杏吧原创

LARGE daily servings of woolly mammoth meat and sabre-toothed tiger liver. Lashings of seal blubber, bison brain and bone marrow. Armfuls of fibrous wild vegetables, nuts and fruits. Use breads, grains and cereals sparingly.

No, you won鈥檛 find these recommendations on the back of your breakfast cereal packet any time soon, but if a handful of researchers had their way, you鈥檇 be seeing the modern-day equivalent: government-sanctioned exhortations to eat plenty of meat and fat 鈥� preferably from wild animals 鈥� and to drastically reduce the consumption of grains and cereals. Fruit and vegetables would continue as a healthy option. That diet, they claim, is closest to the one our bodies have adapted to through aeons of evolution as hunter-gatherers. And it will protect us better from the diseases of affluence, such as obesity, diabetes and cardiovascular disease.

The Food Pyramid (see diagram), the icon the US government plasters on every flat surface in an effort to inspire Americans to eat less meat and fat, 鈥渋s badly in need of repair,鈥� says Loren Cordain, an evolutionary biologist at Colorado State University in Fort Collins, and the key proponent of the eat-more-meat diet.

While some nutrition researchers are adamant that the Cordain camp has got it completely wrong, others believe that their ideas warrant some serious consideration. 鈥淧eople that ask weird questions and think out of the box never get on the [dietary guidelines] committees,鈥� says Joan Slavin, a nutrition researcher at the University of Minnesota in St Paul. 鈥淚t鈥檚 time for a discussion.鈥�

And as the academics debate whether or not to debate the issue, low carbohydrate diets have become the latest health fad. Take Doctor Atkins鈥檚 New Diet Revolution. Atkins argues that we are poorly adapted to deal with carbohydrates and so recommends a diet rich in proteins and fats as the secret to a skinny, healthy life. The book has spent three years on the New York Times best-seller list, and Al Gore and Jennifer Aniston are rumoured to be among its converts.

The idea that people would be healthier if only they ate what they鈥檝e evolved to eat is scarcely new 鈥� the French philosopher Jean-Jacques Rousseau championed this view back in the 18th century. But you hit a problem when you try to work out exactly what this diet should be. No one questions the idea that early hominoids, much like modern chimpanzees, ate mainly plants. But what about the period from very roughly 1.7 million years ago, when humans learned to hunt big game and changed their destiny forever?

Last year Boyd Eaton, a radiologist at Emory University in Atlanta with an interest in preventive and evolutionary medicine, got together with Cordain to answer this question. Together they reconstructed the diets of 229 hunter-gatherer societies, such as the Tiwi in Australia, and the Ona and the Blackfoot in the Americas. As their starting point, they used the 鈥淓thnographic Atlas鈥�, compiled during the 19th and 20th centuries by missionaries and anthropologists anxious to record a dying way of life as colonisation and agriculture steamrollered the last remnants of traditional hunter-gatherer societies.

Far from subsisting mainly on plants, most hunter-gatherers, the two researchers concluded, ate huge quantities of animal food. Over half of the hunter-gatherer groups got at least two-thirds of their calories from animal sources, with a whopping 28 to 58 per cent of that coming from fat. Only 4 per cent of the groups studied got more than two-thirds of their calories from plants. The Copper Eskimo and the Iglulik of the northern polar regions won hands down for what seems by modern standards to be artery-clogging excess 鈥� up to 95 per cent of their calories came from seal, whale, musk ox, caribou and other animals.

Cordain also argues that only minute amounts of the plant portion of ancient diets could have come from grains. The farming and processing technologies needed to turn grains into a ready source of nutrients only became available 12 000 years ago, at the most, in the Middle East and Mediterranean, and a mere 200 years ago in Australia with the arrival of Europeans. Before then it would have been too much effort to gather and grind wild grass seeds until they were digestible. Cereal grains may be the foundation of modern dietary guidelines around the world, but the reasoning goes that they could only have dominated the human diet for a minuscule portion of our evolution.

