Avocados contain plenty of MUFAs, most of which is oleic acid. (Photo by Muffet)
Everyone knows avocados are high in fat, but does that make them healthy or unhealthy? In this post, we'll look at how avocados affect cholesterol levels.
Avocados are technically large berries of the avocado tree, each berry containing a single seed usually called the avocado stone. There are several different cultivars, but on average avocados contain about 15 grams of fat, 9 grams of carbohydrates and 2 grams of protein per 100 grams. Most of the fat (~10 grams) is monounsaturated, while the rest is roughly half saturated and half polyunsaturated.
Most health enthusiasts are either pro-saturated fatty acids (SAs) or pro-polyunsatured fatty acids (PUFAs) – and if you're a long-time reader of this blog, you already know which category I lean towards. Monounsatured fatty acids (MUFAs), however, seem to represent something of a "neutral" group of fats to many. Olive oil, for example, is high in MUFAs, and almost all studies find it either beneficial or at least neutral: generally, LDL decreases and HDL either increases or stays the same.
So if olive oil is good for you, what about avocados? Since both are high in MUFAs, specifically oleic acid, one might expect to see similar results. In rats, adding avocados to their diet seems to increase HDL and decrease triglycerides (link). Avocado leaf extracts appear to be especially effective (link).
Avocados, cholesterol & healthy subjects
Experiments on humans, unfortunately, are not always as unequivocal. The earliest human study I could find compared an avocado-enriched diet with a diet high in complex carbohydrates and low in fat – namely, the lipid-lowering diet advocated by the American Heart Association (link). The study included 15 women who were randomly assigned to one of the diets for three weeks, followed by 3 weeks on the other diet. Only the avocado diet, on which the women ate between half and one and a half avocados per day, resulted in a statistically significant decrease (~8%) in total cholesterol levels. This was due to a lowering of LDL without affecting HDL, whereas the complex carbohydrate diet lowered HDL levels by ~14%. So much for the heart-healthy effects of low-fat diets.
The second study included 16 healthy volunteers who were fed three different diets for 2 weeks: a high-MUFA diet consisting of 30% fat (75% of which came from avocados), a free diet including avocados, and a low-saturated fat without avocados (link). Both the high-MUFA diet and the low-SA diet reduced total cholesterol and LDL. However, the low-SA diet also also reduced HDL and increased triglycerides, while the other two diets reduced triglycerides. Again, the low-fat diet with an emphasis on limiting saturated fats was the most harmful for cholesterol levels.
I find it somewhat surprising that the free diet with avocados apparently also reduced HDL, even though the authors say the volunteers ate the same amount of avocados as during the high-MUFA diet. Unfortunately I don't have access to the full paper, so I'm not sure what the free diets were like in reality. Perhaps the volunteers simply ate more during the free diet, which could have skewed the results, or maybe the SA/PUFA ratio was significantly lower on the free diet for some reason.
Avocados and people with high cholesterol
Another study included 13 patients with high LDL cholesterol (link). The patients were given a standard vegetarian diet, a vegetarian diet enriched with avocado or a free diet that included avocados. The standard vegetarian diet consisted of 70% carbs, 20% fat and 10% protein, while the vegetarian avocado diet was 60% carbs, 30% fat and 10% protein. The vegetarian avocado diet reduced LDL, whereas the free diet increased it slightly. Only the standard vegetarian diet significantly reduced triglycerides – however, it also reduced HDL more than the other two.
The results of this study seem to contradict the two earlier studies, since simply adding avocados to the diet resulted in slightly lower HDL and slightly higher LDL – in other words, their cholesterol levels worsened. Genetics may play a role here, as some individuals who are predisposed to higher LDL levels seem to react negatively to foods that generally improve the cholesterol ratio. The authors themselves state:
"Low-fat, carbohydrate-rich vegetarian diets may be harmful to hypercholesterolemic patients. The avocado addition to a vegetarian diet does not correct these undesirable effects. To obtain beneficial effects on lipid profile with avocado, lower amounts of carbohydrates and polyunsaturated fatty acids are probably needed."
Yet another study compared the effect of an avocado-enriched diet on healthy subjects and patients with slightly elevated cholesterol levels (link). In healthy participants, total cholesterol decreased by 16% on the avocado diet. In participants with high cholesterol, total cholesterol decreased similarly, with LDL and triglycerides decreasing by 22% and HDL increasing by 11%. The authors conclude that a high-MUFA diet containing avocados improves lipid profile in healthy and especially in mildly hypercholesterolemic people.
Compared to low-fat diets, diets containing moderate to high amounts of monounsaturated fatty acids from avocados seem to result in better cholesterol levels. In healthy people, replacing carbohydrates with avocados generally lowers LDL without affecting HDL, similar to olive oil. In people with high cholesterol, replacing carbohydrates with avocados appears to reduce LDL and, in some cases, increase HDL.
The controversial result is the lower HDL in free diets with avocados. Does simply adding an avocado to your diet actually make cholesterol levels worse? This is a tricky question, since adding an avocado would mean an increase in total energy intake, unless it also means you eat less of something else – which would be the case, unless avocados somehow increase appetite. One possibility is that when the participants added avocados (and thus MUFAs) to their diet, they reduced their consumption of other fatty acids while keeping total energy intake the same. Reducing saturated fatty acid intake could result in lower HDL, although this doesn't necessarily explain the higher LDL. Without knowing what the participants actually ate during their free diet periods, it's difficult to say what the cause is.
While it's generally taken for granted that a) olive oil reduces LDL and has a neutral or positive effect on HDL and b) this effect is due to the high MUFA content of olive oil, there are differences in the food sources of MUFAs. Both avocados and olive oil are high in oleic acid, but olive oil contains squalene, whereas avocados do not. Squalene is a precursor in cholesterol synthesis and is metabolized to cholesterol in the body. Avocados, on the other hand, contain beta-sitosterol, which lowers LDL.
Finally, genes play a major role in cholesterol levels. Apolipoprotein E genotype affects how individuals react to dietary fatty acids and also cholesterol-lowering drugs like statins. Without knowing the genotypes of the participants in the studies, it's hard to say how generalizable the results are.
For more information on diet and cholesterol, see these posts:
Want to Increase Your HDL Cholesterol by 50%? Sage Tea May Be the Answer
High HDL Cholesterol Reduces Risk of Dying in Men
Hibiscus Tea Increases HDL, Lowers LDL and Triglycerides
The Twinkie Diet: Thoughts on Weight Loss and Cholesterol