The ‘superfood’ idea is new, and has no strict definition (out of millions of scientific articles, I could find only 17 that had ‘superfood’ in the title). The medical term – ‘nutraceutical’ – is much more widely used (70,000 references). Both are used to describe foods or substances extracted from food that may have medically beneficial effects. Although superfoods and nutraceuticals are similar, superfoods have almost never been subjected to scientific studies and are mostly food fashions heavily promoted as having some medical-like benefits. On the other hand, nutraceuticals have often been studied in the laboratory, sometimes very extensively. Most studies are in test-tubes, and some of them in animals, but even though some of the chemicals are biologically impressive and hold out hope that they will be of value in conditions like Type 2 diabetes, almost none have been clinically tested in humans, mostly because it is very difficult to patent these natural substances.
Claims for the health benefits of superfoods are widely promoted, and include weight loss, lowering cholesterol levels and reducing blood glucose levels in Type 2. Additional claims for some include reduced risk of heart disease and Alzheimer’s, and even anti-ageing and reducing the risk of cancer. Some are macronutrients – that is, food products that form a large part of our diet – for example, protein or carbohydrate. Examples here would be ‘white meat’ (contrasted with ‘red meat’) or ‘fruit’. However, usually they are individual food items, mostly specific vegetables or fruits (often berries). Sometimes they are grouped together as menu items, such as ‘superfood salad’. Then there is a very interesting group of individual plants, spices and herbs that contain fascinating substances with blood-glucose-lowering effects, though frustratingly there are no clinical trials sufficiently large to be convincing. The exceptions here are guanidine and galegine, blood glucose-lowering chemicals from which metformin was developed, and which were originally isolated from a flowering plant, Goat’s rue. Finally, there is a huge group of minerals, trace elements and vitamins that are widely believed to have specific anti-diabetic properties.
I’d like to have space to discuss all the superfoods/nutraceuticals, but this topic alone would require a huge book, so I’ll take a few examples that happened to be in vogue while I was writing this chapter. I’ll use the background from earlier chapters to help us assess pros and cons, and hopefully equip you to assess the next generation of claims when they arrive, as they most surely will.
Key point: Nutraceuticals are foods, food products or substances contained in food that have biological effects when tested in the laboratory. Superfoods are popular versions of nutraceuticals. Promoted as having medical benefits, they have rarely been studied scientifically.
Let’s step back from all the arguments about which superfood is more super than the last, and return to the concepts we discussed in previous chapters. The body tries to digest and then use everything we eat and does so very efficiently, but recall that nearly all the food that can be absorbed into the bloodstream is converted sooner or later to glucose, which is stored or used under the watchful eye of brilliant insulin produced by the pancreas.
Let’s take the avocado, a ‘superfood’ from 2016 onwards – at the time of writing a ‘megafood’ that’s become so popular in the West that it was blamed for adverse environmental and economic effects in Mexico where it is a major fruit crop. (This is often the case with popular superfoods, and is another reason why we should always carefully consider the pros and cons of these foods.) Medically, though, could avocado, eaten in sufficient quantities, provide enough healthy monounsaturated fats to give the same benefits as extra-virgin olive oil in the PREDIMED study described in Chapter 5? One myth we can dispatch immediately is that avocados are full of ‘cholesterol’: only animals synthesise cholesterol, so it forms no part of any plant. Avocados are not high-cholesterol; they are zero-cholesterol. A whole avocado (without its stone) weighs about 150 grams. Here’s a table of the components of two whole avocados – a big portion, so it would likely be in a large salad, or perhaps guacamole, but I wouldn’t want to be accused of being mean.
Table 6.1 Components of avocado
| Nutrient | Proportion | Actual amount contained in two avocados |
| Water | 73% | 220 ml (two thirds of a standard drink can) |
| Fat | 15% | 45 g |
| Carbohydrate | 5% | 15 g |
| Protein | 2% | 6 g |
Conclusion: Avocados are not ‘superfoods’ in the quantities usually eaten, and by themselves can have no health benefits, though they are a good (but small) component of a healthy diet.
Key point: Avocados contain unsaturated fatty acids similar to those in olive oil, but in much smaller amounts. They’re nice to eat, but have no special health benefits.
