Ask the Expert with Dr. Walter Willett: Cholesterol

walter_willettSince 1980, The U.S. Department of Health and Human Services (HHS) and the U.S. Department of Agriculture (USDA) have been advising Americans on what to eat for optimal health. Published every 5 years, the Dietary Guidelines are a reflection of the most up-to-date research.

 In December 2014 the Dietary Guidelines Advisory Committee (DGAC) released a preliminary document stating that “cholesterol is not considered a nutrient of concern for overconsumption,” suggesting that dietary cholesterol has little impact upon a person’s actual health status. Though the final report from the DGAC isn’t due until later in 2015, we checked in with Harvard T.H. Chan School of Public Health’s Walter Willett for some clarification on dietary cholesterol.

1. Cholesterol has long been considered a “bad” thing to eat. Even the first edition of the guidelines says avoid foods with too much cholesterol, because “eating extra saturated fat and cholesterol will increase blood cholesterol levels in most people.” How did this recommendation come about?

 The theory that cholesterol is a major factor in heart disease was attractive, but it turns out it was oversimplified. There was no direct evidence to support the link between egg consumption, for example, and heart disease.

2. Does that mean cholesterol-laden foods like cheese, red meat and eggs were wrongly vilified, or should people still limit consumption? Should we still care about cholesterol amounts in our food?

Some of those foods are definitely important to limit, but not simply because they have cholesterol in them. Poultry, for example, has a moderate amount of cholesterol, and so does fish – in fact all animal products do. Poultry and fish seem to be relatively healthy foods, and fish provides essential omega-3 fatty acids. That’s the problem with just looking at something as simple as one dietary factor like cholesterol; it doesn’t tell us the overall health impact of a food, and so we should be focusing more on the foods.

3. Even if dietary cholesterol doesn’t raise blood cholesterol levels, is it possible that other foods – like refined carbohydrates  – might raise cholesterol levels?

 Dietary cholesterol doesn’t raise blood cholesterol levels very much. It’s not that there’s no effect on blood cholesterol levels; there’s a small effect. It can raise both good and bad cholesterol in the blood so that makes it more complicated, and that’s why we need to look at the whole food, not just cholesterol content.

 The other main factor that increases blood cholesterol levels is saturated fat in the diet, and that actually has more of an impact than cholesterol in the diet. Then there are other aspects of the diet that can reduce cholesterol levels, specifically the bad cholesterol levels – for example unsaturated fat reduces blood cholesterol, and fiber can reduce blood cholesterol levels. Refined starches don’t have a major effect on bad cholesterol but they drop the good cholesterol. There are other aspects of diet that affect blood cholesterol, both the good and the bad parts of it.

4. For people who eat egg-substitute products that are both fat- and cholesterol-free, thinking that’s the “healthy” option, would you recommend switching back to real eggs?Slised boiled egg

It probably doesn’t make a huge difference for most people whether they eat eggs with or without the yolks. However, there is a caveat for people with diabetes. We have seen in repeated studies that higher egg consumption does increase risk of heart disease so we suggest that people with diabetes keep egg consumption and cholesterol intake low. Whether there are some other groups that might be more sensitive is still not clear.

5. As we see nutrition messages change over time – for example, fat used to be considered bad before we realized the health benefits of certain types of fats – and now the recommendation on cholesterol is changing, what should the general public take away from these kinds of shifts in dietary recommendations? What would you say in response to the recent New York Times article entitled “The Government’s Bad Diet Advice?”

The important point is to have the best possible evidence, and we shouldn’t be basing dietary guidance on just guesses or beliefs. In the case of both the egg issue and the total fat issue we were basically starting with virtually no direct evidence. When the evidence did start to come in – and there were different lines of evidence from our studies based on large cohorts and also short term studies investigating metabolic changes – it showed that people who consume more eggs did not have a higher risk of heart disease even after adjusting for any other factors, and that total fat in the diet was not related to heart disease risk or cancer risk. So it took those long term studies to show that those were not important factors, and that allowed us to modify the recommendations. We were really in a state 35 years ago in which we had very little direct evidence and we were basing guidelines on guesses and indirect evidence from very small, short term studies.

There are multiple problems with the article in The New York Times about the new Dietary Guidelines report that has done away with low-fat recommendations. It seems as if the author didn’t really read or understand the research that was mentioned. Although the Times story blames epidemiological research for recommendations to increase carbohydrate, the opposite is true. It’s actually largely our work which has led to the change eliminating the restriction on total fat intake. The author then goes on and makes her recommendations that we should be eating more red meat and high-fat dairy products, when there’s no evidence at all to support that, much less randomized trials, to support that conclusion.

One claim of the New York Times article was that we can only rely on clinical trials for information on diet and health. In theory we might like to have clinical trials to answer all such questions, but for issues that involve long term effects of diet that occur over many years and decades – for example, effects on heart disease and cancer – clinical trials have mainly been a failure because keeping people on specific diets over many years is difficult. So the theoretically perfect study will often just not be possible; therefore we have to use a combination of kinds of evidence. For most questions the best evidence will come from a combination of large cohort studies tracking the dietary habits and disease occurrence of participants over many years and small, short term studies in which a small number of participants are fed different diets and intermediate variables like blood pressure and cholesterol fractions are measured. This combination of evidence has been successful in identification of trans fat and soda as risk factors, has exonerated total fat and cholesterol as major risk factors, and also underlies guidelines on body weight and physical activity.