What makes superfoods super?

Introduction

According to Professor David Bender the concept of superfoods was developed in the USA in 2003-4 and introduced into Britain in an article in the Daily Mail in December 2005 entitled “The top 10 super foods”. Superfood is an informal designation and so there is no precise definition or definitive list of superfoods. If you type superfood into google then you get many different lists which vary in the number and choice of foods included. Bender produced a list of about 40 foods by amalgamating the various lists that he found in 2007. The longest current list that I found was of 52 superfoods “that you need in your diet” which can “help you fight disease, feel more energetic and even lose weight” .

Superfoods are essentially ordinary foods that are rich in particular nutrients, polyunsaturated fatty acids or dietary fibre or rich in other plant chemicals (phytochemicals) that have been claimed to be antioxidants or to have other beneficial effects upon health.

I am often asked about whether a particular food is good or bad or whether one vegetable or one fruit is better than another. I find the whole idea of good foods and bad foods to be flawed. What matters is whether diets are good or bad; one “good” or “bad” food does not a good or bad diet make. An occasional hamburger, sausage, coconut dessert, bag of chips (fries) or cream cake does not make a basically sound diet bad so neither should one expect an occasional portion of broccoli, goji berries, tofu, flaxseeds or salmon to make a bad diet good. The very use of the term “superfood” unrealistically implies it is something that is transformative.

What is the theoretical basis of claims for superfoods?

Many superfoods are accurately said to be rich sources of particular vitamins or minerals, but this abundance of nutrients is only likely to benefit those who would otherwise have an inadequate intake of that nutrient. Large excesses of vitamins and minerals serve no useful purpose and in some instances can be harmful. People who take vitamin and mineral supplements tend to be those who are health conscious and eat nutrient-rich diets i.e. they have the least need for them. Likewise, one might expect anyone buying something because it is promoted as a superfood, to also be health conscious and to eat a generally nutrient-rich diet.

There is no evidence to support the belief that taking extra antioxidants in the form of supplements or antioxidant rich foods reduces disease risk or improves life expectancy (see previous post “the antioxidant myth”). Evidence from clinical trials of antioxidant supplements suggests that these are more likely to do net harm than net good. People who normally eat antioxidant rich diets do tend to be healthier and live longer but this does not mean large intakes of antioxidants per se are beneficial. Association does not mean causation and there are many other alternative explanations for this observation. Extra antioxidants from food or supplements do reduce short-term experimental measures of “oxidant stress” but this is not evidence of long-term holistic benefit. The design of such experiments often guarantees a “successful” outcome i.e. foods known to have high levels of antioxidants show evidence of short term antioxidant effects in people or animals. Some of these short term trials of antioxidant-rich foods are sponsored by those trying to market the food e.g. trials of watercress  sponsored by the Watercress Alliance.

There has been much speculation about the potential health benefits of plant chemicals and even claims that some e.g. glucosinolates in brassica vegetables like broccoli could be the source of new drugs. Almost all of this evidence is highly speculative or preliminary and towards the bottom of the hierarchy of evidence in medical decision-making. It is certainly not sufficient to justify superfood status.  Of course, many important drugs have their origins in these phytochemicals but these drugs have almost always come from non-food plants. We have probably learned to avoid eating plants containing substances with strong pharmacological activity. I asked my pharmacology colleagues to name any major drugs that have their origins in a plant with an authentic culinary use – I got almost no convincing examples. These phytochemicals are not essential nutrients; some may have beneficial effects but we simply do not have the evidence to say which, if any, are “good for us” or what these benefits are.

Some foods are promoted as good (low-calorie) protein sources. Previous posts 1 and 2 have made clear that protein is a very unlikely limiting nutrient in human diets and so a dietary deficiency of protein is extremely improbable. We do not need to eat superfoods to get more protein.

Some foods like nuts and seeds are designated as superfoods because they are sources of “essential” polyunsaturated fatty acids. A simple deficiency of these fatty acids is all but impossible. However, there is very strong evidence that replacing some of the saturated fat in the diet with unsaturated fatty acids reduces the bad cholesterol (LDL) in the blood and so lowers the risk of cardiovascular diseases  . Most common vegetable oils like sunflower oil and corn oil are rich in so-called omega-6 fatty acids whilst olive oil and rapeseed oil are rich in monounsaturated fatty acids. A major dietary shift from highly saturated fat sources i.e. meat fat, dairy fat and tropical oils (coconut and palm oil) to more unsaturated fats especially those rich in polyunsaturated fatty acids, like sunflower or corn oil, will lead to favourable changes in blood cholesterol levels and reduce long-term cardiovascular disease risk.

Fish oils from oily fish like salmon and sardines are rich sources of omega-3 polyunsaturated fatty acids and are the only significant sources of long chain omega-3 fatty acids like EPA and DHA. These long chain omega-3 fatty acids are claimed to reduce risk of heart attacks and also to reduce inflammation in conditions like arthritis. Flaxseed oil is an example of a vegetable oil that is rich in shorter chain omega-3 fatty acids but very little is converted to long chain EPA and DHA in people.

