Avocados have had their fair share of negative PR. From legitimate concerns over their environmental impact to more spurious claims that they prevent millenials from getting on the housing ladder, the rich, creamy berry also known as the “alligator pear” has been widely maligned in the press. Being a fruit unusually high in fat (albeit "healthier" monounsaturated fat), avocados have even been accused of promoting weight gain.

A new large trial, the results of which were recently published in the Journal of the American Heart Association (JAHA), suggests the contrary: eating an avocado every day for 6 months does not cause weight gain and, far from being unhealthy, may slightly improve your cholesterol levels.

What did the study set out to find?

The study, called the Habitual Diet and Avocado Trial (HAT), followed a group of 1,008 people, all of whom had a high waist circumference (defined as ≧ 35 inches in men and ≧ 40 inches in women). “But what’s the significance of having a larger waistline?” I hear you ask.  

Waist circumference, it turns out, is a decent marker of how much visceral fat - the type of fat that sits around internal organs - a person carries. As explained in the Sex hormones and visceral fat article, carrying an excessive amount of visceral fat tissue (reflected by a larger waist circumference) is harmful to our health. This type of fat, which sits deep in the abdomen around organs such as the liver, intestines, and pancreas, secretes inflammatory molecules that damage our cardiovascular system, worsen our control of blood sugar levels, and increase our risk of diseases such as Type II diabetes, heart disease, and breast cancer.

The aim of the HAT trial was to figure out if eating an avocado a day could help people shed visceral fat, lower their waist circumference, and improve markers of cardiovascular health such as blood pressure and blood lipid (fat and cholesterol) levels.

Avocados were not chosen by the researchers at random or because they’re trendy on toast.* Rather, previous studies have hinted that including avocados in your diet may help weight loss and reduce the risk of metabolic syndrome (a cluster of high blood pressure, high blood sugar levels, unhealthy blood lipid levels, and large waist circumference). For example, the National Health and Nutrition Examination Survey (NHANES), which monitored about 5,000 individuals’ diets from 2001-2012, found that people who ate avocados tended to weigh 3.4 kg less and had a 1.2 cm smaller waist circumference than those who eschewed the fleshy, Mexican fruit.

There’s good biological reasons to suspect that avocados are good for you too. Avocados are particularly rich in oleic acid, a monounsaturated fatty acid (MUFA) that is also the main fat in olive oil. High intakes of MUFAs such as oleic acid have been shown to reduce levels of ‘bad’ LDL cholesterol and cut the risk of cardiovascular disease. In fact, the World Health Organisation (WHO), American Heart Association (AHA), and the Dietary Guidelines for Americans (2020-2025) all suggest replacing saturated** and trans fats with MUFAs (and polyunsaturated fatty acids (PUFAs) too) to limit your risk of cardiovascular diseases such as heart attack and stroke.

Augmenting this, avocados may also enhance the absorption of beneficial phytonutrients from other fruits and vegetables, such as lycopene (the antioxidant that gives tomatoes their red hue) and beta-carotene (the carotenoid in carrots which, as we’ve explained before, is used to make Vitamin A). One this note, one study found that sprucing up your Old El Paso Thick ‘N’ Chunky salsa*** with avocados can quadruple the absorption of lycopene from the tomatoes.

So, with the far-from-wacky hypothesis that eating avocados could reduce visceral fat, improve blood cholesterol, and ameliorate other measures of cardiovascular risk (e.g. blood pressure), the researchers set out to test their hypothesis.

* Although it’s worth pointing out here that the HAT Trial was funded by the Hass Avocado Board, who also supplied the avocados. This institution did not, however, have any access to the study data.

** And yes, we’re aware of the controversy over whether saturated fat actually drives cardiovascular disease.

*** This was actually the brand of salsa used in the study and I wasn’t paid in tacos by Ol’ El Paso himself in return for product placement. Other brands of salsa are available.

How was the study conducted?

The Habitual Diet and Avocado Trial (HAT) took the form of a randomised, controlled, interventional study (also known as a clinical trial). Let’s take some time to explain what each of these terms mean (albeit in reverse order). (Note: you can probably gloss over this section if you know the basics of randomised controlled trial methodology).

