Healthy Foods - Garlic, Green Tea, Oily Fish?
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 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:
- 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?
- Because something works in controlled conditions in the laboratory, with animals (the basic science which is the source of many media stories), doesn’t always mean it will work like that in real life, with human beings.
- It is expensive to carry out rigorous, long term trials, comparing people's eating habits. 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. For example a study published in 2016 suggested that consumption of raw garlic reduced the risk of lung cancer in a Chinese population. Clinical trials have been fewer and produced more mixed results, although a majority of the small number of clinical trials reported appear to have had some positive effect, suggesting the value of more research here.
A 2015 review of published research into allium vegetables (like garlic and onions) suggested possible protective effects againts stomach cancer, colorectal (bowel) cancer and esophageal cancer, although research findings did vary. However, the authors concluded, 'if garlic consumption does reduce the risk of cancer, the amount needed to lower risk remains unknown.'
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.
Some research suggests that garlic helps reduce blood pressure. According to a 2012 review of two clinical trials, possibly not sufficiently to have a significant health value. However, a 2015 review of the research findings was more positive. It reported that garlic had a significant lowering effect on blood pressure, with no serious harmful side effects reported in the trials.
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 very high consumption, particularly on an empty stomach, can have side effects.
Green tea is low in calories and high in anti-oxidants. In principle this means it might possibly help boost the immune system and induce cancer cell destruction. However, the National Cancer Institute reports that the evidence for a possible protective effect against cancer is inconclusive. We haven't found much evidence for the claims that green tea helps reduce the risk of colds and flu but a 2011 study from Japan suggests it may help reduce the risk of flu in school children.
However a 2013 review of published research concluded that both green tea and black tea appear to help reduce blood pressure and cholesterol, so may help reduce the risk of cardiovascular disease (like heart attacks and strokes) - while suggesting longer term studies were needed to confirm this. NHS Choices confirms that green tea appears to reduce blood pressure and cholesterol but isn't sure if it does this sufficiently to have health benefits. However, it does suggest that a green tea mouthwash may help protect against tooth decay.
Three cups (or two mugs) a day is normally a safe amount. If possible drink green tea between meals (not after the meal, 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. 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 (A & D) and minerals, as well as 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.
Population studies have suggested health benefits, such as relatively low death rates from cardiovascular disease (like heart attack and stroke) in populations who eat a lot of fish, like Eskimos. And the US National Institutes of Health report that people who eat fish and other seafood have a lower risk of several chronic diseases.
The National Rheumatoid Arthritis Society reviewed 23 studies and concluded that fish oils appeared to have a fairly modest but consistent benefit for rheumatoid arthritis patients, helping reduce joint swelling and pain and how long morning stiffness lasted. While a 2017 review of published research, which it described as being of moderate quality, reported significant benefit for rheumatoid arthritis patients (although not for osteoarthritis patients).
However, omega 3 on its own may not, as previously believed, protect against cardiovascular disease. A review of research published in 2014, examining the effect of omega-3 supplements, concluded that they might possibly have cardiovascular benefits but that their benefits will be minimal, if any. Instead the authors recommend following a Mediterranean Diet instead.
It may be that researchers have been looking in the wrong place. A systematic review of published research, reported in the BMJ 2012 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 different vitamins and nutrients 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 may reduce the risk of type 2 diabetes.
There may be a downside to omega 3 consumption. Two recent studies have suggested a link between high levels of omega-3 in the blood and higher levels of prostate cancer - although there isn't yet a proven cause and effect relationship and it wasn't clear if the omega - 3 levels came from supplements or eating fish.
To put the question of oily fish versus omega-3 supplements in context, Dr Howard Lewine of Harvard Health Publications, commented, 'If we could absolutely, positively say that the benefits of eating seafood comes entirely from omega-3 fats, then downing fish oil pills would be an alternative to eating fish. But it’s more than likely that you need the entire orchestra of fish fats, vitamins, minerals, and supporting molecules, rather than the lone notes of EPA and DHA.'
- ‘Superfoods’ are a media exaggeration.
- However, some types of food appear to be healthier than others.
- Garlic may help protect against high blood pressure and (although evidence here is mixed) possibly even some forms of cancer.
- Green tea may help lower cholesterol and blood pressure and possibly provide some protection against flu.
- Fish oils probably help patients with rheumatoid arthritis but otherwise the benefits of omega-3 supplements are being questioned.
- Oily fish provide a mix of fish oils, vitamins and minerals which may help protect against stroke and vascular dementia.
Published July 2014. Reviewed and updated by Delia Morick July 2017. Next review date June 2020.