A high content of dietary fibre is also used to confer superfood status particularly upon whole grain cereals. Oats, beans (including baked beans) and barley are good sources of the soluble types of fibre which may have beneficial effects upon blood cholesterol levels as well as the more well-known beneficial effects upon bowel function and bowel health.

Examples of superfoods

These are examples largely taken from the two lists mentioned in the opening paragraph.

A number of berries and other fruits feature in many superfood lists including acai berries, apples, Goji berries, prunes (dried plums), strawberries, kiwi, bananas, pineapples, cherries, blackberries, grapes, blueberries, cranberries, oranges and mangoes. A healthy recommended diet should contain at least five varied daily portions of 80g of fruits and vegetables (see earlier post). Any or all of these foods could usefully make up part of that regular five-a-day. Fruits are important sources of several essential nutrients like vitamin C, vitamin B6, folate, potassium and beta-carotene (vitamin A). They also contain dietary fibre and are low in calories. There are claims that many of them contain high levels of powerful antioxidants like:

  • Quercetin a flavonoid or polyphenol found in apples and berries
  • Anthocyanins and other flavonoids in acai berries, cherries, blackberries
  • Resveratrol a natural phenol found in grapes (also found in small amounts in red wines)
  • Beta-carotene (vitamin A) and other carotenoids like lycopene and lutein in mangos, cantaloupe melon, grapefruit, oranges, apricots, watermelon and tomatoes.

These claims are technically accurate but should be viewed in the light of the lack of any demonstrable benefits for increased intakes of antioxidants. Several vegetables are also promoted on the basis of high antioxidant content like sweet potato, carrots, peppers, kidney beans, artichoke, pumpkins and green leafy vegetables. Even tea and dark chocolate are promoted as good sources of antioxidants.

Cruciferous vegetables like cabbage, brussel sprouts, bok choy, spinach, broccoli, kale, watercress and cauliflower are good sources of essential nutrients like vitamin C, vitamin K and folate. They are low in calories and are sources of dietary fibre so they can usefully be part of our five-a-day. These vegetables also contain a group of sulphur-containing compounds called glucosinolates which are promoted as potential cancer preventing agents. This possible anti-cancer activity is linked to their antioxidant activity and their ability to activate the body’s so-called phase II enzymes that detoxify many foreign chemicals including potential carcinogens. Activation of these enzymes may also speed up the breakdown of some prescribed drugs and thus reduce the drug’s effectiveness. Claims for the anti-cancer activity of glucosinolates are quite speculative and based upon low-level evidence such as their effect upon isolated cancer cells or experimentally induced animal cancers. According to a 2012 fact sheet produced by the US National Cancer Institute, there is no consistent evidence from cohort studies (the highest level of observational studies) that high intakes of cruciferous vegetables is linked with reduced risk of prostate, breast, colorectal or lung cancer. An announcement that a UK company was starting trials of a synthetic broccoli glucosinolate for treating brain haemorrhage, and breast and prostate cancer attracted media attention in 2014 . I could not find any published results from these trials and their outcome would in any case probably have little effect upon the case for promoting cruciferous vegetables as superfoods.  In very large amounts these cruciferous vegetables can interfere with thyroid function but this is probably not a practical problem in countries like the UK.

There is evidence to support the inclusion of at least five daily portions (80g) of a variety of fruit and vegetables in our diets but little real basis for choosing specific varieties promoted as superfoods. There is no reason to believe that exotic and expensive varieties are likely to be more beneficial to health than cheaper and more familiar ones.

Peanuts, almonds, walnuts, Brazil nuts, avocados, sunflower seeds like other many other nuts and seeds have large amounts of energy stored as oil. This oil is highly polyunsaturated and so it is a good replacement for some of the more saturated fat in the diet. This also applies to oil extracted from these seeds and nuts to make cooking and spreading fats. These foods are good sources of vitamin E which is a fat soluble antioxidant. Brazil nuts are also promoted as a source of the mineral selenium which also has antioxidant functions. They thus have nutritionally positive features but no unique beneficial effects that would justify any one being singled out as a superfood.

All of the following have been put forward as superfoods because they are high in protein and relatively low in calories: eggs, scallops (any white fish meets this criterion), lentils, black beans, fat-free milk, Greek yoghurt, soy milk, turkey breast and mushrooms. Whilst this description is accurate it should be borne in mind that Western diets almost always have surplus protein and, even in developing countries, protein is an unlikely limiting nutrient.

