Don't Drink (oil) and Fry (in the sun) - the link between polyunsaturated vegetable oil and skin cancer

However, dietary ALA inhibited the increase in erythema score after UVB irradiation compared with LA. The peroxidizability index of the skin total lipids was significantly higher, but UVB-induced prostaglandin E2 (PGE2) production was significantly lower in the group fed an ALA-rich diet compared with the group fed an LA-rich diet. 

When polyunsaturated fat intake had been increased, tumorigenesis had been exacerbated and a remarkable persistence of immunosuppression remained measurable. There was no difference apparent in the CHS responsiveness in mice fed 20% sunflower oil or saturated fat in the absence of UV irradiation, indicating that the persistent immunosuppression was likely to have been induced by the carcinogenic irradiation regime.[2]

Diet 1 is 20% hydrogenated cottonseed oil, diet 5 is 20% sunflower oil

Now 20% sunflower oil is exactly the kind of exposure you'd get if you replaced other fats with oil in a low fat diet, as the experts want you to. It's an amount of LA that the epidemiologists at Harvard Chan have no problem with, using as they do data collection methods unreliable enough to produce false negatives as well as false positives, and not controlling for UV exposure anyway. Note that the control here - hydrogenated cottonseed oil - is 69.25% trans fat, the rest all SFA.

Here's another, where there are two controls - 12% menhaden oil (basically fish oil) and 0.75% corn oil.[3]
The most interesting finding is that while menhaden oil is protective, crossing over to 12% menhaden oil from 12% corn oil is not; it's about the worst thing you can do; crossing over from 12% corn oil to 0.75% corn oil seems safer.

In summary, we have shown that (1) high dietary level of an omega-6 FA source (corn oil) enhances photocarcinogenic expression, both with respect to tumor latent period and multiplicity; (2) that this lipid-induced exacerbation of cancer expression occurs at the post-UV initiation, or promotion, stage of carcinogenesis; (3) modification of diet to low lipid level (corn oil), after UV-initiation, negates the enhancement of cancer expression exhibited by high level of corn oil; (4) high level of dietary menhaden oil containing omega-3 FA, when compared to an equivalent level of corn oil, inhibits photocarcinogenic expression; and (5) menhaden oil mediated inhibition of UV-carcinogenesis appears to occur during the initiation stage and by a mechanism dissimilar to that exerted by low levels of corn oil.

These data suggest a complexity of dietary lipid effects upon cancer expression that perhaps has not been widely appreciated.

But here's my favourite - finally someone uses butter and ghee as controls.[4]


They're measuring ear swelling in response to a hapten test, which I assume is some sort of irritant. Low swelling means the immune system is suppressed, and this predicts that the risk of cancers is increased, just as it did in the other experiments. Note this took 4 weeks of a 20% fat diet - the protective effect of butter wasn't obvious after only 2 weeks, but the harmful effect of sunflower oil was.

Now, how do we know that this applies to human populations or has anything to do with the high rates of melanoma (the most fatal skin cancer) in New Zealand or Australia?
There is a natural experiment we can look at. NZ, unlike Australia, is not a land of extreme natural sunlight - it's the "land of the long white cloud", so that travel to the tropics greatly increases UV exposure. (However, we do have greater UV variability at our lower latitude, described by NIWA's Richard Mackenzie here.) Kiwis didn't travel much in the past, especially to the tropics, with one important exception; around 140,000 men (mostly) served overseas in WW2, around 10% of the population, and most of them served in places like Greece, Crete, Egypt, Libya, Italy and the Solomon Islands. Wearing shorts and short-sleeved shirts - US naval officials in the Solomons were appalled by the lack of protection the NZ sailor's uniform gave against flash burns - with only a Tommy helmet for shade (too hot and heavy to wear all the time). Sunburn on arrival in the combat zone is mentioned in memoirs - in the Armed Forces you stay where you're told and work where you're told, shelter or not.

Soldiers of the 2nd NZEF, 20th Battalion, C Company marching in Baggush, Egypt, September 1941.

The NZ diet, and British Army rations, at this time were very low in omega 6. In NZ, margarine wasn't even legalised until 1972. We cooked with butter, hydrogenated coconut oil, and beef and lamb dripping - even chicken and pork were unusual foods until the 70's. Some time during that decade the use of oil (mostly soy and corn oil at first) took off and was entrenched by the 1980's.
Now if exposure to the excess UV rays of the tropics, rather than the combination of UV and oil, caused skin cancer we'd expect to see an age-specific curve in skin cancer mortality in NZ that rose after the war (starting in the mid-1950's would match the usual cancer latency) then dropped as that generation began to die off from all causes, and a generation raised on "slip slop slap" UV protection took over.
Instead we see this:



And if you check different age groups in this part of the Mortality Trends website, you find the same pattern (most obvious where there is highest mortality), despite the different rates of overseas service in each age group, and the dying off of each generation - mortality from skin cancer climbs rapidly from the 1970's, when the NZ nutritional transition to high-PUFA oils began, and has not declined at all as the Greatest Generation dies off and the lesser slip, slop, slap-happy generations take its place (melanoma mortality in NZ has remained stable from 2001 to the present, I can't find earlier stats specific for melanoma but of course it is the most lethal skin cancer). This data is a better fit for diet than it is for UV exposure. (Of course there's the ozone hole, dating from the 70's, but "The ozone hole does not have a large effect on the concentration of ozone over New Zealand. However, when the ozone hole breaks up in spring, it can send ‘plumes’ of ozone-depleted air over New Zealand." The relative unimportance of this is mentioned on page 6 of Mackenzie's paper.)
Which is not to say that UV exposure has unlimited safety, of course this is not the case, but that heart-healthy vegetable oil and margarine advice has made even limited exposure more dangerous than it needs to be. Much the same phenomenon we see with alcohol and liver disease (and, who knows, alcohol and the risk of other cancers).