An interventional study (or clinical trial) is one that tests out a new intervention, be that a new drug, supplement, psychological therapy, exercise program or, in the case of the HAT trial, an additional avocado to your daily diet. Interventional studies can be contrasted with observational studies, whereby researchers do not introduce an intervention, but merely look for associations between “exposures” (say, amount of avocados eaten) and outcomes of interest (say, incidence of heart disease).

So, the researchers in the HAT trial decided to introduce an intervention - they gave subjects one avocado per day for a period of 6 months. Subjects receiving avocados are formally termed as being in an ‘intervention group’ - which in this trial was called the Avocado Supplemented Diet group. In addition to avocados, handy instructions on how to properly ripen, cut, and peel them (avoiding the perils of the infamous “avocado hand”) as well as suggested recipes were given to the subjects, who were otherwise told to follow their usual diet.

Of course, as the budding scientist in you will no doubt enquire, “How do we tease out the specific effect of avocados in someone’s diet?” This is where the second term “controlled” comes in. A controlled study means that some subjects are assigned to a control group who do not receive the intervention. People in the control group may instead receive a placebo (e.g. a sugar pill or dummy drug), another standard treatment whose effect is already known (e.g. another drug, standard diet), or nothing at all.

In the HAT trial, half the subjects were instructed to just follow their usual diet and not consume an avocado each day. This control group was called the habitual diet group. By including this control group, the researchers could more confidently say whether any health effects seen in the Avocado Supplemented Diet group were indeed caused by avocados themselves and not by some other factor. Furthermore, subjects in both avocado and control groups were selected on the proviso that they ate 2 or fewer avocados per month, allowing researchers to more accurately assess the impact of adding avocados to someone’s regular diet.*

But, the budding scientist in you once more retorts, “Surely everybody’s different, how can we be sure that subjects in both the avocado and habitual diet groups are broadly similar? Perhaps another factor, say age, blood cholesterol level, or consumption of apples, varied significantly between the groups - any health effects attributed to avocados may then, in reality, be caused by this other factor.”

These other factors, such as age, cholesterol level and consumption of apples, are examples of what are known as confounding variables or confounders - other hidden variables that can impact our outcome of interest. Confounders can distort the true relationship between our intervention (in this case, avocados) and health outcome (in this case, changes in amount of visceral fat). How should we deal with these confounded nuisances?

The truth is that we don’t necessarily know what factors will turn out to be important for a health outcome and nor can we control for every potential confounding variable. Does listening to Led Zeppelin, for example, unduly influence whether a person loses weight when eating avocados? If so, are the researchers supposed to ensure both the intervention and control groups listen to Stairway to Heaven the same amount of times each day?

One strategy to counteract the problem of known and unknown confounders is simply to ensure they’re likely to be equally balanced across the intervention and control groups. If age, ethnicity, or listening to Led Zeppelin do in reality affect weight loss when eating avocados, we can minimise the impact of these confounders by ensuring that both the avocado-eating intervention group and habitual diet control group contain roughly the same age distributions, ethnic make-up, and number of Led Zeppelin fans.

And this leads us (finally!) to the third term to be described: “randomised”. Randomisation simply involves randomly assigning subjects to either receive the intervention or be in the control group. In the HAT trial, for instance, subjects had an equal chance of being randomly allocated to the avocado group or the habitual diet control group. By randomising subjects in this way, the researchers could hopefully balance known and unknown confounders across the groups. As can be seen in Table 1 below, both the Avocado Supplemented Diet and Habitual Diet control groups had broadly similar baseline characteristics (with roughly equal distributions of age, sex, cholesterol levels etc.).

Source: Lichtenstein, A. H., Kris‐Etherton, P. M., Petersen, K. S., Matthan, N. R., Barnes, S., Vitolins, M. Z., ... & Reboussin, D. M. (2022). Effect of Incorporating 1 Avocado Per Day Versus Habitual Diet on Visceral Adiposity: A Randomized Trial. Journal of the American Heart Association, e025657.

Naturally, there is still a chance that one group, say the avocado group, ends up with older subjects, more Asian subjects, or more die-hard 1970s British rock fans, but, crucially, these differences will be due to chance and not due to systematic bias on behalf of the researchers. In this respect, one of the benefits of randomisation is that it limits selection bias - it prevents researchers from advertently or inadvertently selecting people with particular characteristics into the intervention or control groups.