Salmon, sardines and other oily fish are rich sources of so-called omega-3 polyunsaturated fatty acids and they are the only significant dietary sources of long chain omega-3 fatty acids like EPA and DHA. There are theoretical mechanisms that suggest the possibility that these long chain omega-3 fatty acids might have anti-inflammatory activity and might also reduce blood platelet aggregation and the tendency for blood to clot. They have thus been promoted as having potential benefits in the management of arthritis and also in reducing the risk of heart attacks. Some early clinical trials did suggest that eating oily fish or taking fish oil supplements reduced the risk of heart attack survivors having another cardiac event and dying in the years after the first attack. Dietary recommendations in both the UK and USA  have suggested that ideally we should eat two portions of oily fish each week. Most of the evidence relating to any anti-inflammatory effect tends to be either very low-level and/or uses very high doses of concentrated fish oils. There is no convincing evidence to support claims of anti-depressive effects  or improvements in cognitive function in the elderly or postnatal depression and intellectual development in babies and children. Many of the more recent trials of fish oils seem to be less than positive about the benefits of eating oily fish or taking fish oil supplements. An article in JAMA Internal Medicine found that almost all clinical trials and meta-analyses involving fish oil supplements and published in top journals reported no beneficial effect (2007-2012) yet over this period sales of fish oils had more than doubled. Many editorials accompanying these largely negative studies tended to be enthusiastic about omega-3 fatty acids – even the experts seem reluctant to accept evidence that fish oils have little or no benefit? Any small effect of fish oils in those at risk of heart attacks may these days be overwhelmed by the impact of mass use of statins. Several vegetable foods like walnuts, flaxseeds (oil) and rapeseed (canola) oil have been promoted as relatively good sources of omega-3 fatty acids but these are short chain omega-3s and there is only limited conversion of these to EPA and especially DHA in humans.

Shiitake mushrooms have been promoted as a good source of vitamin D. All mushrooms grown in sunlight have good amounts of vitamin D and even those grown in the dark have good amounts if exposed to UV light or sunlight after harvesting. This is the only substantial natural vegetarian source of dietary vitamin D. Most diets contain very limited amounts of vitamin D and we rely upon production in the skin when it is exposed to summer sunlight (UV) to meet most of our need. Vitamin D deficiency is very prevalent even in affluent countries especially amongst those with limited sunlight exposure or pigmented skin. A good cheap vegetarian source of vitamin D is therefore an important characteristic and maybe all suppliers should consider UV irradiation of mushrooms to boost vitamin levels. Oily fish are also good sources of vitamin D and in the USA milk is usually fortified with vitamin D (see previous vitamin D post).

Soy foods like tofu and soy milk contain substances called isoflavones which have very weak female sex hormone (estrogenic) activity. Soy foods and supplements have been promoted as having potential to reduce symptoms caused by the menopausal decline in oestrogen production. It has also been suggested that high soy consumption in younger women might reduce the effects of high oestrogen levels in younger women by competing for oestrogen receptors but only weakly stimulating them. It has been claimed that this damping down of natural oestrogen responses might reduce the risk of oestrogen-dependent breast cancers. Evidence from many trials using very high doses of isoflavones is inconclusive about whether they reduce menopausal symptoms like hot flushes; they are clearly much less effective than conventional Hormone Replacement Therapy (HRT)  .These trials often report a large beneficial effect of the placebo which makes it difficult to detect any small effect of the “real” treatment. Epidemiological observations tend to be consistent with a protective effect of high soy consumption against breast cancer but this evidence falls far short of demonstrating cause and effect .

Plants of the onion or Allium family contain substances called s-alkylcysteine sulphoxides. When garlic cloves are crushed they release one of these sulphur-containing substances called allicin for which many health claims have been made. Garlic supplement studies are probably of little relevance to use of garlic in cooking because allicin is unstable when cooked and even decomposes in raw crushed garlic. There is some evidence that concentrated garlic supplements with high allicin content taken for some months may have a slight cholesterol-lowering effect  and a small blood pressure lowering effect . Garlic supplements do not appear to reduce the frequency or duration of colds . Garlic supplements should contain mg quantities of allicin. One 4g clove of garlic can release 5-20mg of allicin (note that 1mg = 1000µg).

Conclusions

Most of the foods listed above as superfoods have useful characteristics from a nutritional point of view so can all be usefully included in a healthy balanced diet. These useful properties are rarely unique to that food. For many of the features highlighted as a justification for superfood status there is no convincing evidence that it has any beneficial effects upon health.

Devil’s Advocate

What constitutes a superfood depends very much upon fashion and what particular nutrient or nutritional feature is currently in vogue; it is essentially a marketing term or a buzz word used by journalists and self-appointed nutrition experts. If one were compiling a list of superfoods a few decades ago then one might choose foods that do not feature in many current superfood lists but one can highlight positive nutritional features for any food and call it a superfood.

  • Lean grilled steak is quite low in calories but is a good source of high quality protein, readily available iron, zinc, vitamin B12 and vitamin B6.
  • Whole milk is a good source of high quality protein and high in energy for growing children. It is a good source of calcium, riboflavin (B2), B12, selenium and iodine and is richer in vitamins A and D than low-fat milk.
  • Liver has been called nature’s most potent superfood because it is a rich source of many essential nutrients like vitamin A, iron, selenium, zinc, folate, B6, B12 and high quality protein. It also contains useful amounts of vitamins D and C. (Animals and people store many vitamins and minerals in their livers)

Many older readers will probably remember a famous advert for a popular chocolate bar that “helps you work, rest and play”. It was promoted as having “all the goodness of milk, sugar, glucose and thick, thick chocolate”. A potential superfood? Maybe only on Mars!!!

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