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?
This article provides some evidence for three types of food that are popularly regarded as healthy or as being superfoods: garlic, green tea, and oily fish.
- Do superfoods really exist?
- Evidence for garlic
- Evidence for green tea
- Evidence for oily fish
- Conclusions
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?
GARLIC
Prevention of cancer
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, while a 2020 meta-analysis of published research suggested that garlic may reduce the risk of colorectal cancer.
However, clinical trials have been fewer and produced more mixed results. The majority of the small number of clinical trials that have been reported appear to have had some positive health effects, suggesting the value of doing more research in this area.
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.
Reducing the risk of cardiovascular disease
Some research suggests that garlic can reduce the risk of cardiovascular disease. A 2020 overview study suggested that based on current research, garlic can significantly reduce the risk of atherosclerosis, hypertension, diabetes, hyperlipidemia, myocardial infarction, and ischaemic stroke.
Reducing blood pressure
Research also suggests that garlic helps reduce blood pressure. A systematic review of research, published in 2019 indicated that garlic supplements lower systolic blood pressure (SBP) to a similar extent as standard anti‑hypertensive medications. This finding follows a systematic review in 2015 which concluded that garlic appeared to be an effective and safe approach for hypertension.
Livestrong suggests two to four grams of garlic (about two to four cloves) per day as a generally safe amount to eat, unless you start to experience side effects.
Caution!
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, such as bad breath, acid reflux, digestive problems, and an increased risk of bleeding.
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Tip: Raw garlic that has been cut or crushed and allowed to sit for about ten minutes is more potent than garlic left uncut or garlic that has been cooked. |
GREEN TEA
Boosts the immune system
Green tea is low in calories and high in catechins (antioxidants). In principle this means it might possibly help to boost the immune system and induce cancer cell destruction. However, Cancer Research UK reports that there is not enough reliable evidence for its use in preventing certain cancers.
Colds and flu
We have found no significant evidence for the claims that green tea helps reduce the risk of colds and flu. However, a 2011 study from Japan which followed more than 2,000 school children through one winter season, suggested it may help reduce the risk of flu.
Another study published in 2018 suggested the possibility of the preventive effects of tea catechins on influenza and the common cold. Although the antiviral activity of tea catechins has been demonstrated in experimental studies, the clinical evidence to support their effect remains inconclusive.
Reduction in blood pressure
Overall, according to a systematic review of the evidence, published in 2020, green tea significantly reduced both systolic and diastolic blood pressure (the top and bottom numbers you see when your blood pressure is measured) over the duration of short-term trials.
Improved oral health
There is some evidence that green tea is good for oral health. A 2021 study reporting that green tea has a beneficial role in the prevention of periodontal and oral diseases by reducing inflammation, preventing the resorption of bone and by restricting the growth of certain periodontal-related bacteria.
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Three cups (or two mugs) of green tea each 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). |
Caution!
Green tea is a blood thinner and can also reduce the absorption of folic acid, a vitamin that is crucial during early pregnancy. So avoid green tea if you are pregnant, or if you are already taking blood-thinning medication.
OILY FISH
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.
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The NHS recommends that a healthy diet should include at least two portions of fish each week, including one of oily fish. |
Low levels of cardiovascular disease
Population studies have suggested that there are health benefits, such as relatively low death rates from cardiovascular disease (like heart attack and stroke) for 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 including cardiovascular disease, some cancers, Alzheimer’s disease and dementia, and rheumatoid arthritis.
Benefit to rheumatoid arthritis patients
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 by helping to reduce joint swelling and pain and shortening the duration of morning joint stiffness. In addition, a 2017 review of published research results, which it described as being of ‘moderate quality’, reported a significant benefit for rheumatoid arthritis patients (although not for osteoarthritis patients).
Fish consumption and a reduced risk of stroke and vascular dementia
Perhaps researchers have been looking in the wrong place, though. A systematic review of published research, reported in the BMJ in 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 eating a food which just might not increase the risk of cerebrovascular disease, like red meat.
Depression and all-cause mortality
Fish consumption is associated with a lower risk of cardiovascular disease, depression, and all-cause mortality (the death rate from all causes of death over a given time). That’s the verdict of a 2020 review of published research, although this noted that the evidence was of a ‘moderate quality’.
Reduction in the risk of diabetes 2
Whether oily fish reduces the risk of type 2 diabetes has been debated. However, a large-scale population study in the UK published in 2021 (with data from 392,287 participants), suggested that ‘consumption of oily fish, but not non-oily fish, was associated with a lower risk of developing type 2 diabetes’.
https://pubmed.ncbi.nlm.nih.gov/33431419/
Omega-3 supplements
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 these supplements might possibly have cardiovascular benefits but that their benefits will be minimal, if any. Instead, the authors recommend following a Mediterranean diet instead.
The most extensive review of research into omega-3’s effects so far (using data from nearly 163,000 participants), published in 2020, concluded that moderate and low-certainty evidence suggests that omega-3 reduces the risk of coronary heart disease only slightly.
An oily fish diet versus omega-3 supplements
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.'
Caution!
The NHS advises a limit of four portions of oily fish a week because oily fish can contain low levels of pollutants. And if you are female and are pregnant, or may become pregnant, the recommended limit is two portions a week.
The way fish is prepared may affect its health effects. For instance, fried fish is associated with a higher risk of heart failure.
Conclusions
- ‘Superfoods’ are a without doubt a media exaggeration.
- However, some types of food appear to be healthier than others, although the research is sometimes of moderate or limited quality.
- Garlic may help protect against high blood pressure and cardiovascular disease and (although the evidence is mixed) possibly even some forms of cancer.
- Green tea appears to lower blood pressure, to be good for oral health, and may possibly provide some protection against flu.
- Oily fish provide a mixture of fish oils, vitamins and minerals which may help protect against cardiovascular disease, depression, and type 2 diabetes.
Reviewed and updated by Kirulagini Sivamathavan and Michael Baber, January 2022. Next review date December 2025