Books: A green party of genocide - The Nazi War on Cancer by Robert N Proctor Princeton University Press, pounds 17.95, 380pp

They were enthusiasts for public health and preventive medicine. They believed that additives, preservatives and junk foods caused cancer, as did petrochemical waste and artificial lifestyles. They campaigned for healthier eating habits (more high-fibre bread, less meat). They set up mass X-ray screening campaigns, notably for tuberculosis and breast cancer. They promoted herbalism, homoeopathy, and other "back to nature" remedies. They were into ecological conservation (including "save the whale") and opposed torturing animals in lab experiments. Who were these enlightened champions of positive health and bio-correctness? They were the Nazis.

It has not exactly been a secret that Hitler and his followers held certain beliefs which tally rather chillingly with today's health drives and environmentalism. But it is thanks to the researches of the American historian Robert Proctor that we can appreciate just how far the Third Reich promoted alternatives in science, medicine and lifestyle, many of which - for instance, the idea that cancers have environmental causes - now have large followings among those in the West who might be called "politically correct".

In particular, Proctor shows in this highly original book that paid-up Nazi scientists, aided by others with Party approval, demonstrated the links between smoking and lung cancer. They even identified the dangers of Passivrauchen (passive smoking). Once a chain-smoker, the Fuhrer himself gave up the habit, calling tobacco "one of man's most dangerous poisons". Nazi propaganda gloated over the fact that, unlike Churchill with his cigars, Hitler was a non-smoker - in fact, a teetotaller and a vegetarian as well. Even his dog, Blondi, was a veggie!

The Third Reich sponsored far-reaching anti-smoking campaigns, banning cigarettes from many public places and from Luftwaffe premises (allegedly, smoking ruined the tail-gunner's eye). On-duty SS officers were not allowed to smoke, and tobacco advertising featuring sportsmen, seductive ladies and fast cars was verboten.

Of course, the anti-smoking crusade, while grounded on science, was wrapped up in the usual appalling Nazi mumbo-jumbo. Tobacco was dubbed the "wrath of the red man" against the Aryan germ plasm, while smoking was associated with "primitives" like gypsies and "degenerates" like Jews. The latter were also reckoned especially liable to stomach cancer, as a consequence of their "greed". Indeed, the race itself was called a "cancerous tumour".

Likewise, the campaign was patchy. Hitler knew that he could not deprive soldiers of their smokes (and anyway, the SS had its own cigarette combine). So he targeted women: those who smoked or drank during pregnancy were singled out, on account of what would later be called "foetal alcohol syndrome".

What drove the Nazis to pioneer new initiatives in medicine and science were their suspicions of orthodox science. Regular science was largely the work of Jews, and hence (as they saw it) utterly suspect. Once Jewish professors had been purged, German science was open house for cranks, quacks - and genuine alternatives.

Impressive in some ways, Nazi cancer science was riddled with contradictions. At the same time as it was claimed that artificial foods, pesticides and modern lifestyles were the key carcinogens, cancer was also defined as strictly racial. Some genetic stocks, like the Jewish, were far more vulnerable than others - although, ironically, it was accepted that Jews, being circumcised, had a much lower incidence of cancer of the penis.

It is such paradoxical details which make Proctor's book - on a sombre subject handled with a light touch - so gripping. What marks out Nazi science, indeed the Nazi mentality at large, are its bizarre contradictions: at once so rational yet mad, so modernistic yet reactionary, so progressive yet barbaric. Did you know, for instance, that the inmates at Dachau were made to produce organic honey?

Or take the Nazi hostility to animal vivisection. Proctor reproduces an astonishing poster showing packs of dogs and cats making the Nazi salute and howling "Heil Goring!" How easy it was to be anti-vivisection if you had no qualms at all about sacrificing those humans whose "lives were not worth living" in the name of scientific progress.

Did the Nazi health crusades do any good? It is hard to say, concludes Proctor. The low female lung cancer figures in the Third Reich were perhaps the product of sexist drives to stop mothers smoking. But if cancer rates were low overall, that owed less to the proclaimed Tabakdammerung than to wartime shortages. German "Camels" were said to be made of camel dung brought back by Rommel!

The strength of this book lies in the way it squarely faces the problem of our own reactions to Nazi ("tainted") science. Proctor comes neither to bury nor to praise it, wholesale, but to understand it.

Not all Nazi science was bad science. Soy beans (restyled "Nazi beans") are good for you, as is wholegrain bread, even if it isn't, as Nazi scientists boasted, the "final solution to the bread question". And just because Hitler was against tobacco, it does not follow that today's health campaigners are "health fascists" or "NicoNazis". Any such conclusion would be hopelessly simplistic and moralistic in a field where, as Proctor rightly insists, none of us can afford knee-jerk reactions, because none of us can afford to cast stones.

Roy Porter's history of medicine, "The Greatest Benefit to Mankind", is published by HarperCollins