Book 2 Listening (1108796), страница 23
Текст из файла (страница 23)
He suggests that itwas responsible for the existence of nervous systems in the first place, and that access tolarge quantities of the stuff was what permitted the evolution of big brains in mankind's morerecent ancestors.According to Dr Crawford, DHA's first job was to convert light into electricity in singlecelled organisms.
This gave them a crude form of vision, allowing them to move in responseto light and shade, but also brought into biology a way of controlling electrical potential. Iforganisms are to be multicellular, cells must be able to talk to each other. Electricalpotentials, the basis of every nervous system, are one way of doing this. And DHA was theenabler.The molecule is certainly ubiquitous.
Some 6oom years after animals becamemulticellular, more than half of the fattyacid molecules in the light-sensitive cells of the humaneye are still DHA, and the proportion of DHA in the synapses of the brain is not far short ofthat, despite the fact that similar molecules are far more readily available. Indeed, DrCrawford thinks that a shortage of DHA is a long-term evolutionary theme. The molecule ismost famously found in fatty fish. He suggests this might explain why, for example, dolphinshave brains that weigh 1.8kg whereas zebra brains weigh only 350g, even though the twospecies have similar body sizes.Furthermore, he argues that the dramatic increase of the size of the brains ofhumanity's ancestors that happened about 6m years ago was not because apes came out ofthe trees to hunt on the savannahs, but because they arrived at the coast and found a readysupply of DHA in fish.Not everyone, it must be said, agrees with this interpretation of history. For one thing,humanity's ancestors do not seem to have been exclusively coastal.
What they do agreeabout, though, is that substituting DHA with other, superficially similar molecules is a badidea.Joseph Hibbeln, a researcher at America's National Institutes of Health, has beenlooking at the supply to babies of DHA from breast milk and at genetic variation in the abilityto produce this molecule from other omega-3s. A study that began in the early 1990s hasshown that children who are breastfed have the same range of IQs, regardless of whetherthey have the ability to make their own DHA. In the case of those fed on formula milk low inDHA, though, children without the DHA-making ability had an average IQ 7.8 points lowerthan those with it.81Nor is intelligence the only thing affected by a Jack of DHA. There is also a body of datalinking omega-3 deficiencies to violent behaviour. Countries whose citizens eat more fish(which is rich in DHA) are less prone to depression, suicide and murder.
And new researchby Dr Hibbeln shows that low levels of DHA are a risk factor for suicide among Americanservicemen and women. Actual suicides had significantly lower levels of DHA in the mostrecent routine blood sample taken before they killed themselves than did comparablepersonnel who remained alive. More worryingly, 95% of American troops have DHA levelsthat these results suggest put them at risk of suicide.America's department of defence has taken note. It will soon unveil a programme tosupplement the diets of soldiers with omega-3s. The country's Food and Drug Administrationmay change one of its policies, too.Thomas Brenna, a professor of nutrition at Cornell University, has written a letter (cosigned by many of the scientists at the meeting) urging the agency to revise its advice topregnant and fertile women that they limit their consumption of fish.
This advice, promulgatedin 2004, was intended to protect fetuses from the malign effects of methyl mercury, whichaccumulates in fish such as tuna. The signatories argue that this effect is greatly outweighedby the DHA-related benefits of eating fatty fish.They may, however, be swimming against the tide. The popularity of omega-6-richfoods based on cheap vegetable oils will be difficult to reverse. Indeed, if another of DrHibbeln's studies proves true of people as well as rodents, it may be self-fulfilling.In this experiment he fed rats diets that were identical except that in one case 8% of thecalories came from linoleic acid (an omega-6 fatty acid) while in the other that value was 1%.These percentages reflect the shift in the proportion of omega-6s in the American dietbetween 1909 and the early 21st century.
In the 8% diet, levels of rat obesity doubled. It turnsout that in rats (and also in humans) linoleic acid is converted into molecules calledendocannabinoids that trigger appetite. Those who eat omega-6s, in other words, want to eatmore food. And since, in the human case, omega-6-rich food is much cheaper than omega-3rich food, that is what they are likely to consume.The way out of this vicious circle is not obvious.
