"Eat to Live, Not Live to Eat"
John H Weisburger, 2000
The olive tree (Olea europaea) has been grown from ancient times for its fruits. It’s a very hardy species that thrives in a Mediterranean climate and consequently was extensively incorporated into the national cuisines of southern European countries. In more recent times it has gained a valued reputation as one of the important components of the healthy Mediterranean Diet (MD) that has been demonstrably effective in reducing the incidence of chronic diseases so prevalent throughout developed countries and increasingly elsewhere. Most fruit production worldwide (about 90%) is used to make olive oil for garnishing and cooking. High consumption is the norm in southern European countries. For example, in Greece, adults consume an average of 80g/d and in Spain, as a result of high olive oil intake, fats may represent over 40% of dietary calories. The following commentary is focussed only on the nutritional aspects of plant-based oils; their comparative organoleptic properties are quite subjective and strongly dependent on historical and national traditions and will not be considered here.
Although established as a revered and proven component of the MD, are there other plant-based oils that are equally beneficial for long term well-being or maybe even better? Does the attention given to distinctions between various virgin oils and other cheaper refined oils have any nutritional basis? Or is it more a case of olive oil users supporting what is a long-cherished culinary tradition in the Mediterranean basin based on availability in earlier times when globalised sourcing of foods was less feasible and there was little scientific knowledge comparing the relative merits of others?
Healthy diets recommended by world authorities should have 20-35% of total daily calories contributed by fats. It’s common knowledge that not all fats are equal. Trans fats should be avoided altogether or at least kept below 1%, and saturated fats which predominate in animal-based foods should be restricted to less than 7% of energy intake. The value of plant-based fats is that they have low saturated fats (apart from palm and coconut) and consist mainly of mono- and poly-unsaturated fatty acids (MUFA and PUFA respectively). Only two fat types are essential for humans – the polyunsaturated omega-6 and omega-3 oils (n-6 and n-3 resp) as we can synthesize all the rest from other nutrients. The problem in Western diets is to ensure sufficient n-3 intake to avoid deficiency symptoms and also contribute to long-term well-being, with 1-3 g/d commonly recommended (of which 10% can be the long chain n-3 marine oil types) as n-6 consumption is usually far in excess of needs; the preferable limit for these is less than 10% energy intake. Total fat intake in a healthy diet should be more than the 3g/d contributed by essential fats to enable, for example, absorption of fat-soluble vitamins and other phytonutrients from the gut food matrix. For many years it was thought that the main contribution of olive oil to the MD was the high concentration of MUFA (73%), principally as oleic acid. But more recently it has been found that non-fatty acid phytochemicals (polyphenols/antioxidants) also have an important role, maybe even the dominant one. This is a nutritional basis for distinguishing between virgin and more refined olive oils as the former have much higher antioxidant levels than the latter.
Oils with high levels of PUFA including the n-6 and n-3 fatty acids are much more prone to degradation/oxidation (rancidity) than MUFA. Given the main use of olive oil is in producing cooked foods, it has the relative advantage with high MUFA of increased thermal stability but the disadvantage of low PUFA, meaning it is low in essential oils (9.8% n-6 and 0.8% n-3). Canola oil is an alternative to olive oil (19% n-6 and 9.1% n-3). Although it only became available in1979 following successful Canadian research and consequently has not benefited from the many centuries of use that olive oil has enjoyed, it currently far exceeds it in world production and consumption. Epidemiological and clinical trial evidence for its positive effects in humans versus onset of chronic diseases is as impressive as olive oil. At a more basic level it has roughly equivalent MUFA content (63% vs 73%) conferring cooking stability, but more importantly a much better content and balance of the n-3 essential fatty acids. The following paper (Exp Biol Med (2008) 233:674–688) makes the case for lowering the ratio of omega-6/omega-3 oils in Western diets.
Several sources of information suggest that human beings evolved on a diet with a ratio of omega-6 to omega-3 essential fatty acids (EFA) of about 1 whereas in Western diets the ratio is 15/1– 16.7/1. Western diets are deficient in omega-3 fatty acids and have excessive amounts of omega-6 fatty acids compared with the diet on which human beings evolved and their genetic patterns were established. Excessive amounts of omega-6 polyunsaturated fatty acids (PUFA) and a very high omega-6/omega-3 ratio, as is found in today’s Western diets, promote the pathogenesis of many diseases, including cardiovascular disease, cancer, and inflammatory and autoimmune diseases, whereas increased levels of omega-3 PUFA (a lower omega-6/omega-3 ratio) exert suppressive effects. In the secondary prevention of cardiovascular disease, a ratio of 4/1 was associated with a 70% decrease in total mortality. A ratio of 2.5/1 reduced rectal cell proliferation in patients with colorectal cancer, whereas a ratio of 4/1 with the same amount of omega-3 PUFA had no effect. The lower omega-6/omega-3 ratio in women with breast cancer was associated with decreased risk. A ratio of 2–3/1 suppressed inflammation in patients with rheumatoid arthritis, and a ratio of 5/1 had beneficial effect on patients with asthma, whereas a ratio of 10/1 had adverse consequences. These studies indicate that the optimal ratio may vary with the disease under consideration. This is consistent with the fact that chronic diseases are multi-genic and multi-factorial. Therefore, it is quite possible that the therapeutic dose of omega-3 fatty acids will depend on the degree of severity of disease resulting from the genetic predisposition. A lower ratio of omega-6/omega-3 fatty acids is more desirable in reducing the risk of many of the chronic diseases of high prevalence in Western societies, as well as in the developing countries.
