Wednesday, April 6, 2011

Linoleic Acid and Melanoma

Dr. Briffa writes: "Could shying away from the sun increase our risk of melanoma?"

Seems a little odd that a condition under which we evolved could cause a fatal disease, right?  You'd think that some other variable would have to have changed to make the sun toxic...  Of course Briffa points out that sunlight isn't toxic, necessarily.

Somebody should look into whether any novel dietary components might be implicated in the rise in melanoma rates.  Oh, wait:

"A population-based case–control study of diet and melanoma risk in northern Italy" (PDF)

"We aimed at examining the association between dietary constituents and risk of cutaneous melanoma."
Well that's helpful.  What did they find, I wonder?  (Or you could re-read the title of this post, but humor me...)

"Results: We found an excess risk of melanoma in subjects with a higher energy-adjusted intake of total polyunsaturated fatty acids and, in particular, of linoleic acid (relative risk = 2.16 for intake in the highest tertile compared with the lowest tertile, P for linear trend = 0.061). Conversely, disease risk was inversely associated with the consumption of soluble carbohydrates (relative risk = 0.34 for intake in the upper vs. the lowest tertile adjusting for total energy intake, P for linear trend = 0.046). No other dietary factors, including alcohol, vitamins and trace elements, correlated with melanoma risk. The association of melanoma risk with linoleic acid and soluble carbohydrates intakes was further strengthened in multivariate analysis, and when analysis was limited to females.

"Conclusions: Overall, these results indicate that an excess energy-adjusted intake of linoleic acid and a lower consumption of soluble carbohydrates may increase melanoma risk"

This doesn't establish causation, of course, but since linoleic acid is already a proven carcinogen, this is not much of a stretch.

What's interesting is the inverse association with soluble carbohydrates, a.k.a soluble fiber.  Stephen Guyenet did a post a while ago on the benefits of fiber in the diet:

"It turns out, butyrate has been around in the mammalian gut for so long that the lining of our large intestine has evolved to use it as its primary source of energy. It does more than just feed the bowel, however. It also has potent anti-inflammatory and anti-cancer effects. So much so, that investigators are using oral butyrate supplements and butyrate enemas to treat inflammatory bowel diseases such as Crohn's and ulcerative colitis. Investigators are also suggesting that inflammatory bowel disorders may be caused or exacerbated by a deficiency of butyrate in the first place."

Butyrate is produced in the bowel by bacteria consuming soluble fiber, as one of the commenters on that post notes.

So if you're eating lots of processed foods, which feature linoleic acid as a preservative, effectively, and little fiber, you're in trouble.  If you're eating a whole-foods paleo diet, where you eating some fruits and veggies, and avoiding linoleic acid, you're reducing your risk, most likely.

This is a follow-up, of sorts, to this post: My Vitamin D Experiment, and is an explanation of why I'm not too worried about the cancer risks of going sunscreen-free.

I'll also note that sunscreen use has been going up at the same time that skin cancer rates have been increasing. (The conclusions of that paper are amusing.  Take a mouse known to get skin cancer from exposure to UV radiation, block UV radiation.  What do you think happens?)

If sunscreen prevents skin cancer, why is incidence still going up?  (This article notes the differences between different types of skin cancer and sun exposure.)

Folks are slathering their kids with enough sunscreen to give them rickets...