Risk: The Science and Politics of Fear – Chapter 10, The Chemistry of Fear

Dans le chapitre 10 de “Risk: The Science and Politics of Fear” Dan Gardner expose les facteurs de risque de cancer couramment sous-estimés et les facteurs de risque couramment sur-estimés.

The same fact take the sting out of the statement Carson thought was so important she put it in italics: ‘Today, more American school children die of cancer than from any other disease.’ By 1963, traditional child killers such as diphteria had been wiped out. More children were dying of cancer than any other disease not because huge numbers of children were dying of cancer but because huge numbers of children were not dying of other diseases. (page 266)

But as age is the primary risk factor for cancer the fact that far more people were surviving childhood and living old age would inevitably mean that more people would get cancer – mostly when they were old – and so the ‘lifetime risk’ would rise. … Data on cancer were still sketchy in that era but in the previous two decades there was an apparent 200 per cent rise in the incidence of cancer among women and a 600 pe cent rise among men, which was mostly the result of a rise in only one type of cancer. Lung cancer ‘is the only form of cancer which shows so definite a tendency,’ the report noted. (page 266)

‘Exposure to pollutants in occupational, community, and other settings is thought to account for a relatively small percentage of cancer deaths,’ says the American Cancer Society in Cancer Facts and Figures, 2006. Of those, occupational exposures – workers in aluminum smelters, miners who dug asbestos under the unsafe conditions of the past – are by far the biggest category, responsible for perhaps four per cent of all cancer. The ACS estimates that only 2 per cent of all cancers are the result of exposure to ‘man-made and naturally occurring’ environmental pollutants – a massive category that includes everything from naturally occurring radon gas to industrial emissions to car exhaust. (page 269)

It’s critical to understand that not all carcinogenic chemicals in the environment are man-made. Far from it. To take just one example, countless plants produce carcinogenic chemicals as defences against insects and other predators and so our food is positively riddled with natural carcinogens. They are in coffee, carrots, celery, nuts, and a long, long list of other produce. Bruce Ames, a leading cancer scientist at the University of California at Berkeley, estimates that ‘of all dietary pesticides people eat, 99.99 per cent are natural’ and half of all chemicals tested – synthetic and natural – cause cancer in high-dose lab animal experiments. (page 270)

Major health organizations agree that traces of synthetic chemicals in the environment are not a large risk factor. What is hugely important is lifestyle. Smoking, drinking, diet, obesity, and exercise: These things make an enormous difference – by most estimates, accounting for 65 per cent of all cancers. (page 270)

‘You’re talking about studying humans and there are a million confounders. A study will say this and another will say the opposite’ … Ames cites the example of a controversy in California’s Contra Costa county. ‘There are a lot of refineries and there is more lung cancer. Ah, the refineries are causing the lung cancer. But who lives around refineries? Poor people. And who smokes more? Poor people. And when you correct for smoking, there’s no extra risk in the county.’ (page 275)

The media, in pursuit of the dramatic story, are another contributor to prevailing fears about chemicals. Robert Lichter and Stanley Rothman scoured stories about cancer appearing in the American media between 1972 and 1992 and found that tobacco was only the second-most mentioned cause of cancer – and it was a distant second. Man-made chemicals came first. Third was food additives. Number 6 was pollution, 7 radiation, 9 pesticides and 12 was dietary choices. Natural chemicals came 16th. Dead last on the list of 25 – mentioned in only nine stories – was the most important factor: aging. … The [American Institute of Cancer Research] noted with regret that only 49 per cent of Americans identified a diet low in fruits and vegetables as a cause of cancer; 46 per cent said the same of obesity; 37 per cent, alcohol; and 36 per cent, diets high in red meat. But 71 per cent said pesticides residues on food cause cancer. ‘There’s a disconnect between public fears and scientific fact,’ said an AICR spokesperson. (pages 277-278)

… cancer is primarily a disease of aging, a fact which has a profound effect on cancer statistics. The rate of cancer deaths in Florida, for example, is almost three times higher than in Alaska, which looks extremely important until you factor in Florida’s much older population. ‘When the cancer death rates for Florida and Alaska are age-adjusted,’ notes a report from the American Cancer Society, ‘they are almost identical.’ (page 281)

Consider chlorine. Treat drinking water with it and it creates by-products that have been shown to cause cancer in lab animals in high doses and may increase the cancer risk of people who drink the water. There’s even some epidemiological evidence that suggests the risk is more than hypothetical. So the precautionary principle would suggest we stop putting chlorine in drinking water. But what happens if we do that? ‘If you take the chlorine out of the drinking water, as was done in South America, you end up with an epidemic of 2,000 cases of cholera,’ says Daniel Krewski. And cholera is far from the only threat. There are many other water-borne diseases, including typhoid fever, a common killer until the addition of chlorine to drinking water all but wiped it out in the developed world early in the 20th century. (page 287)

