HDL

Any change in diet inevitably affects our body chemistry. Many people make the change to a plant-based diet expressly to improve their blood lipid levels and indeed, LDL-cholesterol is usually greatly reduced by this alteration in food intake. However, some other aspects of blood lipids may change in ways that may at first glance appear to be negative. Two examples of this are HDL-cholesterol levels and triglyceride levels. In this article we’ll focus on the type of fat known as HDL, the “good” cholesterol.

HDL (high density lipoprotein) is only one cholesterol component of our total blood cholesterol. Others are LDL (low-density lipoprotein) and VLDL (very low-density lipoprotein). HDL-cholesterol has long been considered “good” cholesterol because it is associated with less heart disease. HDL can act as a scavenger, picking up LDL-cholesterol and taking it out of the arteries and back to the liver where it is broken down and excreted through the intestines. HDL also has the ability to act as an antioxidant and reduce inflammation. Research is ongoing regarding the actual extent of this effect in human beings (1,2).

In recent years this topic has turned into something of a conundrum. Drugs that raise HDL have not resulted in lower cardiovascular disease risk. Statins for instance reduce LDL and increase HDL but the increase in HDL does not seem to correlate with reduced heart disease, especially when LDL is sufficiently lowered (3,4). The class of drugs known as fibrates, one example of which is niacin, can raise HDL by 10 to 15% but studies have not been able to show any reduction of cardiovascular-caused death by niacin alone (3,5). Torcetrapib, which had the ability to raise HDL by 72% and was hoped to be the answer to lipid dysfunction, was found to result in increased risk of cardiac events and death. Upon this finding studies on the drug were immediately terminated and its development was halted. (6)

Increasingly evidence is revealing that two variables seem to be important when it comes to the effect of HDL on cardiovascular disease risk. First of all it appears that it is the quality of HDL particles that is important and not its quantity. Secondly, studies have shown that very high HDL levels actually increase the risk of premature death from cardiovascular disease to the same extent as do low HDL levels. Let’s look more closely at these aspects.

Simply the presence of HDL particles in the bloodstream does not guarantee that they are in good working order. In other words, even if HDL levels are in the healthiest range, the HDL particles may not be functioning properly and consequently will not result in a significant beneficial effect on heart disease. Research has shown that, in a state of chronic inflammation such as that found in atherosclerosis, diabetes, kidney disease or arthritis, the ability of HDL to clean LDL out of arteries is interrupted. HDL particles in such inflammatory environments not only lose their antioxidant capabilities, they actually begin to support the oxidation of cholesterol, adding to the inflammatory process (3,7,8). The presence of other types of cholesterol in the bloodstream can also have a detrimental effect. A 2016 study illustrated that HDL-cholesterol may not protect against heart disease if LDL-cholesterol and triglycerides are high (9).

Adding to the riddle are new studies that demonstrate that very high HDL blood levels actually increase the risk of premature death, not reduce it as has been generally accepted. In August, 2017 a study was published in which the health and HDL-levels of 52268 men and 64240 women were monitored over a period of up to twenty years. This investigation revealed that the association between HDL-cholesterol concentration and early death from all causes was U-shaped in both men and women. In other words both very high and very low HDL-cholesterol concentrations are linked to a high rate of premature death. This investigation observed that the concentration of HDL-cholesterol associated with the lowest all-cause mortality was 1.9 mmol/L in men and 2.4 mmol/L in women. Researchers stated that “the common belief that the higher the concentration of HDL-cholesterol the better does not hold for extreme high concentrations.” (10). Additionally a 2016 genetic analysis of over 300,000 people looking at a gene variant that is associated with elevated HDL-cholesterol levels found that those with the variant had significantly higher risk of heart disease (11).

Happily there is good news to be found here. Though people eating a healthier diet such as a plant-based diet tend to have lower than average HDL levels, they also have lower rates of heart disease.
Healthy diets do lower blood lipid levels. A 2017 review of 49 observational and interventional studies illustrated that vegetarian diets lower total cholesterol, LDL and HDL levels compared to omnivorous diets. Vegan diets show the greatest lowering effect (12).
Healthy diets do result in lower risk factors for heart disease. A 2013 study looked at the consequences of a low-fat, plant-based lifestyle on HDL levels in the bloodstream. Over 5000 people participating in a Complete Health Improvement Program (CHIP) were examined for the effect of the lifestyle change on body mass index (BMI), blood pressure, lipid profile and fasting plasma glucose. The CHIP program uses a low-fat plant-based diet with the ultimate goal of no animal protein consumption. Results after only thirty days on the program showed a 3.2% decrease in BMI, a 5.2% decrease in both systolic and diastolic blood pressure, a 7.7% decrease in triglycerides, an 11.1% decrease in total cholesterol, a 13% decrease in LDL-cholesterol, and an 8.7% decrease in HDL levels. Interestingly, 323 participants who were classified as having metabolic syndrome at the start of the study were all completely free of the syndrome after thirty days of the new lifestyle. Conclusions from the CHIP study stated that even though a low-fat plant-based diet may decrease HDL levels, other indicators of cardiovascular risk improve and rates of heart disease are lowered (13).

