In the news: will eating grapes protect you against getting sunburned?Last Updated : 08 December 2022
Recent news stories reported that eating grapes could protect against UV damage (UV radiation is part of the natural energy produced by the sun) that can increase the risk of malignant melanoma, the most serious form of skin cancer. While it is well known that a diet high in fruit, vegetables, legumes, whole grains and nuts and seeds is beneficial to health, could eating grapes help us ditch the sunscreen? Before doing so, here are a few things to keep in mind when reading the headlines.
The study behind the headlines
The study behind the news was carried out by a group of researchers from the US.1 Their aim was to investigate whether eating grapes could enhance the resistance of the skin to UV light. Blocking or preventing exposure of the skin to UV light is the primary prevention strategy for skin cancer. They chose the grape due to its high content of phytochemicals (a group of natural components found in fruits and vegetables that may have beneficial health effects), more specifically the polyphenol resveratrol, which past animal research has suggested may protect against skin cancer.
The study included 29 healthy adults (average age of 43 years). Each person first completed a 14-day period where they began to restrict intake of certain foods and supplements (e.g., berries, dark chocolate and red wine). These foods were excluded from the diet because they contain similar phytochemicals as grapes which could potentially distort the results. Next, they completed a 14-day period of consuming a grape powder drink which was equal to 3 servings or 378g of grapes a day. Finally, each person completed another 14-day restricted diet period. Between each stage, participants received controlled UV exposure to a small section of their skin and the resulting redness was monitored. Urine, stool and blood samples were also taken.
The results showed that 9 of the 29 participants (31%) demonstrated greater resistance to UV-induced skin redness following grape consumption. Biochemistry tests showed that the 9 responders had lower levels of 3 particular urinary molecules compared to the 20 non-responders, which may indicate certain genetics influencing skin sensitivity. Furthermore, 3 of the 9 participants also had a sustained response (30 days after stopping the consumption of the grape powder) and showed a different gut microbiome pattern. The researchers concluded that ‘a segment’ of the population may have greater resistance to UV exposure as a result of grape consumption and that there may be a gut-skin relationship influencing this response.
What to keep in mind when reading the study’s conclusions?
- The trial involved few participants and only a small percentage responded to the consumption of the grape powder.
This was a small sample of 29 healthy US adults, who were all of the white or Hispanic ethnicity. Among this very specific sample, only 31% demonstrated possible UV resistance from consuming grapes. In fact, 36 people had started the trial, but 7 had dropped out for various reasons, although reported not related to the study intervention. Were they to be included in the analysis – and with no results available had to be assumed as non-responders – the response rate would only be 25%. Hence being a unique small sample, with a low response rate, it is a rather big jump to meet the media reports that eating 3 servings of grapes a day could protect people from sun damage.
- The experimental scenario of the study cannot be compared with real-life consumption.
Participants were consuming a concentrated grape drink daily for 14 days while at the same time restricting or abstaining from an extensive list of other foods. This included those thought to affect skin sensitivity (e.g., citrus, celery, red vegetables, olive oil) and those containing high amounts of certain phytochemicals (e.g., berries, cocoa, red wine), in addition to excluding multivitamins and other supplements. As such this does not reflect a real-life situation, where people are expected to eat a variety of fruit and vegetables as part of a balanced diet, and not restrict themselves to one type such as 3 servings of grapes every day.
- It is unknown what components within grapes may have brought about an effect.
Related to this, it is unknown what phytochemicals within grapes may have been causing any potential UV protection. It is unknown what the ‘required dose’ would be, and whether people would have to eat 3 servings a day to get the desired effect. Likewise, it is unknown whether this could be a unique effect of grapes, or whether other foods containing phytochemicals, could give the same effect.
- There is no evidence that grapes protect against melanoma or other skin cancers.
Finally, despite the media extrapolation to potential protection against malignant melanoma, this has not been investigated. It is unknown whether the effects of reduced skin redness seen in this small sample of people would translate to the risk of skin cancer.
What do authorities say?
- The World Health Organization (WHO) advises that a healthy diet to protect against non-infectious diseases (including cancer) should include at least 400g (5 portions) of fruit and vegetables a day, whole grains, legumes, less than 10% energy intake from free sugars, less than 30% from fats (preferably unsaturated), and less than 5g of salt.2 The WHO recommend that fruit and vegetable intake is varied.
- The World Cancer Research Fund (WCRF) report on diet, nutrition and skin cancer (2019) reviewed the evidence for different foods.3 The only food item with ‘limited – suggestive’ evidence for decreasing skin cancer risk was coffee. The WCRF state that the biological mechanisms linking coffee consumption to cancer were uncertain, but does say that coffee contains various active compounds, including certain phytochemicals. Meanwhile, there was ‘strong evidence’ that beta-carotene supplements were unlikely to affect risk. All other food items or supplements assessed had ‘limited evidence – no conclusion’. The WCRF also emphasise that UV exposure is the primary cause of skin cancer, with various other risk factors established (e.g., genetics, certain medications and infections).
- EUFIC provide information on polyphenols, discussing how they are not an absolute dietary requirement and that there is no official recommendation for intake.4 As they say, much of the evidence supporting a protective effect on cancer, as well as other diseases, comes from laboratory studies in animals or human cell cultures, making it difficult to establish their relevance to human health. These studies have also typically tested at much higher doses than would normally be consumed in the individual diet.
- Pezzuto, J. M., Dave, A., Park, E. J., Beyoğlu, D., & Idle, J. R. (2022). Short-Term Grape Consumption Diminishes UV-Induced Skin Erythema. Antioxidants, 11(12), 2372.
- World Health Organization. (2020). Healthy diet. Retrieved from https://www.who.int/news-room/fact-sheets/detail/healthy-diet
- World Cancer Research Fund. (2019). Skin cancer. Retrieved from https://www.wcrf.org/diet-activity-and-cancer/cancer-types/skin-cancer/
- EUFIC. (2015). Polyphenols. Retrieved from https://www.eufic.org/en/whats-in-food/article/polyphenols