It’s strange that while nice-tasting and fashionable fruits such as avocados are often cited as ‘superfoods’, they don’t seem to be of much value, and other more ordinary products, for which there is more plausible evidence, never make it into the headlines. Examples here are lady’s fingers (okra) and bitter melon, both cheap and tasty vegetables, and which potentially have a role in lowering blood glucose levels.
Okra (‘ladies’ fingers’) is widely used as a diabetes treatment in countries where it is eaten as a vegetable. There are good reasons why it may have some effect on blood glucose levels. First, it is high in soluble fibre, which is why it’s gooey when cooked. Soluble fibre reduces blood glucose, and a pure soluble fibre, guar gum, was in vogue in the 1980s and 1990s as a prescription drug in Type 2, though you needed to take a large amount each day (around 15 g), and even a good portion of okra (100 g, 3½ ounces) contains much less, perhaps 6 g. However, okra also contains a specific chemical that reduces the amount of glucose absorbed from the gut (very much like a drug called acarbose, a natural product derived from a bacterium, which was also popular 15–20 years ago). Okra may also lower cholesterol and blood pressure (see the section on the DASH diet in Chapter 7), but because there are no clinical trials, we can only recommend it as a very good vegetable in the general diet. Remember also that, like medication, if okra did have good effects on blood glucose, you’d need to eat it every day. Even the keenest okra consumer might have a bit of a problem achieving this. The daily routine obviously applies to all nutraceuticals.
The bitter melon (karela), widely eaten in Indian and south-east Asian cuisines, is also used in traditional medicine as an anti-diabetic agent. In some ways it is similar to okra: it has even higher levels of soluble fibre, and also contains active blood glucose-lowering chemicals which have been isolated and studied in the laboratory. One of these substances works at the point on the cell surface where insulin molecules bind – the insulin receptor. It increases the efficiency of the receptor, and therefore increases uptake of glucose from the circulation and reduces blood glucose levels. It also contains a fascinating substance that inhibits certain natural steroids, and which was seriously investigated by drug companies a few years ago as potentially a very effective blood glucose-lowering drug.
But – critical question – is bitter melon meaningfully effective in reducing blood glucose levels, and how much of this genuinely bitter food do you need to consume? As usual, these are the really difficult questions and unfortunately there are no well-conducted clinical trials. Until these trials are done – and holding our breath until they are may not be a wise strategy here – it should, like okra, be considered a valuable vegetable to include in the diet. Again, the portfolio approach is probably the best: increase the amounts you eat of these entirely safe vegetables, thereby gaining the benefits of the high-vegetable and high-fibre diet, while they possibly have some effect in reducing blood glucose levels. But low glucose level – hypoglycaemia – has been reported when these natural foods are taken in combination with some conventional diabetes medications. Always be aware of the risk.
Key point: Enjoy okra and karela (bitter melon) in your newly varied diet. They may help to lower blood glucose and cholesterol a little, but you probably wouldn’t want to eat them daily.
The goji berry is a typical ‘small’ superfood that has been used for many years in traditional Chinese medicine, where it is added to herbal soups. We’ve seen that the large avocado doesn’t contain sufficient amounts of any macronutrient (fat, carbohydrate or protein) to make any difference – unless you are an addict. The ‘small’ superfoods (usually berries) would therefore need to be consumed by the kilo if they were to have any chance of doing the same as olive oil. In addition, like the intriguing okra and karela, they may contain specific drug-like agents that help blood glucose, blood pressure, cholesterol or something else that’s important to outcomes in diabetes. Below is a list of claims for the benefits of goji berries. Most small superfoods have a remarkably similar list of supposedly extraordinary therapeutic properties:
Antioxidants usually come at the top of the list. Most fruits and vegetables contain these fascinating substances. Test-tube experiments and studies in animals often show that antioxidants improve levels of anti-inflammatory compounds in the blood and tissues, but nobody has ever been able to find specific and measurable health benefits (other than the known benefits of a high-vegetable diet, such as the Mediterranean diet). Some of the other claims, for example improving immune function, fighting cancer and boosting fertility, are false, and a drug company claiming the same when there was no watertight evidence for a conventional drug would find themselves in court and billions out of pocket.
But like many of these foods, goji berries are interesting. There are warnings about interactions between goji berries and warfarin (a blood-thinner/anticoagulant), and some medications used in diabetes (sulfonylureas, for example, gliclazide), or for treating blood pressure. In the quantities most people will eat the berries it’s unlikely they will have any serious side-effects, but it does remind us that like many other plants they contain bioactive chemicals. They haven’t been identified yet. So yes to goji berries: a sprinkle over the porridge is tasty and pretty, but superfood – no.
