Diet-related chronic diseases are considered a serious public health problem (1). The World Health Organization points out that the two main causes of death that have prevailed for more than 15 years in the world are ischemic heart disease and stroke, which have caused 15.3 million deaths (2). Hypercholesterolemia, hypertriglyceridemia, increase in low-density cholesterol (LDL), and a reduction of high-density cholesterol (HDL) are the general parameters of dyslipidemia. They have been related to complications such as cardiac damage and atherosclerosis (3–5). Also, the evidence relates dyslipidemias with heart disease and the risk of stroke (6, 7). The diagnosis of dyslipidemias corresponds to the alteration of one or more of the following parameters: total cholesterol (TC) ≥ 240 mg/dl, LDL ≥ 160 mg/dl, HDL ≤ 40 mg/dl, and triglycerides (TGs) ≥ 200 mg/dl (7). In both animals and humans, several studies have reported the effects of food compounds to improve the signs and symptoms of various chronic diseases, such as diabetes, hypertension, dyslipidemia, and oxidative stress (8–17).

Avocado (Persea americana Mill) is an oleaginous fruit highly distributed and consumed worldwide. This fruit is native to Central America and South America; its domestication as food dates back more than 9,000 years (18). It belongs to the Lauraceae family; the genus Persea includes three species: Persea schiedeana, Persea parvifolia, and P. americana. The fruit of P. americana (common avocado) is made up of the peel, pulp, and seed (19). The color of the shell can be light or dark green or purple; smooth; rough; and also bright or opaque (20).

Avocado is increasingly consumed, especially for its pulp, which, in addition to its flavor, contains a large number of vitamins, minerals, and especially monounsaturated fatty acids, and in lesser amounts polyunsaturated acids. While other components of avocado (peel, seed, and leaf) are usually discarded as waste, a variety of functional compounds from the waste have been identified as polyphenols, organic acids, and flavonoids (21–23). Avocado peel contains a high concentration of bioactive compounds such as phenolic acids, flavonoids, catechins, and procyanidins; nevertheless, the content of compounds will depend on the extraction methods (24, 25). Melgar et al. examined raw material of bone and lyophilized shell and carried out a hydroethanolic extraction (ethanol:water 80/20). The extracts obtained were redissolved in 80% ethanol solution for phenolic characterization (final concentration 40 mg/dl). Twenty-nine phenolic compounds, 14 flavan-3-ols (epi) derived from catechin, nine flavonoids (derivatives of quercetin, kaempferol, and isorhamnetin glucoside), and six phenolic acids such as chlorogenic and coumaric acid derivatives (25) were found.

Functional compounds from avocado waste have shown an antioxidant effect through the reduction of pro-oxidative substances (26–28): hepatoprotective, gastroprotective, lipid-lowering, and hypoglycemic effects (8, 9, 15, 29). Other studies had evaluated some of its effects in vitro on human colon cancer cells (30, 31), on gut health in rats (32), and on hypercholesterolemic rats (33, 34). Some others have been related to cognitive function such as improvement in learning and memory processes in subjects exposed to diets high in fat and sugars (14, 16). Also, there is importance to promote research with ecological vision. The environmental impact is a preoccupying issue, because the consumption of food generates a large amount of waste. According to the World Bank, in 2016, the world generated 2,010 million tons of waste, driven by extremely rapid urbanization and the growth of populations. It is predicted that the world will produce 3.4 billion tons of waste by 2050 (35). Derived from this, it is important to look for some alternatives for the reuse of this type of waste, which can also have a positive impact on health.

For that, the objective of this review was to identify studies that have reported the effect of avocado waste on blood lipids in animal models.

The search of information was based on the following databases: EBSCO, PubMed, Science Direct, and Springer Link. The search terms were “Avocado peel” AND combined with the next words: “lipid”/“rats”/“in vitro”/“triglycerides”/“cholesterol.” “Avocado seed” AND “lipid”/“rats”/“in vitro”/“triglycerides”/“cholesterol.” “Avocado leaf” AND “lipid”/“rats”/“in vitro”/“triglycerides”/“cholesterol.” The search was carried out from January to December 2020; articles published between the years 2000 and 2020 were included.

Only original articles that included investigation of extracts of peel, seed, or leaf avocado and its effects on blood lipid in animal models and in vitro techniques were considered. The articles excluded were those that did not evaluate the effect on blood lipids.

Through the search, we identified a total of 5,898 articles, of which 5,889 were excluded. Articles were discarded due to duplications, reviews, evaluation of different effects, and using another source. From the papers, 4,352 were excluded due to an evaluation of other compounds of avocado and other effects. Also, 1,537 were reviews. Finally, nine articles that addressed the inclusion criteria were selected. Figure 1 shows the flowchart of the election process of the articles included.

