You've probably read about ‘superfoods.’ Garlic, green tea and oily fish are three examples. Are they really good for us - and if so, why? And if they can really affect our bodies, are there any possible side effects?
Do superfoods really exist?
We should be wary of ‘superfood’ stories in the media. There are three main reasons for this:
- How can researchers be sure that one specific food makes a difference? For example one large Japanese study appeared to show that drinking plenty of green tea halved the risk of advanced prostate cancer, However the men who drank more green tea also ate more soya, more fruit and more vegetables. So what was making the difference?
- What works in controlled conditions in the laboratory, with animals (the basic science which is the source of many media stories), doesn’t always work like that in real life, with human beings.
- It is expensive to carry out rigorous, long term trials, which compare one group of people who eat a particular food with an equivalent group who don’t. And research hasn’t always used consistent doses and quality.
The NHS provides a fuller explanation in its assessment of ‘Miracle Foods’.
What is the evidence for some of the foods popularly regarded as healthy?
Population studies have suggested that garlic may help prevent cancer – including seven different population studies reported by the National Cancer Institute in the USA. Clinical trials have been fewer and produced more mixed results. However a majority of the small number of clinical trials reported appear to have had some positive effect, suggesting the value of more research here.
Medline Plus reports that the Natural Medicines Comprehensive Database rates garlic as being possibly effective for: high blood pressure, hardening of the arteries, tick bites and fungal infections of the skin (via a garlic gel) – but possibly ineffective for diabetes, H.pylori and high cholesterol.
The University of Maryland Medical Center suggests 2 – 4 grams (cloves) per day as a generally safe amount to eat, unless you start to experience side effects.
Because garlic thins the blood it should be avoided if you are pregnant, about to undergo surgery or taking blood thinners. And avoid very high consumption, particularly on an empty stomach, as this can have side effects.
Green tea is low in calories and high in anti-oxidants (hence possibly helping boost the immune system and induce cancer cell destruction).
Some of the health claims haven’t been supported by evidence. For example it doesn’t seem to help prevent the symptoms of cold and flu or to help with weight loss and it isn’t yet proven to protect against Alzheimer’s.
However, research suggests it may help:
- lower cholesterol
- lower blood pressure
- reduce the risk of developing certain cancers (including breast cancer in women under 50 and prostate cancer) - but with mixed results for other cancers.
Three cups (or two mugs) a day is normally a safe amount. If possible drink green tea between meals, as it interferes with the absorption of iron from food.
Green tea is a blood thinner and can also reduce the absorption of folic acid. So avoid it if you are pregnant or already taking blood thinning medication. To prevent caffeine overdose, don’t overdo it. It is also probably safer to avoid it if you already have heart problems, kidney disorders, stomach ulcers or are receiving chemotherapy for prostate cancer.
Mackerel, tuna, salmon and trout are some of the oily fish readily available. They are rich in vitamins and minerals, including omega 3 fatty acids.That’s why the NHS recommends a healthy diet should include at least two portions of fish a week, including one of oily fish. However, the NHS also advises a limit of four portions of oily fish a week because it contains low level pollutants. And if you’re female and are pregnant or may become pregnant the recommended limit is two portions a week.
There’s current debate about the value of omega 3 fatty acids. Population studies have suggested health benefits, such as relatively low death rates from cardiovascular disease, in populations who eat a lot of fish, like Eskimos. And the US National Institutes of Health reports general agreement that we should consume more omega- 3 (found in oily fish) and less omega -6 fatty acids (found in many vegetable oils used for cooking) to promote good health.
Omega 3 may help reduce the risk of dementia, as part of a healthy diet. That’s one conclusion from a French study reported in 2007. It also appears to have a consistent but modest benefit in treating arthritis.
However, omega 3 may not, as previously believed, protect against cardiovascular disease or cancer. That’s according to a BMJ review published in 2006.
It may be that researchers have been looking in the wrong place. A systematic review of published research, reported in the BMJ found no evidence of a link between omega 3 consumption and reduced risk of stroke and vascular dementia (cerebrovascular disease).
However, what the review did find was a moderate but significant association between fish consumption and a reduced risk of stroke and vascular dementia. They suggest two possible explanations. First, fish contains a range of vitamins and nutrients (including vitamins D and B and essential amino acids and trace elements) and it may be these rather than omega 3 which have the health benefits. Second, each time someone eats fish they are not eating a food which might increase the risk of cerebrovascular disease, like red meat.
Similar findings emerged from a 2013 review of the incidence of type 2 diabetes. The review found no significant link between omega 3 from fish and the incidence of type 2 diabetes. However eating oily fish did reduce the risk of type 2 diabetes.
And the value of omega 3 should not be completely discounted. A 2014 meta analysis concluded that, for lowering blood pressure among people with hypertension who weren’t already taking treatment, omega 3 was as effective, and in some cases more effective, than other lifestyle related interventions.
- ‘Superfoods’ are a media exaggeration.
- However, some types of food appear to be healthier than others.
- Garlic may help protect against high blood pressure, hypertension, hardening of the arteries and possibly even cancer.
- Green tea may help lower cholesterol and blood pressure and reduce the risk of breast cancer (if you’re under 50) and prostate cancer.
- Oily fish may help protect against cardiovascular disease and type 2 diabetes.
Nadine Richards and Michael Baber
Published 18 June 2014, Review datw June 2017