I was corresponding with JoAnn Farb who told me something interesting she has noticed about carrot juice protecting against sunburn:

“I discovered on several occasions that drinking large amounts of carrot juice and/or to a lesser extent — simply eating huge amounts of raw greens, reduces any observable changes in skin color (burning or tanning) when exposed to sun.

“I first noticed this on a canoe trip — years ago — I should have burned — was not using sunscreen and everyone else who didn’t use it did burn badly. But I had been drinking lots of carrot juice every day before hand. Then again in Australia, in January, in Sydney on the beach, Sarina [her daughter] was one year old at the time and pretty much nursing for all her calories. Each day over the noon hour I took her to the beach and let her crawl around naked — and I was in a swim suit. At first I just did ten minutes and then we put full clothing on. Each day I saw no burn and no tan. I increased the time more — until we were fully exposed for well over an hour and saw absolutely no reactions on our skin. There were juice bars everywhere; I was drinking two to three large glasses of fresh carrot juice every day.

“I have certainly had my share of sunburns at other times, from much less exposure, but have tested this repeatedly. Not only will carrot juice protect me and my family from sunburns, but if we have not been drinking carrot juice and do happen to burn, we will drink large amounts right away and the burn fades quickly.”

I have long known about the orange tint that people who drink a lot of carrot juice can get, but I was not aware of any sunburn protection. I checked it out and found research showing that beta-carotene does protect against sunburn:

Köpcke W, Krutmann J. Protection from sunburn with beta-Carotene — a meta-analysis. Photochem Photobiol. 2008 Mar-Apr;84(2):284-8. Epub 2007 Dec 15.

The study found that taking beta-carotene for 10 weeks, in amounts of about 57 mg per day, protected against sunburn. One cup of carrot juice has about 22 mg of beta-carotene.

The researchers said the SPF of beta-carotene was about 4. They thought the benefit from beta-carotene could be due to its antioxidant potential or interference in other parts of the biochemical pathways leading to sunburn.

JoAnn went on to ask:

“I have always wondered, since Michael Hollick mentions the pinking of the skin as a way to get an indication of how much vitamin D one is potentially making, does consuming all this carrot juice in some way inhibit vitamin D production? Or is the pinking of the skin merely a general way to gauge melanin amounts but otherwise not really related to biosynthesis of D?”

I’m afraid I do not know the answer to this and I could not find any research on it. I did, however, find one study in which higher intakes of beta-carotene were associated with less bone mineral loss in the elderly:

Sahni S, Hannan MT, Blumberg J, Cupples LA, Kiel DP, Tucker KL. Inverse association of carotenoid intakes with 4-y change in bone mineral density in elderly men and women: the Framingham osteoporosis Study. Am J Clin Nutr. 2009 Jan;89(1):416-24. Epub 2008 Dec 3.

It appears that in this case, beta-carotene was not noticeably interfering with any vitamin D formation in these people, though this is a very indirect way to try to answer that question.