The n-6/n-3 ratio for olive oil is about 12.2/1 and for normal canola it is 2.1/1 with enhanced n-3 forms having an even lower ratio. With comparable impressive evidence for their health conferring properties over animal-based fats, it may thus be possible to achieve further improvements in the MD by substituting canola oil for olive oil. Additional considerations are that canola has approximately half the saturated fat content as olive oil (7.4 vs 14%) and it’s cheaper than the various speciality virgin olive oils. But realistically, traditional customs are very resistant to change. So if individuals choose to keep olive oil as a part of the MD it is important to realise that the whole fruit, de-bittered by the Spanish brining technique, has the same oil with more healthy fibre, lower calories with 50% contained water, and for black olives, about 20 times (!) the antioxidant level as extra virgin olive oil. Such a large difference in nutrient content between different parts of a food (eg after peeling, de-seeding, juicing or oil expression) is unusual but some differences are the norm and should always be considered when deciding on the nutritional merit of a given processing technique. Two obvious factors contributing to such a disparate outcome in this instance are that water soluble nutrients will not be included in separated oil, and other phytonutrients that are extensively membrane-bound will also be left behind with oil expression. So committed MD olive oil consumers might expect enhanced health outcomes by substituting or supplementing part of their oil intake with table olives. If olive oil was to be the sole source of n-6/n-3 fatty acids in the diet, then for men, they would have to consume about 210g /d to meet recommended n-3 levels versus 16g/d for canola which also provides a superior n-6/n-3 ratio.
Both olive and canola oils are valuable foods, but when used at elevated temperatures for frying, they are nevertheless still subject to finite chemical degradation to a number of unhealthy substances that occur more extensively above 150°C. This is why food outlets such as fish and chip shops have to totally replace their oil periodically when deterioration has progressed beyond certain levels. Production of these substances is minimal when the oil is used only once for cooking a given meal, provided the temperature/time is not excessive. However with the higher temperatures/rapid cooking used in popular Asian stir frying techniques (oil taken to the smoke point and beyond) more dangerous carcinogens are formed, some volatile. As a result, a several-fold higher incidence of lung cancer is seen in cooks frequently using this technique, especially where there may be poor or ineffective ventilation. Also, with higher temperature/cooking frying time combinations, the degradation of valuable phytochemicals can be profound. This may be made even worse when water-based boiling follows the frying.
Fruits mainly store energy as carbohydrates but avocados, like olives, predominantly use lipids. Lipids are the norm for seeds and nuts, but again there are exceptions such as carbohydrate storage in chestnuts. The edible portion of avos consists of 73% water, 15% lipids, 8.5% carbohydrates and 6.7% fibre, and while having a slightly higher calorie content (160kcal/100g) than typical fruits, they are a nutrient-rich food containing significant amounts of minerals and other important phytochemicals. The 15% total lipids (fats) is made up of approximately 2.1% saturated (SFA), 9.8% monounsaturated (MUFA) and 1.8% polyunsaturated (PUFA) fats, so fat content is largely of the healthy non-saturated types. Furthermore, the ratio of omega-6 to omega-3 polyunsaturates is favourably less than one. On a population level they're not as frequently eaten as more common fruits such as apples or oranges for a variety of reasons, some of which are - increased cost, availability, buttery texture, lack of sweetness and concern that higher calories would lead to unwanted weight gain. The following US study (Nutrition Journal 2013, 12:1) demonstrates that this last concern is unwarranted, and in fact avo consumers had lower body mass index and better levels of various markers of well-being compared to non-consumers.