A note on metabolic syndrome (14);
Metabolic syndrome greatly increases the risk of chronic diseases such as diabetes, cardiovascular disease, chronic kidney disease and dyslipidemia.
This condition is diagnosed in a person that displays at least three of the following symptoms :
High blood pressure (130/85 or over or receiving medication for high blood pressure)
High blood glucose levels (5.6 mmol/L or over or receiving medication for diabetes)
High triglycerides (1.7 mmol/L or over or receiving medication to reduce triglycerides)
Low HDL-Cholesterol (less than 1 mmol/L in men or 1.3 mmol/L in women)
Large waist circumference (more than 102 cm in men or 88 cm in women)

A very recent study from May, 2018 consisted of a systematic review and meta-analysis of controlled trials on the effect of a Portfolio plant-based dietary pattern on cardiovascular disease prevention, LDL-cholesterol level and other established risk factors for cardiovascular and metabolic disease. The NCEP (National Cholesterol Education Program) Step II diet is a commonly used diet plan in the USA and other countries for the lowering of cholesterol. It allows 30% of calories or less from fat, less than 7% of calories from saturated fat and less than 200 mg of cholesterol daily. The Portfolio diet uses the same standards as the NCEP Step II diet with the additional criteria of including only plant protein and emphasizing viscous fiber, plant sterols and nuts. Investigators found that, compared to an NCEP Step II diet alone, the Portfolio diet reduced LDL-cholesterol by 17% and also reduced total cholesterol, non-HDL-cholesterol, apolipoprotein B, triglycerides, systolic and diastolic blood pressure, C-reactive protein (a measure of inflammation) and estimated 10-year coronary heart disease risk. This review found no effect from the Portfolio diet on HDL-cholesterol levels or body weight (15).

Even more recently, a June 2018 study conducted a systematic review and meta-analysis of randomized controlled trials to summarize the evidence for the effects of a vegetarian diet on blood sugar control and cardiovascular risk factors in diabetics. Results showed that vegetarian dietary patterns significantly lower fasting blood glucose, HbA1c (a measurement of long-term blood glucose levels), LDL-cholesterol, non-HDL-cholesterol, body weight, BMI and waist circumference (16). Hana Kahleova, one of the authors of this study pointed out that “The link between diabetes and cardiovascular disease is strong. Sixty to seventy percent of people who have type-2 diabetes die of heart disease. The good news is that this study shows that the same simple prescription–eating a plant-based diet–can reduce our risk for heart problems and improve type 2 diabetes at the same time.” (17)

Additionally a 2016 study looked closely at HDL levels and early death. This observational study included 631,762 participants and discovered that lower HDL-cholesterol levels were more likely to occur in individuals of low income, unhealthy lifestyle and with other cardiac risk factors and medical problems. In these situations low HDL also showed some association with higher risk of death from cancer and other causes. The researchers concluded that low HDL-cholesterol is not the cause of these health problems but only a symptom. The underlying cause of the low HDL-cholesterol itself must be established in order to determine the real reason for premature mortality (18).

In conclusion, the take home messages are clear.

1. A drop in HDL after a change to a plant-based lifestyle is to be expected and should not be a source of worry.
2. The presence of chronic diseases which introduce inflammation into the body can result in HDL in poor working order and little to no benefit for cardiovascular disease.
3. There is a sweet spot in HDL-cholesterol levels which is about 1.9 mmol/L in men and 2.4 mmol/L in women. As these levels change significantly in either direction, the benefits from HDL lessen. Extreme low HDL levels (less than 1 mmol/L) or extreme high HDL levels (beginning at 2.5 to 2.99 mmol/L in men and 3 to 3.45 mmol/L in women) not only show no health benefits but actually become risk factors for increased heart disease and increased death from all causes (10).
4. Moving towards a completely plant-based diet is the best way to lower harmful lipids in the blood. There is an immense amount of evidence available for the link between both high total cholesterol blood levels and high LDL-cholesterol blood levels and increased risk of early death. Numerous prospective epidemiological cohort studies have demonstrated a continuous, log-linear relationship between increasing levels of plasma cholesterol and increasing risk of coronary heart disease (19,21). In fact, for every 1% reduction in cholesterol there is a 2% reduction in the risk of heart disease (20).

Think about it. Reduce your cholesterol by a third and reduce your risk of a heart attack by two-thirds. Certainly a big step in the right direction.

 

SOURCES:

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2 Vergeer, M., Holleboom, A.G., Kastelein, J.J.P., Kuivenhoven, J.A. The HDL hypothesis: does high-density lipoprotein protect from atherosclerosis? J Lipid Res. 2010 Aug; 51(8): 2058–2073.
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