If pesticides were banned, agricultural yields would decline. Fruits and vegetables can reduce the risk of cancer if we eat enough of them, which most people do not do even now. And so banning pesticides in order to reduce exposure to carcinogens could potentially result in more people getting cancer. (page 288)

Il est sans doute vrai que, comme Dan Gardner l’a noté tout au long de son livre, les médias ont tendance à exagérer la gravité des événements, à plus forte raison lorsque la promotion de la peur permet d’engranger d’énormes profits. Mais la situation actuelle est certainement moins réjouissante que ce qu’il pourrait nous laisser croire. Au chapitre 7, Fear Inc., l’auteur note un détail crucial :

On current trends, more people would get cancer than ever, and more would die from it. But what the ad didn’t mention is that this is because the population is growing – more people means more cancer – and it is aging, which means more cancer because aging is by far the biggest risk factor for cancer. The ad also failed to note that the death rate from cancer is falling and expected to fall further, nor did it mention that the incidence rates of most types of cancer – after taking population aging into account – are flat or falling. (pages 174-175)

Voici le problème. L’obésité est en expansion dans le monde, et l’obésité est aisément responsable de 30% des cancers, et cela, sans compter les autres risques de santé induits par l’obésité (Adams et al., 2006, figure 1 & 2, discussion p. 776 & 777). Néanmoins, la découverte par Adams et al. (2006) selon laquelle un BMI de 25, relativement à un BMI de 20, diminue les risques de mortalité a été vivement contestée par Zajacova et al. (2011, figure 1). La relation entre BMI et mortalité est monotone et non en forme de U.

Obesity Trends in the U.S. [1985 – 2006]*

National, regional, and global trends in body-mass index since 1980: systematic analysis of health examination surveys and epidemiological studies with 960 country-years and 9.1 million participants

Obesity Prevention, Nutrition and Physical Activity

Nearly 30 percent of cancers are attributable to obesity, including colon cancer, breast cancer and cancers of the esophagus, stomach, pancreas, uterus and kidney. Obese individuals have a 50 percent higher risk of dying from cancer than their healthy-weight counterparts.

Overweight, Obesity, and Mortality from Cancer in a Prospectively Studied Cohort of U.S. Adults

In a prospectively studied population of more than 900,000 U.S. adults (404,576 men and 495,477 women) who were free of cancer at enrollment in 1982, there were 57,145 deaths from cancer during 16 years of follow-up. We examined the relation in men and women between the body-mass index in 1982 and the risk of death from all cancers and from cancers at individual sites, while controlling for other risk factors in multivariate proportional-hazards models. […] On the basis of associations observed in this study, we estimate that current patterns of overweight and obesity in the United States could account for 14 percent of all deaths from cancer in men and 20 percent of those in women.

[…]

The numbers of deaths among men were sufficient to permit only the death rates from all cancers to be examined separately for the two highest body-mass-index categories of 35.0 to 39.9 and 40.0 or more. The relative risks of death for these categories, as compared with the group of men of normal weight (body-mass index, 18.5 to 24.9), were 1.20 (95 percent confidence interval, 1.08 to 1.34) and 1.52 (95 percent confidence interval, 1.13 to 2.05), respectively (Table 1). We observed significant positive linear trends in death rates with increasing body-mass index for all cancers, esophageal cancer, stomach cancer, colorectal cancer, liver cancer, gallbladder cancer, pancreatic cancer, prostate cancer, kidney cancer, non-Hodgkin’s lymphoma, multiple myeloma, and leukemia (Table 1). As compared with men of normal weight, men with a body-mass index of at least 35.0 had significantly elevated relative risks of death from cancer, which ranged from 1.23 (95 percent confidence interval, 1.11 to 1.36) for death from any cancer to 4.52 (95 percent confidence interval, 2.94 to 6.94) for death from liver cancer (Table 1). […] The results for the total population of women were similar. Women with a body-mass index of at least 40.0 had a relative risk of death from any cancer of 1.62 (95 percent confidence interval, 1.40 to 1.87), as compared with women of normal weight (Table 2). Significant positive linear trends in death rates were observed for colorectal cancer, liver cancer, gallbladder cancer, pancreatic cancer, breast cancer, cancer of the corpus and uterus, not otherwise specified, cervical cancer, ovarian cancer, kidney cancer, non-Hodgkin’s lymphoma, multiple myeloma, and “other” cancers (Table 2). The highest relative risk we observed was for death from uterine cancer (relative risk, 6.25 for women with body-mass index of at least 40.0; 95 percent confidence interval, 3.75 to 10.42).

[…]

We estimated the proportion of all deaths from cancer in the U.S. population that are attributable to overweight and obesity to be from 4.2 percent to 14.2 percent among men and from 14.3 percent to 19.8 percent among women (Table 4).

