There aren’t many well-done nutrition studies. Many studies are observational and rely on questionnaires to figure out what people eat. There are very few randomized trials because these are hard to carry out. They take time, cost a lot of money, and the logistical demands are enormous. Even major studies can have problems, as we discovered in the 2013 PREDIMED study, which (apparently) demonstrated a reduced risk of cardiovascular disease in people who followed a Mediterranean diet. The study was retracted, as it was shown that patients were not properly randomized and a corrected version was later published.
That is why it is always satisfying to find a nutritional study that is well done. One that particularly interested me was a study published in the Journal of the American Heart Association that looked at whether incorporating avocados in the diet could lower blood cholesterol. The question is an interesting one since elevated cholesterol is very common and lowering it is undeniably beneficial for the prevention of cardiovascular disease. As I have written previously, in the mainstream medical community there is no cholesterol controversy. So, a relatively easy dietary intervention to lower cholesterol would be very welcome.
The researchers enlisted 45 obese individuals and fed them three different diets in sequence for 5 weeks each, with 2-week wash out periods between diets, presumably to give people a break. One diet was low-fat, one was moderately low-fat, and one was moderately low-fat with the addition of a daily avocado.
Here is what happened. No one lost any weight, a revelation that unfortunately got buried towards the bottom of the paper. None of the diets resulted in any improvement in diabetes or blood pressure. They did, however, lower cholesterol. And they lowered not just LDL, often called the “bad cholesterol,” but they also lowered Apolipoprotein B, a protein that is also a marker for cardiovascular risk. But while all the diets lowered patients’ cholesterol, it was the fact the avocados lowered cholesterol even more that garnered the most attention.
At the start of the study, participants had a “bad cholesterol” (LDL) level of 3.31 mmol/L. With the low-fat and moderately low-fat diets, LDL decreased to 3.08 mmol/L. But in the avocado group, LDL cholesterol fell to 2.94 mmol/L. The difference in LDLbetween those who ate avocados and those who did not was about 0.14mmol/L. If that seems like a small number, it is. Indeed, such a small difference is largely meaningless.
The point here is that there is a difference between statistical significance and clinical significance. When researchers talk about statistical significance, they mean that they proved a measurement was not due to chance or random error. There is always some degree of uncertainty regarding any measurement in science. For example, a cholesterol level of 3.08 could easily have been 3.07 or 3.09. Proper statistical analysis can determine if the uncertainty surrounding a measurement is small enough so that it does not invalidate the results. In this study, the uncertainty was small enough to allow the researches to be confident that their results were real and not due to random statistical noise. But just because results are real, doesn’t mean that they are meaningful.
While the result of the study was statistically significant, it was not clinically significant. It is doubtful that a 0.14-point change in cholesterol will have an effect on anyone’s health. In medicine, when we treat cardiac patients, we generally look for changes 10 or 20 times greater than that, so the cardiovascular benefit of a 0.14-point drop in LDL is going to be negligible.
The study was also partially funded by the Hass Avocado Board (yes there is such a thing as an avocado lobby), which makes you wonder not only about the potential for bias, but also about why we live in a world where we apparently need avocado lobbyists.
In the end, avocados do lower cholesterol, but only by a vanishingly small, clinically irrelevant amount. When the study first came out, people asked me if it was true that avocados lowered cholesterol. It was, but remember that just because a study is well-done, doesn’t mean its results should be taken to heart.