Avocados contain monounsaturated fatty acids, dietary fibre, essential nutrients and phytochemicals. However, no epidemiologic data exist on their effects on diet quality, weight management and other metabolic disease risk factors. The objective of this research was to investigate the relationships between avocado consumption and overall diet quality, energy and nutrient intakes, physiological indicators of health, and risk of metabolic syndrome. Avocado consumption and nutrition data were based on 24-hour dietary recalls collected by trained National Health and Nutrition Examination Survey (NHANES) interviewers. Physiological data were collected from physical examinations conducted in NHANES Mobile Examination Centres. Diet quality was calculated using the USDA's Healthy Eating Index-2005. Subjects included 17,567 US adults and#8805; 19 years of age (49% female), including 347 avocado consumers (50% female), examined in NHANES 2001–2008. Least square means and standard errors were determined using appropriate sample weights, with adjustments for age, gender, ethnicity, and other covariates depending on the dependent variable of interest. Avocado consumers had significantly higher intakes of vegetables, fruit, diet quality, total fat, monounsaturated and polyunsaturated fats, dietary fibre, vitamins E, K, magnesium, potassium and vitamin K, and lower intakes of added sugars. No significant differences were seen in calorie or sodium intakes. Body weight, body mass index and waist circumference were significantly lower and HDL-cholesterol (the good one) was higher in avocado consumers. The odds ratio for metabolic syndrome was 50% lower in avocado consumers vs. non-consumers. We conclude that avocado consumption is associated with improved overall diet quality, nutrient intake, and reduced risk of metabolic syndrome. Dietitians should be aware of the beneficial associations between avocado intake, diet and health when making dietary recommendations.
Metabolic syndrome is a significant risk factor for cardiovascular disease and diabetes. It increases markedly with age and is present in more than 42% of adults over the age of 60 in the US. Consequently a 50% reduction in this incidence through consumption of avos is very important for individuals and for societies in terms of lost productivity, quality of life and overwhelming medical costs. Like any epidemiologic study there can be many confounding variables that might have contributed to the reported results. But the 50% reduction was found after allowing for contributions due to age differences, gender, ethnicity, poverty-income ratio, physical activity, smoking and alcohol intake. Only 347 out of 17,567 people (ie 2%) stated they regularly consumed avos (70g or about half a fruit/d). This proportion seems absurdly low but it is in a society where andlt;3% of US men and andlt;6% or women meet recommended daily fruit and vegetable intake. This is such a long way from being ideal, is it any wonder that metabolic syndrome is at epidemic proportions there?
Other studies addressing the unfounded concern about possible weight gain or negative effects with avos have been reported. A US 2013 clinical trial found that addition of half an avo to a standard lunch was associated with improved meal satisfaction and decreased desire to eat over the next 5 hrs, and substitution for an equivalent number of calories in the lunch led to a significant reduction in blood insulin level over the next 3 hrs. An Australian crossover trial in men studied the effects of two iso-caloric diets – one high SFA (milk, butter, cream, cheese and fatty meat)/low MUFA and the other high MUFA (avos, nuts and olive oil)/low SFA for 4 wks on each diet. Even though calories were the same in each diet, fats provided by MUFAs led to lower bodyweight and fat mass. And another Sth African study found that consumption of 200g/d avos within an energy-restricted diet in overweight and obese adults did not compromise weight loss when substituted for an equivalent number of calories in the daily diet. One underlying mechanism that helps understand these bodyweight outcomes is that MUFAs, the principal component of avo lipids, are more likely to be oxidised in the body (increased thermogenesis) whereas SFAs are more prone to fat storage. At a broader level than all the above, an increasing number of clinical and pre-clinical studies in the last 20yrs have now reported positive effects on cardiovascular health, DNA damage protection, osteoarthritis, eye and skin health and various cancers.
Some other features of avos worth considering are:
Type 2 diabetes has become a major health issue in Australia where more than 1 million adults had been diagnosed with diabetes in 2010, with a projection for 1.5 million by 2030. In the US the problem is much worse, but as happens with so many phenomena, the rest of the world is following not far behind. For US adults older than 19 years of age in 2005-2006, the total incidence of diabetes (diagnosed, undiagnosed and pre-diabetes) was 42.3% and for those over 64 it was 72%! As many as 80% of patients with type 2 diabetes will develop and possibly die of macro-vascular disease. This results in major loss of life expectancy and quality of life, and medical expenses and lost productivity in the US were estimated to cost US$174 billion in 2007.
The main drivers of these alarming statistics and outcomes are obesity, populations living longer than in the past with increased likelihood of development of chronic diseases, and urbanisation leading to more sedentary activities. The first and third factors are modifiable by lifestyle changes, and one of the best ways to minimise the onset of overweight/obesity is through smarter dietary habits. Fruit and vegetable substitution for more nutrient-empty but high-calorie foods has been documented as an effective strategy to combat weight increases in numerous studies over many years. With the ready availability of all sorts of handy kitchen appliances nowadays, juicing fresh fruit has become a popular means of meeting fruit intake for many people. But does this affect their nutritional and health qualities compared to eating the fresh fruit whole? The following paper investigated this question and found while whole fruit had the expected beneficial result, juice significantly increased the incidence of diabetes (Diabetes Care (2008) 31:1311–1317).