I won’t repeat the claims for blueberries, acai, mulberries, chokeberries, maqui and countless other exotic ‘superfood’ berries because there are only minor variations on the goji list They have no specific actions in Type 2 diabetes, and are small and so expensive that they’re only ever eaten in tiny quantities, so can have no benefits other than a fraction of any of your fruit and veg portions a day. But there’ll never be any shortage of new rare berries enthusiastically endorsed as the next amazing superfood.
Key point: Antioxidants in berries and other foods are insufficient to deal with the body’s oxidative stress.
There is a great deal of medical interest here, because herbs and spices often contain high concentrations of biologically active substances, and they are therefore of great interest to the drug industry. In addition, like the vegetables we’ve just discussed, they have found their way into traditional treatments around the world. Some have been studied in clinical trials in diabetes, usually to explore their effects on blood glucose levels, but only in small numbers of people, so it’s very difficult to spot whether there is a genuine effect. Let’s discuss two that have achieved superfood status – cinnamon and turmeric – and have been more widely investigated as nutraceuticals.
Cinnamon hit the headlines in the mid-2000s and rapidly established itself as one of the earliest specific superfoods for reducing blood glucose levels in Type 2 diabetes. Since then, it’s sunk in the popularity stakes, but studies still surface occasionally. The clinical trials highlight problems with studies of all plant derivatives. First, because many plants exist in different species and varieties it’s very difficult to standardise the preparations used in a clinical trial. We say ‘cinnamon’, but there are four slightly different species used as spices in different parts of the world. Cinnamomum cassia (the main import into the USA) seems to be more effective in lowering blood glucose than the Sri Lankan forms – Cinnamomum verum or Cinnamomum zeylanicum. Botanical details apart, it’s clear that ‘cinnamon’ powder or extract may be a variable mixture of different plants. So, in addition to all the other reasons why clinical trial results vary, in this case the medically active substance being tested may not even be the same. Second, as well as the variation in the plants themselves, there is no agreement on the best dose, and different studies have asked participants to take doses that range between 1 g and 5 g a day. You see the problems. Finally, because cinnamon is widely available and cheap, major pharmaceutical companies are not interested in producing standardised products which would be needed for reliable studies. (Recall that drugs need to undergo clinical trials in thousands of patients to observe any meaningful effects and the largest trials of herbs and spices have only included about 100 people.)
The general view is that any variety of cinnamon has no meaningful effects on either fasting blood glucose levels or glucose levels after meals, and no studies have shown an improvement in long-term glucose levels (that is, a reduction in HbA1c). The highly authoritative USA National Center for Complementary and Integrative Health (see References, page 220) doesn’t recognise cinnamon for the treatment or prevention of diabetes. So, let’s enjoy it as … cinnamon (but sadly not in the form of a very high-sugar iced bun).
Turmeric is a yellow spice widely used across south and south-east Asia as a curry spice and natural food colourant, and also in traditional medicine. Its rebirth as a potential treatment for diabetes occurred in the early 2000s. Its active ingredient is curcumin, a relatively simple chemical isolated nearly 200 years ago. It has multiple interesting effects on biochemical pathways that may be relevant in Type 2 diabetes (and in cancer and neurodegenerative diseases), but although there are many studies in test-tubes and animals, there are no meaningful clinical trial results in any of these conditions. It is quite difficult to understand how it has become so spectacularly popular, even in the highly commercialised and competitive world of superfoods.
The list of spices and herbs supposedly effective in reducing blood glucose in Type 2 diabetes is huge, and includes:
None is supported by any clinical trial results that demonstrate useful blood glucose lowering properties, but many of them add wonderfully to the taste of food (and when used properly can reduce the need for seasoning with salt). Use and eat them all, and if clinical trial results do emerge that confirm some form of benefit, then we can always convince ourselves that that we were taking at least some of the right treatment all along. Putting all your therapeutically hopeful eggs in the fenugreek basket, however, doesn’t seem sound.
Key point: In spite of promising chemistry, no herbs or spices have been shown to meaningfully and consistently reduce blood glucose levels.