Eating fish is all very well, but theoceans are under enough pressure as it is. Biotechnology might be brought to bearcreatinggenetically modified crops such as soyabeans with higher levels of DHA. Until that day,though, the best advice is probably that which was posted over the oracle at Delphi: "Nothingin excess". (From The Economist, May 29, 2010)Script 47. Nutrition and healthProtection racketEating lots of fruit and vegetables may not help stave off cancer, after all.For snivelling children and recalcitrant carnivores, requests that they should eat fiveportions of fruit and vegetables every day have mostly fallen on deaf ears. But those who didcomply with official advice from charities, governments and even the mighty World HealthOrganisation (WHO), could remind themselves, rather smugly, that the extra greens theyforced down at lunchtime would greatly reduce their chances of getting cancer.Until now, that is.
Because a group of researchers led by Paolo Boffetta, of the MountSinai School of Medicine in New York, have conducted a new study into the link betweencancer and the consumption of fruit and vegetables, and found it to be far weaker thananyone had thought.In the past, veggie-associated reductions of cancer-risk rates as high as 50% had beenreported. But it appears that some of these early investigations may have been biased by theuse of "case-control" studies.
Such studies try to identify the factors contributing to cancer bycomparing people who have the disease with those who do not, but are otherwise similar.The problem is that they can easily be biased if researchers do not adequately establish thatthe two groups being compared are, indeed, otherwise similar.Walter Willet, at the Harvard School of Public Health, says it appears that earlierinvestigations were more likely to use health-conscious people as their controls.
These types82of people are, unsurprisingly, more likely to agree to be interviewed about their health thanslob by couch potatoes.Dr Boffetta and his colleagues have therefore carried out a different kind of study,known as prospective cohort study, which they report in the journal of the National CancerInstitute. Their work follows a group of individuals over time and looks at how different factorscontribute to different outcomes - in this case, the development of cancer. Analysis of dietarydata from almost 500,000 people in Europe found only a weak association between high fruitand vegetable intake and reduced overall cancer risk.According to Susan Jebb, of the British Medical Research Council's CollaborativeCentre for Human Nutrition Research in Cambridge, the new study suggests that ifEuropeans increased their consumption of fruit and vegetables by 150g a day (about twoservings, or 40% of the WHO's recommended daily allowance), it would result in a decreaseof just 2.6% in the rate of cancers in men and 2.3% in women.
Even those who eat virtuallyno fruit and vegetables, the paper suggests, are only 9% more likely to develop cancer thanthose who stick to the WHO recommendations.On the face of it, that is quite a blow to the smug salad eaters, and the health lobby'sspin-doctors were out in force in the wake of the paper's publication, to play down itsconclusions. Before racing to the food-recycling bin with the contents of an ageing fruit bowl,they pointed out, there are a number of other factors that nutritionists would urge that youconsider.One is that this kind of study has attempted to adjust for every possible factor that mightcontribute to the relationship, and isolate only the contribution that fruit and vegetables make.This means that if people who turn away from fruit and vegetables end up eating moreprocessed meats or foods high in fat instead, they probably will increase their cancer risk,even though the direct cause is not the consumption of less fruit and veg.More importantly, there is still good evidence that fruit and vegetables protect againstheart disease and strokes by reducing blood pressure.
A separate investigation of the peopleinvolved in Dr Boffetta's study suggests that those who eat five servings a day of fruit andvegetables have a 30% lower incidence of heart disease and strokes than those who eat lessthan one and a half servings. It is also possible that some specific foods, such as tomatoes,broccoli and other cruciferous vegetables, do offer protective effects against particular kindsof cancer.As a consequence, the best advice is probably still to eat your five a day. But forsnivelling children and recalcitrant carnivores the fleeting thought that you might not have towas nice while it lasted.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
