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
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?
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.
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.
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.