Finally, let’s turn to a varied group of supplements and minerals thought for many years to be useful in Type 2. Some have positive clinical trial results, but none has been approved because – as with the foods discussed earlier – trials haven’t been performed in sufficient numbers of patients, the doses aren’t standardised, and the results of small preliminary studies have not been consistent. I’ll choose a few that people with Type 2 have mentioned to me recently.
Vinegar – specifically apple-cider vinegar – is often taken to reduce blood glucose levels. Does it? Probably yes, but only slightly. There have been some reasonable studies in non-diabetic people and Type 2s. How does it work? Probably through the acetic acid in vinegar reducing the conversion of carbohydrate to glucose, especially if the carbohydrate is high on the glycaemic index. Vinegar therefore lowers glucose levels only during and after meals – not overnight. A clever experiment showed that it’s the acid that matters, not specifically the acetic acid found in vinegar, because sodium acetate, a salt that isn’t itself an acid, has no blood glucose-lowering effect. The trials have been very short – a few days – so any effect on longer-term blood glucose measurements isn’t known, but it looks as if it could reduce blood glucose levels after a meal by about 20% (for example, from 12 to 10 mmol/l) and if this was a consistent effect it could be meaningful. One trial used two teaspoons three times a day during each meal. Because all vinegar contains acetic acid, there’s no need to buy expensive apple-cider vinegar: any vinegar will do. If you’re going to try vinegar, make sure you don’t take tablets that claim to be vinegar – they don’t always contain what they say, and there’s one report of ‘vinegar’ tablets causing ulceration of the oesophagus.
Key point: A couple of teaspoons of vinegar with food might help reduce peak blood glucose levels after meals containing high GI carbohydrates.
Resveratrol is found in grape skins. It’s undoubtedly a very powerful chemical, and in the laboratory has some anti-cancer action, though there are no clinical studies. It’s widely touted as a supplement for Type 2 diabetes. In 2016 a trial in Type 2s found that it did not help insulin resistance or liver fat (features of the metabolic syndrome, Chapter 3). There are no studies on blood glucose levels, but I’d be surprised if it turned out to have any useful effects.
We need these three fascinating metals in tiny (‘trace’) amounts in the diet. An atom of each sits at the middle of complicated biological substances, for example enzymes, which help chemical reactions to continue. Deficiency of any of these can occur, but extremely rarely, because such small amounts are needed (for example, in one study men, though not women, were found already to be taking more than the recommended amount of chromium).
Chromium and vanadium are needed for metabolising carbohydrate, fat and protein, and about 20 years ago vanadium salts (vanadate) were researched in the hope they would help blood glucose levels in diabetes. The chromium story goes back even further – it was identified as a ‘glucose tolerance factor’ back in the 1950s. Chromium is naturally found in some foods, especially meat, wholegrain products, broccoli and red wine. There have been several investigations of chromium salts (usually chromium picolate) in small numbers of people with Type 2 diabetes. The bottom line is that chromium supplements have almost no effect on blood glucose levels. Cholesterol levels don’t fall, and chromium doesn’t help weight loss either.
Interestingly, there is one specific food that seems to have high chromium levels – brewer’s yeast. You can get brewer’s yeast tablets in any health food store, and yeast extract – Marmite in the UK, Vegemite in Australia – is mostly brewer’s yeast. So, although there’s no point in eating Marmite to help blood glucose levels or boost your chromium intake, eat it if you like it. In 2017 researchers found that some indicators of brain activity improved after Marmite, but not after peanut butter, and its effect on an important brain neurotransmitter (GABA) may be the reason. But not all countries think Marmite is great. For a long time, Denmark didn’t allow sales of vitamin-foods such as Marmite, but in 2014 gave up and Danes can now eat it. Medically nothing has been proved, but no harm if you love it. This is the kind of discussion we should be having about most ‘superfoods’: if they are relatively low-carbohydrate and mesh nicely with the Mediterranean diet then they may be a tasty addition to meals. But individually none of them will save our lives.
