The small-scale study by researchers from Penn State looked at 45 participants aged 21 to 71 with overweight or obesity.

All participants were asked to follow a two-week diet which mimicked the average American diet and gave everyone a similar nutritional start to the study. They were then randomly assigned to a sequence of three diets for five weeks each.

These diets included a low-fat diet, a moderate-fat diet that included one avocado a day, and a moderate-fat diet without avocados, but which was supplemented with extra healthy fats to match the amount of monounsaturated fatty acids found in the avocados.

The findings, published in the Journal of Nutrition, showed that after five weeks on the avocado diet, participants had lower levels of small, dense low-density lipoprotein (LDL) cholesterol particles, also known as "bad" cholesterol, than before starting the study or after completing the low- and moderate-fat diets.

In addition, after following the avocado diet, participants had significantly lower levels of oxidized LDL particles. The researchers explain that just as oxygen can damage food, as when a cut apple turns brown, oxidation processes are also bad for the human body.

Participants also had higher levels of the antioxidant lutein after following the avocado diet.

"When you think about bad cholesterol, it comes packaged in LDL particles, which vary in size," said study author Penny Kris-Etherton. "All LDL is bad, but small, dense LDL is particularly bad. A key finding was that people on the avocado diet had fewer oxidized LDL particles. They also had more lutein, which may be the bioactive that's protecting the LDL from being oxidized."

"A lot of research points to oxidation being the basis for conditions like cancer and heart disease," Kris-Etherton continued. "We know that when LDL particles become oxidized, that starts a chain reaction that can promote atherosclerosis, which is the build-up of plaque in the artery wall. Oxidation is not good, so if you can help protect the body through the foods that you eat, that could be very beneficial."

"Consequently, people should consider adding avocados to their diet in a healthy way, like on whole-wheat toast or as a veggie dip."

However, Kris-Etherton added that more research is still needed.

"Nutrition research on avocados is a relatively new area of study, so I think we're at the tip of the iceberg for learning about their health benefits," she noted. "Avocados are really high in healthy fats, carotenoids-which are important for eye health-and other nutrients. They are such a nutrient-dense package, and I think we're just beginning to learn about how they can improve health."