L’obésité entraîne un gros coût pour les soins médicaux et, fait intéressant, elle semble affecter davantage les individus des classes défavorisées, et par conséquent, à faible QI, même si le lien entre low-SES (ou medium-SES) et obésité ne semble encore valide que chez les populations blanches, alors que le lien entre low-SES et obésité chez les populations noires américaines est désormais passé de positif à négatif, et qu’il semble varier sensiblement au cours des décennies (Youfa Wang, 2011, page 27; Youfa Wang & Qi Zhang, 2006, figure 3; Youfa Wang & May A. Beydoun, 2007, figure 3).

Predicting obesity in young adulthood from childhood and parental obesity

The probability of being obese as a young adult increased with the age of the obese child and was higher at all ages for the group of very obese children (Table 2). After six years of age, the probability of obesity in adulthood exceeded 50 percent for obese children, as compared with about 10 percent for nonobese children. Obesity at one or two years of age was not associated with an increased risk of adult obesity. Overall, there were no significant differences between the sexes in the risk of adult obesity associated with childhood obesity (data not shown). The risk of adult obesity was significantly greater if either the mother or the father was obese (Table 2). There were no significant differences in these risk estimates between boys and girls (data not shown).

Genetic Epidemiology of Obesity

Twin, adoption, and family studies have established that obesity is highly heritable, and an individual’s risk of obesity is increased when one has relatives who are obese (27–29). Heritability estimates ranged from 16 percent to 85 percent for body mass index (30–34), from 37 percent to 81 percent for waist circumference (35–37), from 6 percent to 30 percent for WHR (38–40), and from 35 percent to 63 percent for percentage body fat (40–43). The Framingham Heart Study reported a moderate heritability estimate for body mass index (40–50 percent) (32). In contrast, the National Heart, Lung, and Blood Institute family heart study and twin studies observed higher estimates of heritability for body mass index (40–80 percent), and they also reported a heritability of 70–80 percent for weight gain (27, 44–46). Davey et al. (46) reported that the heritability estimate exceeded 90 percent for abdominal fat accumulation in an Indian population, while a family study in an Old Order Amish community showed a heritability of 37 percent for waist circumference and 13 percent for WHR (35). A twin study and HERITAGE (HEalth, RIsk factors, exercise Training, And GEnetics) Family Study reported similar heritabilities of 63 percent and 62 percent, respectively, for percentage body fat (41, 42), while the maximal heritability estimate in a Taiwanese population was 35 percent (43).

Obesity accounts for 21 percent of medical care costs

The Cornell study reports that an obese person incurs medical costs that are $2,741 higher (in 2005 dollars) than if they were not obese. Nationwide, that translates into $190.2 billion per year, or 20.6 percent of national health expenditures. The study appeared in the January issue of the Journal of Health Economics (31:1). Previous estimates had pegged the cost of obesity at $85.7 billion, or 9.1 percent of national health expenditures.

Il serait donc curieux que, comme l’affirme Dan Gardner, “the incidence rates of most types of cancer – after taking population aging into account – are flat or falling”. Il faut des années, des décennies pour que les cellules saines évoluent en cellules cancéreuses, pour proliférer et aboutir à la formation d’une tumeur, du fait des mauvaises hygiènes de vie pendant un long laps de temps : insuffisance de fruits et légumes, excès de viandes rouges, d’alcool, de tabac, de poids corporel, etc. On dit aussi que le manque d’activité est un facteur de risque de cancer, mais je pense qu’il y a des facteurs confondants comme l’obésité ou la malnutrition (à ne pas confondre avec sous-nutrition). La plupart des cancers détectés impliquent des personnes âgées. Si l’on contrôle l’âge de la population, population qui effectivement vieillit dans les pays développés, on est susceptible de réduire les incidences de cancer qui n’apparaissent que très tardivement dans la vie. Si l’on étudiait l’impact de l’obésité sur l’incidence du cancer en limitant l’échantillon aux enfants de 5-20 ans, on ne trouverait certainement aucune corrélation entre obésité et cancer. Je n’ai aucune raison de croire que l’âge est un facteur indépendant de risque de cancer. Il y a sans doute beaucoup de facteurs confondants, notamment l’hygiène de vie, à savoir, nous accumulons les risques liés à nos comportements et habitudes au cours de notre vie, et pour cette raison précise, le risque de cancer augmente avec l’âge.

La société n’est pas aussi malade que ce que les médias veulent nous faire croire, mais elle est plus malade que ce que Dan Gardner semble nous laisser croire. Si les taux de cancer sont en expansion dans le monde, ce n’est sans doute pas tant parce que nous vivons plus longtemps en moyenne, mais parce que nous nous alimentons assez mal.

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