Cancer, diabetes, heart disease, dandruff... can these things really be cured, or at least prevented, by what we eat? As books that give answers go, there's one classic that often gets overlooked - the dictionary. So next time you're wondering whether a £10 tub of the latest miracle food can really stave off cancer, diabetes and heart disease, and get rid of a podgy belly in time for summer, run your finger down to the word "miracle" where you will find this definition: "an extraordinary and wondrous event" - so far so good - "that cannot be explained by natural or scientific laws".
"Whether it's coconut oil, chia seeds or apple cider vinegar," says Duane Mellor, an assistant professor in dietetics at the University of Nottingham and a spokesperson for the British Dietetic Association, "there is no scientific evidence to suggest that if you top up your diet with any 'miracle' or special food that you'll get any of the promised effects. The idea is almost entirely a marketing vehicle, but when people read claims online, they start to think differently and can start believing it." One of the reasons people might believe the hype is because as with any good miracle - or magic trick - the success lies in smoke and mirrors. With miracle foods, while the magical health food salesman is conjuring a few extra coins out of our pockets, we're left bamboozled by scientific terminology.
"Many products tend to be accompanied by all sorts of horrendous scientific jargon, like 'maintains cognitive function'," says Mellor, "which are watery, scientific-style claims that people tend to read as being something meaningful to human health. Then there's antioxidants and free radicals, which are some of the most feared and misunderstood words used."
Free radicals are unstable elements that come spinning off any oxygen-using chemical reaction in the body. They are unstable because they are missing an electron and, in a bid to restabilise themselves, they steal an electron from elsewhere. This could be from the fats in cell membranes or from your DNA. The damage they do when bullying other elements into handing over an electron is called oxidative stress, and this can be associated with heart disease, cancer and diabetes.
Free radicals, however, are also involved in beneficial processes. They help to destroy invading bacteria and play a part in cell communication. To limit their role to only those things that benefit us, our bodies make things called antioxidants that, much like people standing outside nightclubs handing out hugs and hot chocolate to pacify drunken revellers, provide free radicals with the electrons they need so they don't cause damage elsewhere.
"But if you look at the antioxidants circulating in our bodies," says Mellor, "by far the most common are the ones we make ourselves - glutathione and uric acid - followed by vitamins A, C and E, which we get from normal food anyway. Many of the antioxidants in things like chia seeds are there to stop the plant oils going rancid, or to protect them from sunlight damage, and may not be that available to our bodies anyway. So although the EFSA [European Food Safety Authority] allows manufacturers to claim that their products are rich in antioxidants - because they are - manufacturers are not allowed to claim any health benefits. If you look carefully, it's sort of legalese what they end up claiming."
Even when used as supplements, antioxidants don't seem to provide any benefit. A large study published in the New England Journal of Medicine that followed nearly 10,000 people over an average of four-and-a-half years showed no benefit from vitamin E supplements in the prevention of heart disease. Studies for other antioxidant supplements have been equally discouraging.
Part of this confusion is because diet is complex. It's tough to tease apart the contribution of individual components because the nutrients in many foods become available to us only when eaten as part of a wider diet: studies have shown that only when we cook carrots can their beta-carotene become more available and; the lycopene in tomatoes is most readily available when they are eaten with oil.
But what about all the other vague claims about foods that can help you lose weight, or support a healthy immune function, or lead to a healthy heart? They all sound good and sort of make sense, don't they? According to Ali Khavandi, a cardiologist at the Royal United Hospital in Bath, these claims are vague for a reason - they are based on experiments carried out on animals or on human cells in a lab. They have not been shown to have any effect on people, and until such effectiveness is shown, he says, we should stay open-minded but cautious about exaggerated claims.
"As doctors I think we've taken our eyes of the prize," he says of the importance of a healthy diet in avoiding the major chronic diseases such as heart disease and diabetes. "For the past few years, at least for heart doctors, I think we've been more interested in the sexier side of preventing disease - new drugs, stents, and operation techniques - and we've left the diet arena a little unmanned. It's now been populated by unqualified people and celebrity health gurus spreading misinformation. As doctors I think we have an obligation to reassert an authoritative voice when it comes to healthy eating."
Yanking the spotlight back from celebrities and fad food products might be a difficult task. "The problem," says Khavandi, "is that the message we try to get across - which is based on proper, robust evidence that has been shown time and time again - is not very interesting to people. They have heard it all before."
The messages he is talking about include the fact that fruit and vegetables are good for you. As are wholegrain cereals and nuts. For fats, which you need, choose unsaturated fats such as olive oil and those directly taken from marine sources such as oily fish. Neutral foods, he says, are saturated fats like butter or coconut oil and unprocessed red meats - eat these in moderation and they're unlikely to do any harm. Stay away from excess white-flour products, processed meats, and trans fats such as vegetable oils and palm oils found in fast foods.
