Watermelon is not only a deliciously sweet, refreshing fruit synonymous with summer days, it also has numerous health benefits. Among the most interesting may be its purported value to hair health. A number of people and media outlets have claimed that watermelon can increase hair growth and combat hair loss. Let’s dive deep into the evidence for such claims.
- There’s a view that watermelon promotes hair growth because it is a source of the amino acid citrulline, which could increase blood flow to the scalp.
- It’s also thought that the vitamin C content of watermelon means it can prevent hair loss by increasing the iron content of the blood and thus increasing oxygen delivery to the scalp, and by increasing the synthesis of collagen.
- These claims are not made without reason, but there is insufficient evidence to definitively prove the effects or the mechanisms accounting for them.
- Using the logic behind these popular claims, there are other nutritional properties which may also make watermelon good for hair, but likewise they have not been scientifically proven.
Basics of watermelon nutrition and hair science
Watermelon is approximately 92% water and 6% sugar, and yet the micronutrients and phytochemicals within it make it a nutritional powerhouse. In the table below is some key nutrition information about watermelon flesh (adapted from the Australian Food Composition database by Food Standards Australia New Zealand and USDA’s Food Data Central).
Keep in mind that there are hundreds of varieties of watermelon out there, each with its own unique nutritional composition. For example the classic red watermelon has a high level of carotenoids, while less are found in yellow, salmon, orange and white flesh varieties. The origin and climate of where a plant is grown will affect these things. Thus the information in this article is based on averages and should not be taken as representative of each watermelon.
|100g||152g (average mass of 1 cup diced watermelon)|
|Energy||31 calories||47 calories|
|Vitamin C||4-8.1mg^ (9-18% RDI*)||6-12.3mg^ (13-27% RDI*)|
|Vitamin A (as retinol equivalents)||49µg (7% female RDI*, 5% male RDI*)||75µg (11% female RDI*, 8% male RDI*)|
* RDI= Recommended Dietary Intake for adults, according to Nutrient Reference Values for Australia and New Zealand.
^Lower vitamin C value obtained from Australian Food Composition Database, higher from USDA’s Food Data Central. Values for other nutrients were very close, and so the average of the two are presented here
Hair follicles are amongst the most metabolically active tissues in the body, constantly producing and growing more hair. Thus nutrition could affect hair growth, although the precise mechanisms and links are complex and unclear. There is an easily understood link between low caloric or protein intake and hair (growth requires energy, and hair is primarily protein). From the above table, however, it is clear that watermelon is a good source of neither. Any effect is more likely mediated by micronutrients.
Watermelon and hair growth
Behind the claim that watermelon can promote hair growth is a bit of fancy footwork and connecting-the-dots between scientific factoids.
One factoid is that watermelon is one of the best food sources of the amino acid citrulline. Citrulline is converted into arginine (an essential amino acid) within the body. Arginine then stimulates nitric oxide production, which causes blood vessels to dilate (i.e. relax), thus increasing blood flow. Studies have demonstrated increased blood flow to many tissues (eg: brain, kidneys) following arginine delivery.
The theory is that watermelon stimulates hair growth through citrulline/arginine/nitric oxide induced increases of circulation to the scalp. However, I was not able to find experimental proof for the scalp receiving increased blood flow with arginine. That is, we can’t immediately assume that there is either increased blood flow to the entire body including the scalp, or if blood is preferentially sent to the scalp.
If we do assume for a moment that watermelon does increase blood flow to the scalp, it is not clear that this stimulates hair growth in itself- regardless of some claims to the contrary.
Future research may prove this effect on hair growth. If it does, it will also need to be determined how much watermelon is required to have this effect, remembering citrulline is an amino acid and is only one component of the 0.6g of protein/100g of watermelon.
Watermelon and hair loss
Watermelon flesh is described as a good source of vitamin C, and this has been taken to mean that watermelon prevents hair loss. There’s logic here: vitamin C helps your body absorb non-haem iron (i.e. the iron from plant foods), which means it can increase the oxygen-carrying capacity of the blood. Therefore, it is reasoned that more oxygen will be carried to hair follicles, reducing the chance of hair loss. Indeed, iron deficiency has been linked to hair loss (eg: it’s commonly seen among females who’ve experienced hair loss). However, we don’t know how iron deficient someone needs to be before hair loss occurs. There is also no definitive answer to how iron affects hair growth, but some studies have suggested that iron is involved in the regulation of genes found in the hair follicle, and that low levels of iron may limit the rate of DNA synthesis in normally rapidly-dividing hair follicle cells.
