A new review of studies looking at the health effects of avocados finds that there is "satisfactory clinical evidence" that the fruit can help to treat metabolic syndrome.

Metabolic syndrome is defined as a cluster of risk factors that can raise the risk of other health conditions, such as type 2 diabetes, heart disease, and stroke.

Risk factors include abdominal obesity, low levels of high-density lipoprotein (HDL) cholesterol - or "good" cholesterol - high triglyceride levels, high blood pressure, and high fasting blood sugar.

The presence of at least three of these risk factors warrants a diagnosis of metabolic syndrome.

According to the American Heart Association, metabolic syndrome affects around 23 percent of adults in the United States.

Adopting a healthful diet is considered one of the best ways to prevent or treat metabolic syndrome. The new review - recently published in the journal Phytotherapy Research - suggests that avocados should form a part of this diet.

Avocados are a fruit from the avocado tree, or Persea Americana, which is native to Mexico and Central and South America.

A number of studies have documented the possible health benefits of avocado. A study reported by Medical News Today in 2014, for example, found that eating half an avocado with lunch may aid weight loss, while more recent research linked the fruit to reduced levels of low-density lipoprotein (LDL) cholesterol, known as "bad" cholesterol.

These benefits have been attributed to the bioactive components of avocados, which include carotenoids, fatty acids, minerals such as calcium, iron, and zinc, and vitamins A, B, C, and E.

For their review, co-author Hossein Hosseinzadeh, of Mashhad University of Medical Sciences in Iran, and colleagues set out to determine how these components might help to combat the risk factors of metabolic syndrome.

Avocado has the strongest effect on cholesterol levels

To reach their findings, the researchers analyzed the results of various in vivo, in vitro, and clinical studies that investigated the effects of avocado on metabolic health.

Hosseinzadeh and colleagues found that the fruit has the strongest impact on lipid levels - that is, levels of HDL cholesterol, LDL cholesterol, total cholesterol, and triglycerides.

As an example, the team points to one study of 67 adults, of whom 30 had a healthy lipid profile and 37 had mild hypercholesterolemia. After adhering to an avocado-enriched diet for 1 week, both groups showed significant reductions in total and LDL-cholesterol and triglyceride levels.

"The reported mechanism of this effect was regulating of the hydrolysis of certain lipoproteins and their selective uptake and metabolism by different tissues such as liver and pancreas," explain the authors.

"Another possible mechanism could be related to the marked proliferation of the liver smooth endoplasmic reticulum which is known to be associated with induction of enzymes involved in lipid biosynthesis."

An 'herbal dietary supplement' to help treat metabolic syndrome

The review also uncovered evidence that avocado is beneficial for weight loss. The researchers cite one study that found overweight or obese adults who ate one avocado every day for 6 weeks experienced significant decreases in body weight, body mass index (BMI), and the percentage of body fat.

Additionally, the team identified a number of studies associating avocado intake with reductions in blood pressure among patients with hypertension, and evidence suggests that the fruit might also help to reduce atherosclerosis - the narrowing or hardening of arteries caused by a buildup of plaque.

Notably, Hosseinzadeh and colleagues found that it is not just the flesh of the avocado that can benefit metabolic health - the peel, seed, and leaves of the fruit may also help.

One study published in 2014, for example, found that a daily dose of oil extracted from avocado leaves led to reductions in total and LDL cholesterol and blood pressure.

Overall, the researchers conclude that avocado may be effective for the treatment of risk factors associated with metabolic syndrome, though further research is warranted. They write:

"In this review article, satisfactory clinical evidence suggested that avocado can be used as herbal dietary supplements for treatment of different components of [metabolic syndrome].

"Although avocado like other herbal products is safe and generally better tolerated than synthetic medications, there is limited scientific evidence to evaluate different side effects because of contaminants or interactions with drugs. Besides, further studies need to be accomplished on the metabolic effects of different parts of avocado for other possible mechanisms."