related to atherosclerotic disease.
It is only through careful reading of the guidelines that we can understand the true description of the main points of cholesterol in this new guideline.
(1) The body uses cholesterol in order to maintain structural and physiological functions, and the body can produce enough cholesterol on its own so that humans do not need to obtain it from food.
(2) Strong prospective cohort studies and RCTs have shown that a low-cholesterol structured diet reduces the risk of cardiovascular morbidity.
(3) In general, fatty meats and high-fat dairy products are not only rich in saturated fat but also contain high cholesterol.
(4) The types of foods recommended by the new guidelines limit saturated fat intake, and cholesterol intake has been reduced. As part of a healthy diet, the lower the cholesterol within the food, the better.
(5) The structure of the diet takes into account the interaction of multiple foods, rather than paying attention to only one independent aspect of the food.
Clearly, more research is needed in humans to demonstrate a quantitative-effect relationship between dietary cholesterol content and blood cholesterol levels. There is a lack of appropriate methods to quantify dietary cholesterol intake in dietary guidelines. Although both the U.S. and Chinese guidelines no longer restrict dietary cholesterol intake, it may not be necessary for the general healthy population to specifically restrict cholesterol intake, but long-term high cholesterol intake can certainly increase circulating cholesterol levels, especially in patients who already have hypercholesterolemia and need to control dietary cholesterol intake. In fact, the daily cholesterol intake is limited to 100-300mg in the healthy dietary recipes recommended by the new guidelines; moreover, the limitation on the amount of animal food is also indirectly limiting the cholesterol intake as shown in the balanced dietary pagoda recommended by the guidelines.