To give a concrete example: imagine if the researchers of the HAT trial knowingly allocated more men to the avocado group, and less men to the control group. We know that men, despite having a lower overall body fat percentage, tend to carry more visceral fat than women. If we then see greater absolute reductions in visceral fat in the avocado group, this may simply be due to the avocado group having more men rather than the fat-busting effects of avocados, per se. Randomisation would prevent this from happening, as men and women would have an equal chance of being allocated to either the avocado or control group.

* Presumably, due to a law of diminishing marginal returns, someone going from eating five to six avocados a day, as well as continuing to be unable to get on the property ladder, would derive less health benefits from eating an additional avocado than someone going from 0 to 1 avocado a day.

What outcomes did the study look at?

Primary outcomes

Recall that the HAT trial principally wanted to address the question, “Does eating avocados reduce the amount of visceral fat a person carries?” This naturally raises further methodological questions, one of which is, “How should we measure changes in amounts of visceral fat?”

Although getting a tape measure and putting it around your belly is a quick and easy way to gauge how much visceral fat you carry, a more accurate, albeit less accessible, way would be to use an MRI (Magnetic Resonance Imaging) scanner and directly assess the volume of visceral fat tissue deep in your abdomen.

This is exactly what the researchers did. Subjects were placed into an MRI scanner on two separate occasions: once before the trial, and then again 6 months later, having either eaten an avocado every day (the intervention group) or stuck to their usual diet (control group). Using some clever calculations, researchers were then able to measure the volume of visceral fat tissue (or, to use the more technical term, visceral adipose tissue (VAT)) in subjects’ abdomens.

If the researchers’ hypothesis was correct i.e. if eating avocados does indeed cause a loss of visceral fat, we would expect to see a much greater drop in the VAT volume after 6 months in the avocado group compared to the control group.

Secondary outcomes

Making full use of access to an MRI scanner, the researchers also looked for collections of fat in the liver to calculate something known as hepatic fat fraction. As with visceral fat around your internal organs, excessive fat stored in your liver (reflected by a higher hepatic fat fraction) is unhealthy and promotes inflammation.  

Various blood tests were also taken before the trial, and at 3 and 6 months into the trial, to track changes in well-established risk factors for cardiometabolic disease - a catch-all term for a group of conditions with related underlying causes, such as cardiovascular diseases (e.g. heart attack and stroke), Type II diabetes, and metabolic syndrome. Blood tests analysed in the HAT study included: total cholesterol, LDL (“bad”) cholesterol, HDL (“good”) cholesterol, triglycerides, hsCRP (an inflammatory marker secreted by visceral fat), glucose, and insulin levels.

Again, if the researchers’ hypothesis was correct, that is if eating avocados does actually ameliorate cardiometabolic health, people consuming avocados would show significantly greater improvement in parameters such as cholesterol, blood glucose etc. compared to those in the habitual diet control group.

What did the study find?

If you’re a glass half-empty person, the biggest takeaway from the HAT trial was that eating one avocado per day for 6 months does not help people get rid of visceral fat.

At the beginning of the study, subjects in both the avocado and control groups had a similar average (mean) visceral adipose tissue (VAT) volume of 3.2 L (litres). By the end of 6 months, both groups accumulated a trivial amount of visceral fat: 0.0074 L in the avocado group and 0.0057 L in the habitual diet control group. (This is shown in the first row of data in Table 3. below).

Importantly, there was no statistically significant difference in VAT volume between the groups at the end of the study, suggesting that a daily avocado does not cause any meaningful change in visceral fat. If you’re looking for a magic bullet superfood that will cut visceral fat, the avocado certainly isn’t it.

Source: Lichtenstein, A. H., Kris‐Etherton, P. M., Petersen, K. S., Matthan, N. R., Barnes, S., Vitolins, M. Z., ... & Reboussin, D. M. (2022). Effect of Incorporating 1 Avocado Per Day Versus Habitual Diet on Visceral Adiposity: A Randomized Trial. Journal of the American Heart Association, e025657.