Based on the results of the search, a relatively low number of research papers were found in which the objective was to evaluate the effect of avocado waste over blood lipids. There was a study that evaluated the lipid profile-lowering effect of AS flour at a dose of 125–500 mg/kg/day in a hypercholesterolemic animal model reporting major decreases by the highest doses of TC (39.1 mmol/L) and LDL (37.7 mmol/L); the lowest increase of TG was presented in the dose of 125 mg/kg/day (0.078); and at the same dose, an increase (0.60 mmol/L) of HDL (37) was shown. Similar results were found when avocado oil was used in a similar model with a dose of 450–900 mg/kg/day of virgin avocado oil (VAO) and simvastatin with a hypercholesterolemic diet. A reduction of TG and LDL was found in the group with a major concentration of VAO (900 mg/kg) and simvastatin (10 mg/kg). Also, in the doses of 450 mg/kg of VAO, there was found an increment in HDL and a significant reduction of LDL (34). Even though a reduction of TC and LDL and an increase in HDL were identified, but unlike in the effect of VAO, the TGs rose (37). An important difference is the period of time of the study: the first was carried out for 6 days, while the second lasted 28 days. It is important to mention that the value of phytosterol reported on VAO was 34 g/10,034 g, with β-sitosterol as a major phytosterol and small amounts of campesterol, stigmasterol, and α5-avenasterol. While in the AS flour, they identified protocatechuic acid of 128.18 μg/g and in other quantities vanillic acid (28.67 μg/g) and kaempferide (9.63 μg/g), among others. Two other studies were found that used AS: in the first study, TC decreased in healthy animals with doses of 2 and 4% (AS), obtaining a decrease of 0.27 and 0.48 g/L, respectively. In the healthy groups who consumed a diet high in sucrose, TC was reported to be elevated in all doses. The other study used diabetic animals, and in 500 mg/kg/day, a significant reduction of TG, TC, and LDL compared with those of the control group was found; also a significant increase of HDL p < 0.01 was shown, and the values demonstrated the best effect even when compared with the positive control who was treated with medication (39).

The hypertensive animal groups showed a decrease in TC, the most evident being 0.27 g/L in a 4% AS dose (38). Results show a wide difference between studies with the same raw material; this may be due to the fact that both the doses and the intervention time are varied (37–39).

In the case of the animal model with metabolic disorders, it was generally evidenced that the consumption of 200 mg/kg/day of avocado peel improved TG and TC levels; the subjects who only consumed the high-sucrose fat diet showed significant increases in those parameters (45). Other investigations utilized differences based on extracts; three of them utilized similar doses administrated in different periods of time (40, 43, 44). Brai et al. (44) informed a diminution of TC at 82.26 mg/dl in a dose of 100 mg/kg/day, while the highest diminution of TC (24.22 mg/dl) was 20 mg/kg/day. Dzeufiet et al. (43) evidenced a significant reduction of TG and TC dose dependently, showing a greater reduction in the dose of 150 mg/kg/day (TG 205.72 mg/dl and TC 98.97 mg/dl); also, the HDL evidenced an increase of 32.52 mg/dl. Kouamé et al. (40) reported non-significant differences in a reduction of TG in a dose of 100 mg/kg/day in aqueous extract and methanolic extract but an increase in the ethanolic extract. Those researchers proved its respective leaf extract in a different pathological model: with hepatotoxicity, hypertension, and diabetes, with varied temporality of 8, 35, and 28 days, respectively. Brai et al. (42) studied the effect of an aqueous leaf extract and a methanolic extract and found an important reduction of 8% of TC in the first one. Another study (41) was made with a hydroalcoholic leaf extract with 0.30 g/kg/day and showed an increase of TG (24.33 mg/dl) and TC (37.79 mg/dl).

There is an important field of research in the area of organic waste. It becomes increasingly relevant due to concern for the environmental impact of the large volume of waste, in addition to the resources necessary for its degradation.

Organic waste has been shown to be an important source of bioactive compounds that can exert beneficial health effects. It is a fact that in the near future, humans must incorporate organic waste in the alimentation and not as an aliment but also as an ingredient, a complement, and even medicine, not only derived from the amount of functional elements that these wastes contain but also because it is necessary for humans to be environmentally responsible and to seek new ways to reuse at a low environmental cost and high impact on health.

In the present review, it was not possible to evaluate the relationship between waste compounds and biological parameters, since they were mostly not reported. In the same way, the doses, exposure times, and pathological models were varied. Therefore, the study of organic waste, specifically avocado, has a lot of potential, and evidence needs to be able to make more forceful comparisons, especially those that suggest a relationship between dose and effect; it is important to highlight that as far as we know, this is the first review to address this problem.

So we can conclude that more research is needed in this field of knowledge; although phytotherapy is already considered an innovative treatment with an important impact on health, the use of biodegradable waste has not yet been explored and completely studied.

JP-L: conception or design of the work, data acquisition, analysis or interpretation of work data, writing of the work, and final approval of the version for publication. AM-M: conception or design of the work, analysis or interpretation of work data, writing of the work, critical review of the content of the manuscript, and final approval of the version for publication. CV-C: analysis or interpretation of work data, critical review of the content of the manuscript, and final approval of the version for publication. All authors contributed to the article and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.