Remember mega-dose vitamin C in the 1970s and ’80s? Vitamins are the permanent superfoods: they never go away, and each of them hits the headlines for a while. About 10 years ago the vitamin D bandwagon started rolling. Vitamin D had always been on the medical radar because it’s so important for maintaining bone strength. The ultraviolet part of the spectrum of sunlight is needed to convert cholesterol (yes, cholesterol) to vitamin D in the skin, so it’s not surprising that people in northern countries are often low in vitamin D, especially in the winter. But we had no idea that so many people didn’t have any detectable blood vitamin D. The story about the ‘sunlight vitamin’ exploded. Studies emerged every week linking vitamin D to countless conditions, especially the metabolic syndrome, hypertension and Type 2 diabetes. Then came the results of clinical studies in these areas – and, frustratingly, there was no benefit. We don’t hear so much about vitamin D these days, and it won’t help any aspect of your Type 2 (glucose, blood pressure, cholesterol).
The other main vitamins, A, the B group, C and E, all have important functions within different organs, but the body controls very carefully the amounts it retains (just as it does with trace elements), presumably because in excess they may be toxic. Type 2 patients with symptoms of numbness, pins and needles or pain in the feet and legs caused by peripheral neuropathy are often given B group vitamins, though there was never any evidence they helped. Doctors seem to love giving vitamins as much as patients love taking them. But there are warnings: in the mid-90s there was a huge trial of ‘antioxidant’ vitamins E and A taken separately or together in smokers who were therefore at risk of lung cancer. There was no benefit – but there was a higher overall death rate in the group taking a precursor of vitamin A (beta-carotene). Standard multivitamin preparations are probably harmless (and a huge clinical trial in Type 2 is currently in progress – see below), but I’d avoid high doses of individual vitamins.
Key point: If you have a good mixed diet, vitamin and mineral supplements aren’t needed, and we still don’t know whether multivitamins carry any health benefits.
Let’s end the chapter on a positive note, and could there be anything more positive than the potential health benefits of chocolate? Cocoa beans, from which chocolate is made, contain high levels of biologically active flavanols. One of the main flavanols in cocoa is theobromine, derived from the botanical name for the cocoa tree, Theobroma – Greek for ‘food of the Gods’. We’ve heard about biologically active compounds in spices and herbs. They haven’t fulfilled their biological promise in outcomes that are relevant to people, so is there anything different about cocoa flavanols? There may well be, because they don’t act on a specific biochemical pathway, but are general and powerful relaxants of blood vessels, and there are many good-quality small studies showing that they increase the pliability and elasticity of arteries. Since arterial disease is partly due to stiff arteries, and arterial disease is the underlying problem in most of the complications of diabetes, flavanoids may help reduce heart attacks and strokes.
The National Institute of Health in the USA was sufficiently impressed by the potential for cocoa flavanols to improve vascular health that it funded a huge clinical trial (the Cocoa Supplement and Multivitamin Outcomes Study, COSMOS for short). It will enrol 18,000 people over 60. Half will take a standardised preparation of 600 mg cocoa flavanols twice a day for five years, the remainder a placebo (dummy) capsule. At the same time, subjects will take either a multivitamin tablet or a placebo. Vascular events, especially heart attacks and strokes, will be carefully documented as the primary outcome, but the trial is sufficiently powerful that it can also track other important outcomes – for example, cancer. It isn’t specifically a trial in people with diabetes, though its large size means that many diabetic people will be included, and we know from other drug studies that any benefits in non-diabetic people are at least matched in people with diabetes. It should report toward the end of 2020 (and it will also help us decide whether it’s worthwhile continuing our multivitamin pill-popping habit).
The flavanoid content of chocolate varies widely and you can’t predict it from the stated percentage of cocoa solids (though milk chocolate and those with low cocoa levels will be higher in sugar, milk and calories). It looks as if you’d need to eat several ounces a day of dark chocolate (70% cocoa or higher) to even approach the flavanoid content of the capsules in COSMOS, and that would pile on your weight. But, as we’ve seen in several instances already, there are certain foods – including chocolate – that you can enjoyably include more frequently in your diet and that may eventually turn out to have health benefits in the long term.
Superfoods and nutraceuticals are always in the headlines. They contain a fascinating world of potential valuable substances, but none has been adequately investigated for its effects on diabetes – either short term on blood glucose levels or long term on the complications of diabetes. A few (for example okra and kerala) could usefully be incorporated into a healthy diet, but we should be eating lots of green veg anyway. Vinegar has a simple chemical effect that may reduce glucose levels after meals. Vitamins and minerals don’t help diabetes. The COSMOS trial will tell us whether medicinal use of chocolate with or without multivitamins can improve vascular outcomes.