And they may be a far from ideal food source, judging by the ill health that accompanied the initial transition to eating more grains and cereals. Comparisons of skeletons dug up on the Atlantic coast of the US are telling. Those that date from around 1000 AD, when Native Americans switched to a corn-based agriculture, are smaller than older skeletons. Studies of skeletons from other early agricultural societies show signs of deficiencies such as anaemia. 鈥淎nywhere this transition occurs, just about, health declines,鈥� says Clark Larsen, a physical anthropologist at the University of North Carolina in Chapel Hill, who studied the Atlantic skeletons.

To Cordain and Eaton, all the evidence suggests that the perfect diet would be roughly 60 per cent animal and 40 per cent plant (mainly fruit and vegetables) versus no more than about 35 per cent animal according to modern dietary guidelines. That conclusion has been raising hackles ever since the two researchers announced their findings at the 10th World Conference on Food Science and Technology in Sydney last October.

A common reproach is that the lifestyles of the hunter-gatherers described in the Ethnographic Atlas may be nothing like that of ancient hunter-gatherers. Many were already partly Westernised 鈥� some used guns, for instance 鈥� while others had been forced from their ancestral lands by farming. What鈥檚 more, much of the Atlas was put together by cultural anthropologists who were enthralled by the hunting of wild beasts, while barely noticing duller enterprises such as plant gathering.

鈥淣one of these people were very interested in diet, and they hadn鈥檛 been trained in dietary [field data] collection techniques,鈥� points out one of Cordain鈥檚 most vehement critics, physical anthropologist Katharine Milton of the University of California in Berkeley.

Kangaroo fat

Cordain concedes that the Ethnographic Atlas is far from perfect, but points out that a handful of carefully executed modern studies of the most recently Westernised groups of hunter-gatherers corroborate the data.

Kerin O鈥橠ea, a nutrition researcher at Baker Medical Research Institute in Melbourne who is now at Monash University, carried out one of the best-known studies. She accompanied a group of urbanised, middle-aged Aborigines into the bush for a stint of living the traditional lifestyle of their youth. Not only did they eat considerable amounts of meat, but when they killed a kangaroo, they鈥檇 extract every last bit of fat from the carcass by eating high-fat organs such as the brain, liver, bone marrow and even abdominal fat itself. On a diet rich in meat it鈥檚 a bad idea to go easy on the fat, it turns out. (See 鈥淗old the bunny,鈥�)

Milton also questions the significance of a high meat diet during one short part of our evolution, especially if you consider that for 25 million years before we learned to hunt, our anthropoid ancestors survived solely on plants. The human gut, she points out, with its slow transit time, is adapted to eating slowly digested plant foods, just like the guts of all other primates. What鈥檚 more, humans can鈥檛 make vitamin C, a typical trait in animals that consume lots of plants.

But, counters Cordain, people also have several qualities that suggest a penchant for eating meat. Like pure carnivores, we lack enzymes to make the amino acid taurine, which is plentiful in meat. By comparison, herbivores have high levels of these enzymes. Humans also have low levels of other 鈥渉erbivore enzymes鈥�, including those for converting beta carotene into vitamin A (abundant in organs like the liver), and lengthening 18-carbon fatty acids from plants into the 20 and 22-carbon fatty acids that animals need. Even the structure of our gut differs slightly from the other great apes 鈥� we have a shorter colon and longer small intestine, more like a carnivore.

Milton is unimpressed. Our anatomical differences could just as easily have arisen from eating plants made more digestible by processing, she says. Meanwhile, the enzyme differences merely suggest that early humans ate enough meat to provide them with certain nutrients, not that the majority of their calories came from animal sources as Cordain suggests.

鈥淭here is no ideal diet,鈥� maintains Milton. Even today, 鈥減eople live on diets that are 90 per cent animal 鈥� or 90 per cent plant 鈥� and get sufficient amounts of all of the nutrients they need.鈥�

But perhaps the most serious obstacle to Cordain鈥檚 theories is that epidemiological evidence seems to suggest that too much meat is bad for you, whereas vegetarian diets based on grains and legumes are good.