But perhaps I’m being too much of a Debbie downer; a negative Nancy, a pessimistic Percival. Glass half-full readers of the study will note that a daily avocado, despite containing about 322 calories (kcals) a fruit*, does not cause weight gain or an increase in visceral fat. Indeed, as one of the HAT trial researchers, Professor Joan Sabate, says, “This is positive because eating extra calories from avocados doesn't impact body weight or abdominal fat…”

Moreover, remember that the HAT trial also assessed changes in parameters of cardiovascular and cardiometabolic health. Two of these parameters were total cholesterol and LDL cholesterol (sometimes known as “bad” cholesterol). High blood levels of total and LDL cholesterol have been associated with a greater risk of cardiovascular disease, particularly atherosclerotic cardiovascular disease (ASCVD), whereby fatty plaques build up in arteries.  Conversely, lowering total and LDL levels can reduce these risks. And that’s where it’s good news for avocado aficionados: the HAT trial showed that people eating an avocado a day experienced a small but nevertheless statistically significant decrease in total and LDL cholesterol levels.

Compared to those in the habitual diet group, at the end of 6 months the avocado eaters enjoyed a 2.94 mg/dL and 2.47 mg/dL greater reduction in total and LDL cholesterol level, respectively. To put these figures in context, before the trial, the subjects had average (mean) total and LDL cholesterol levels of 188 and 112 mg/dL, respectively. To add even more context, a healthy total cholesterol level is considered to be between 125 and 200 mg/dL, while a healthy LDL cholesterol blood test result should read below 100 mg/dL. Amidst this numerical hinterland then, a 2-3 mg/dL reduction in cholesterol levels (roughly 1.5 - 2.2%) is quite small, but certainly not to be sniffed at.

For one, disregarding the economic and logistical ramifications of sourcing a daily supply of the fruits, the act of merely eating an avocado in addition to your normal diet is fairly low-effort. Contrast this with 12 or more weeks of aerobic exercise, which is shown in one meta-analysis to effect reductions in LDL cholesterol of around 5%.

Avocados’ cholesterol-cutting effects are likely due in part to their fibre content, with a single avocado containing roughly 3.3 grams of soluble fibre. Numerous studies show that eating 5-10 grams of soluble fibre per day can lower total and LDL cholesterol levels, a phenomenon due to soluble fibre’s complex effects on cholesterol absorption and synthesis (see diagram below).

Source: Surampudi, P., Enkhmaa, B., Anuurad, E., & Berglund, L. (2016). Lipid lowering with soluble dietary fiber. Current atherosclerosis reports, 18(12), 1-13.

Avocados are also rich in phytosterols - cholesterol-like compounds that make up the membranes of plant cells - which can also prevent cholesterol being absorbed in the intestine and incorporated into lipoproteins (specialised particles that transport fat and cholesterol around the body).

To sum up then, if you’re looking for a specific food to eat everyday for 6 months in addition to your normal diet, you can do much worse than the not-so-humble avocado.

*Which is more than that in a 44 gram bar of milk chocolate (235 kcals)! Of course, not all calories are created equally, the same chocolate bar contains 22.7 grams of sugar and 8.14 grams of saturated fat (compared to just 0.66 grams of sugar and 2.18 grams of sat fat in an avocado).  

What were the limitations of the study?

It’s a truism, but no study is perfect. We cannot test all 8 billion inhabitants of the world under precisely controlled conditions to assess anything, let alone whether avocados help you shed visceral fat.

If you’ve ever read a scientific paper, you’ll have noticed that there is often a section at the end entitled ‘Limitations.’ In this section, researchers typically discuss methodological weaknesses and the limits to what can be safely concluded from their study.

In the HAT trial, one obvious weakness (with a not-so-obvious solution) was the lack of blinding - subjects in the Avocado Supplement Diet group knew they were eating avocados. Blinding involves preventing subjects and researchers from knowing whether or not they’re receiving the intervention. This is crucial because the mere knowledge that you’re in the intervention or control group can alter subsequent behaviour and possibly bias outcomes. For example, people aware that they’re receiving a real, say, cholesterol-lowering drug as opposed to a placebo may respond by altering their diet, perhaps by eating more fatty foods, for example.

Of course, how do you give subjects an avocado a day for 6 months without them knowing it? Correspondingly, how do you not give the control group a daily avocado without them knowing it?

Another thing that can distort the relationship between an intervention and health outcome, which we discussed earlier, is confounding variables. The HAT trial didn’t monitor changes in medication use (or, it seems from reading the study, physical activity), both of which are potential confounders that can impact visceral fat and cardiometabolic parameters over 6 months. The researchers also mention that the beginning of the COVID-19 pandemic occurred during the study, which again might have disrupted subjects' normal dietary patterns, exercise regimes, and lifestyles, influencing changes in visceral fat etc.

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