Despite these links between iron deficiency and hair loss, we do not have any data directly connecting them to vitamin C. Vitamin C deficiency has been linked to abnormalities in body hair- but that’s a different issue.
It is also said that vitamin C prevents weakening or loss of hair, via its crucial role in collagen formation. The primary protein of hair is keratin- not collagen, as is the case within skin– but collagen hydrolysate (especially from fish scales) has been seen to have antioxidant activity. This could be significant as oxidative stress can damage hair follicles. This antioxidant effect of collagen is also hypothesized to combat premature greying caused by oxidative stress.
There seems to be a bit of assumption about watermelon in this case, as there’s a couple of unproven steps between vitamin C being obtained from watermelon increasing collagen synthesis, and this collagen then being broken down or otherwise protecting hair from oxidative stress. I haven’t come across any research proving such events occur.
It’s also interesting to note that vitamin C itself is an antioxidant- and yet I didn’t come across any media claiming that watermelon directly protects against hair loss via its antioxidant effects. (If you have seen such claims, feel free to let us know.)
I would also query how powerful an effect watermelon can have on hair loss, should these links with vitamin C be proven in time. The 4-8.1mg of vitamin C provided by 100g of watermelon is dwarfed compared to the roughly 52mg in 100g of orange. (Of course, there may be further variations in vitamin C content according to the variety of fruit, but these figures are based on selected average available data).
Red watermelon is also a good source of lycopene (a carotenoid that is the strongest antioxidant found in fruit. Lycopene is also what gives tomatoes their colour.) Watermelon lycopene has a high bioavailability, meaning it is readily absorbed and used by the body. It is possible that this lycopene could exert antioxidant pro-hair effects, but this would need to be researched.
Watermelon is also a decent source of vitamin A. In mice, vitamin A has been seen to activate hair follicle stem cells. This effect has not been seen in people, however, and is likely moderated by a complex set of interacting factors. In contrast, over supplementation of vitamin A has been seen to actually lead to hair loss in humans. You’re far more likely to achieve such high levels of vitamin A from supplementation than eating watermelon, but it goes to show you don’t want to eat it in huge excess for its hair-related benefits.
The evidence that watermelon promotes hair growth and reduces hair loss is unconvincing. It is entirely possible that it does, but so far the claims are based more on basic scientific knowledge and extrapolation than actual proof. Feel free to include watermelon in your diet for its general health benefits and deliciousness, and if happens to lend its hand to your hair all the better, but don’t count on it as *the* answer.
What beauty topic would you like to know more about, from a health point of view?
Abedin, M. Z., Karim, A. A., Latiff, A. A., et al. (2014). Biochemical and radical-scavenging properties of sea cucumber (Stichopus vastus) collagen hydrolysates. Natural product research, 28(16): 1302–1305. https://doi.org/10.1080/14786419.2014.900617
Almohanna, H.M., Ahmed, A.A., Tsatalis, J.P. et al. (2019). The Role of Vitamins and Minerals in Hair Loss: A Review. Dermatol Ther (Heidelb). 9: 51–70. https://doi.org/10.1007/s13555-018-0278-6
Food Standards Australia New Zealand. Australian Food Composition Database- Release 1.0. https://www.foodstandards.gov.au/science/monitoringnutrients/afcd/Pages/default.aspx. (Accessed 2020, July 22).
Guo, E. L., & Katta, R. (2017). Diet and hair loss: effects of nutrient deficiency and supplement use. Dermatology practical & conceptual, 7(1), 1–10. https://doi.org/10.5826/dpc.0701a01
Klahr, S. (1999). Can L-arginine manipulation reduce renal disease? Seminars in Nephrology. 19(3):304-309
Morikawa, E., Moskowitz, M.A., Huang, Z., Yoshida, T., Irikura, K., and Dalkar, T. (1994)
L-arginine infusion promotes nitric oxide-dependent vasodilation, increases regional cerebral blood flow, and reduces infarction volume in the rat. Stroke. 25:429-435. https://doi.org/10.1161/01.STR.25.2.429
Seiberg M. (2013). Age-induced hair greying – the multiple effects of oxidative stress. International journal of cosmetic science, 35(6), 532–538. https://doi.org/10.1111/ics.12090
Trüeb R. M. (2015). The impact of oxidative stress on hair. International journal of cosmetic science, 37 Suppl 2, 25–30. https://doi.org/10.1111/ics.12286
U.S. Department of Agriculture (2020) FoodData Central Search Results: Watermelon, Raw. https://fdc.nal.usda.gov/fdc-app.html#/food-details/786754/nutrients. (Accessed 2020, July 22).