The purpose of this study was to examine the association between fruit, vegetables and fruit juice intake and development of type 2 diabetes. A total of 71,346 female nurses aged 38–63 years who were free of cardiovascular disease, cancer, and diabetes in 1984 were followed for 18 years, and dietary information was collected using a semi-quantitative food frequency questionnaire (FFQ) every 4 years. The FFQ contained 16 questions on fruit consumption, 28 on vegetables and 3 on potato consumption which was a separate category. Median intakes of fruit, vegetables and juice for the bottom and top 20% were 0.5 and 2.5, 1.5 and 5.2, and 0.2 and 0.7 servings/day respectively. Diagnosis of diabetes was self-reported and then confirmed using standard diagnostic criteria. During follow-up, 4,529 cases of diabetes were documented, and the cumulative incidence of diabetes was 7.4%. In the statistical analysis of the data, allowance was made for the possible confounders of body mass index, family history of diabetes, smoking, post-menopausal hormone use, alcohol intake, physical activity and others that have been related to diabetes. It was found that an increase of three servings/day in whole fruit consumption was associated with an 18% lower hazard of diabetes. An increase of 1 serving/day in green leafy vegetable consumption was associated with a 9% lower hazard of diabetes, whereas the same change in fruit juice intake was associated with an 18% increased hazard of diabetes.
Some considerations that may have led to this unfavourable finding for juices are the following:
The moral of the story is to eat the whole fruit if and when you can.
Most people are aware of the term ‘Glycemic Index' (GI) which can be applied to different foods. Essentially it gives an indication of how rapidly the glucose component of a food is absorbed in the body relative to glucose itself. High GI foods produce very fast and high peak blood levels of glucose, whereas those with low GI produce lower and more gradual elevation. In the former situation there is a correspondingly rapid and large rise in insulin blood level, that if continued over many years can result in insulin resistance, diabetes and other negative health outcomes. Recommendations to eat foods with low GI minimise this outcome and allow glucose and insulin to quickly return to the base-line state. Minimally processed plant-based foods such as fruit and vegetables with their high fibre content can slow absorption of glucose from the food matrix and help achieve this goal.
A similar high peak in blood lipids occurs after meals containing foods with significant fat content. Again, the preferred outcome is to have slower longer absorption with less severe disruption of the resting state, and fibre contained in fruit and vegetables can assist here also. Elevated lipid, glucose and insulin levels after food represent pro-oxidant and pro-inflammatory stressors but in a healthy person these effects are kept in balance by anti-oxidant chemicals in food and in the body. If not counteracted by antioxidants then, for example, LDL-cholesterol (the bad cholesterol) can become oxidised and it is then more likely to attach to the interior walls of arteries, leading on to atherosclerosis and cardiovascular disease. It is in provision of antioxidants that there is a distinction between fresh fruit and vegetables, as fruit contain on average several times the antioxidant levels as vegetables. Also, because many vegetables are not very palatable in the raw state they are usually processed in some way before eating, and this can lead to varying levels of antioxidant loss or degradation, further accentuating the differences.
There is now overwhelming epidemiological evidence for the general value of fruit and vegetables in minimising the chronic diseases that are all too common in Western countries. What the study below examines is whether these beneficial effects are independent of when they're consumed with respect to other foods, or are they more effective when taken with meals (British Journal of Nutrition (2010), 104, S1–S14).
There is increasing evidence that the postprandial (after meal) state is an important contributing factor to chronic disease. This review assesses the role of fruit phenolic (antioxidant) compounds to protect health and lower disease risk through their actions in mitigating fed-state metabolic and oxidative stressors. Two main questions are posed: first, what is the role of plant foods, specifically fruits rich in complex and simple phenolic compounds in postprandial metabolic management; and second, does the evidence support consuming these fruits with meals as a practical strategy to preserve health and lower risk for disease?
An overview of the postprandial literature is presented, specifically on the effect of fruits and their inherent phenolic compounds in human subjects on postprandial elevated blood lipids, glucose and insulin, and associated events such as oxidative stress and inflammation. Among the identified well-controlled human trials using a postprandial paradigm, half used wine or wine components and the remaining used various berries. Notwithstanding the need for more research, the collected data suggest that consuming phenolic-rich fruits increases the antioxidant capacity of the blood, and when they are consumed with high fat and carbohydrate ‘pro-oxidant and pro-inflammatory' meals, they may counterbalance their negative effects. Given the content and availability of fat and carbohydrate in the Western diet, regular consumption of phenolic-rich foods, particularly in conjunction with meals, appears to be a prudent strategy to maintain oxidative balance and health.
If you're in the habit of having a dessert after dinner, you could be doing yourself a big favour down the track if you substitute fresh fruit for the more indulgent ice-cream, cheese cake etc. And what about a fruit salad or smoothie to go along with the rest of your lunch? Fruit will not only address the antioxidant story above but also add into the deal their high fibre content and low calories.