Simple enough advice on the face of it, but with sensationalised articles emerging daily about the benefits or dangers of specific foods, people get confused and lose sight of the simple messages. A complication nowhere more true than with cancer.
"There is certainly no such thing as an anti-cancer diet," says Justin Stebbing, a consultant oncologist and professor of cancer medicine and oncology at Imperial College London. "But I have patients asking me things about these foods all the time." He puts a finger on why cancer-busting food is such an appealing concept. "As a patient, disease makes you lose control. People immediately want to regain that control and a very easy way for them to do that is by diet, and they can get all sorts of things off the internet. We should understand that the internet is a double-edged sword and if we're looking for information we should go to reputable sites."
Such sites, says Stebbing, are NHS Choices, Cancer Research UK, Diabetes UK and the British Heart Foundation, which all give clear, evidence-based dietary recommendations. Duane Mellor has another simple rule of thumb for distinguishing cherrypicked claims from bona fide scientific evidence. "The EFSA is very clear - and very strict - about what health messages it allows companies to use in the marketing," he says.
"If you see a claim on a blog, and if it's persuasive and looks good, ask yourself why has the company not used it in their marketing? If the product really did prevent cancer or heart disease, do you not think it'd be plastered all over the packaging?"
Mythbuster: the facts about five 'miracle foods'
1. Coconut oil
The claim: Coconut oil is a saturated fat. While not the heart-clogging evil they were once thought to be, it would take a leap of faith to proclaim that they are good for you. A recent review of studies suggested that saturated fats raise levels of both good and bad cholesterol. The oil is predominantly a medium-chain triglyceride that, proponents state, might carry benefits for weight loss, but this claim has not been shown in human studies. Other suggestions for the benefits of coconut oil include helping blood glucose regulation and preventing strokes and Alzheimer's - again, none of these benefits have been shown in people.
Dietetics professor Duane Mellor's verdict: Probably best to enjoy a little coconut oil in a Thai dish occasionally rather than using it daily!
2. Apple Cider Vinegar
The claim: Doubtless a tasty condiment, but has been anecdotally linked with an eye-wateringly long list of potential health benefits in areas including: digestive disorders, sore throats, high cholesterol, indigestion, preventing cancer, dandruff, acne, energy boosting, cramps, and helping with blood sugar control. The EFSA, however, hasn't approved any of these claims. Many of the studies have been on animals or in laboratories using human cells.
Mellor's verdict: Vinegar is probably best kept as a condiment. Use it on salads instead of high calorie oils and mayonnaise and to add flavour to sauces to help reduce salt intake - it might help, not because of anything it contains, but because it would be replacing less-healthy foods.
3. Manuka Honey
The claim: A medical-grade version of this honey is used in sterile wrappings. As with most honeys it has hydrogen peroxide, which gives it its antibiotic qualities. It also has methylglyoxal, an antibacterial component, in much higher quantities than found in other honeys. Studies have suggested that manuka honey might help to ease symptoms of infections such as coughs, but it's not clear whether the honey is having an antimicrobial effect or whether it is just soothing like all syrups.
Mellor's verdict: Any of the claims for eating manuka honey, all of which have been rejected by regulators, are vague. Any health benefits must be balanced against the very high quantities of sugar compared with the very small amounts of these proposed active compounds.
The claim: This is another proposed miracle food for which regulatory agencies, this time the US National Institutes for Health, say there is not enough scientific evidence to support any health claims. Rejected claims include those relating to metabolic and heart disorders (eg blood pressure control and diabetes), and also mental health disorders such as anxiety, depression and ADHD. It does have useful nutrients - calcium, niacin, potassium, magnesium, B vitamins, iron, and essential amino acids - but the jury is out on whether your body can use these nutrients in plant form.
Mellor's verdict: Spirulina shouldn't be relied on as a source of nutrients. Rather than taking such supplements, it would be better to spend your money on vegetables and fruit - this will help to make your whole diet better rather than adding a supplement and not thinking about the food you actually eat.
5. Chia seeds
The claim: Packed with antioxidants, but many of these are of plant origin so less likely to be available to us. They have high omega-3 content, too, but our bodies are not great at using omega-3 oils from plants - it's best to get these oils from oily fish such as salmon. But for people who don't eat fish, chia and other seeds can be a good substitute. Other potential pluses are linked to their high protein and fibre content, which have led some to suggest they might help you lose weight by reducing hunger. However, two trials to date have shown no evidence of any benefit in terms of weight loss or reduced risk of heart disease.
Mellor's verdict: Chia seeds can add an interesting texture to bread. Linseed and hemp seed are also rich in omega-3 fatty acids, so chia is not unique and should be enjoyed more for its effect on texture rather than any particular health effects.