Take, for example, a new meta-analysis of 28 000 vegetarians and 48 000 non-vegetarians from five landmark studies, including the Oxford Vegetarian Study (The American Journal of Clinical Nutrition, vol 70, p 516S). Over a 10-year period, the vegetarians were 24 per cent less likely than non-vegetarians to die of ischaemic heart disease, including heart attacks. A mountain of smaller studies backs up this finding, convincing many nutrition experts that Cordain, not to mention those fad high-protein diet gurus, are plain wrong.

鈥淚f Americans even remotely think they鈥檙e supposed to eat more animal fat and protein,鈥� concludes Milton, 鈥渢hey鈥檙e absolutely crazy.鈥� Even O鈥橠ea, who鈥檚 studied the high meat and fat diets of Aborigines, thinks it鈥檚 out of the question for your average sedentary city dweller to consider eating more meat.

Indeed, it was epidemiological studies that initially persuaded the US Department of Agriculture in the early 1990s to devise the Food Pyramid. Its goal was to spur Americans to lay off their burgers and chips in favour of diets low in cholesterol and saturated fats. Reach that goal, government nutritionists believed, and you鈥檇 improve the nation鈥檚 blood lipid levels and stem the rising tide of heart disease, obesity and other diseases of affluence.

But although people like Cordain and Eaton agree that the goal is a good one, they believe that the USDA and the health authorities in Australia and Britain are going about achieving it in entirely the wrong way.

For one thing, Cordain says, those epidemiological studies are flawed. They compare one bad diet (vegetarian) with other bad diets 鈥� essentially, a hotchpotch of every non-vegetarian Western diet you can think of, including the ever-popular Twinkies, Big Macs, and fizzy drink with not-a-vegetable-in-sight diet. The diet they should be testing is one that is not only heavy on animal foods and light on carbohydrates, but is made up of radically different types of these foods.

Heart-gagging

This is what distinguishes Cordain鈥檚 hunter-gather diet from the low carbohydrate fad diets. 鈥淭hey鈥檝e got one thing right,鈥� he says. 鈥淭hey elevate protein at the expense of carbohydrate. But unfortunately, that is only a small part of the whole picture.鈥� Take meat from domestic livestock. It contains heart-gagging amounts of cholesterol and saturated fats, and has high ratios of unsaturated omega-6 fatty acids to omega-3 fatty acids 鈥� another risk factor for cardiovascular disease. Substitute meat from wild animals such as deer or rabbit, which contains much less fat overall, and especially less saturated fat, and you鈥檒l reduce those risks. With game meat hard to come by, there鈥檚 always fish. And as a last resort, suggests Cordain, you should at least stick to free-range chicken or trim every last shred of fat from your beef.

Epidemiological studies on sedentary Westerners who eat game meat simply don鈥檛 exist, but other studies certainly suggest that eating animals isn鈥檛 always bad for you. A second look at that meta-analysis reveals that fish eaters suffer no more heart disease than vegetarians 鈥� and less than pure vegans. A now-famous 10-year study of 2000 Greenland Eskimos who subsisted mainly on fish and wild marine mammals reported not a single heart attack. And the Harvard Nurses Study, which followed 80 000 female nurses for 14 years, found that when other risk factors such as intake of saturated fat are equal, women on a high protein diet (in this case about 24 per cent of calories coming from protein) had lower rates of heart disease than women eating less protein. The reason for the drop in heart disease? High protein diets appear to improve blood lipids.

But embracing the full hunter-gatherer diet is not just a matter of eating the right sort of meat. You also have to radically cut back on the sorts of carbohydrates that flood the body with glucose, replacing them with meat, fat, fruits and vegetables. A constant deluge of glucose may predispose us to insulin resistance, a disorder in which the body鈥檚 tissues increasingly ignore insulin鈥檚 call to absorb glucose from the blood. Insulin resistance, in turn, encourages obesity, adult onset diabetes and even hypertension, which can lead to heart attacks and strokes. But whereas current guidelines merely recommend avoiding cakes and sweets, Cordain鈥檚 team maintains that you should cut out all processed carbohydrates 鈥� including those supposed mainstays of healthy eating, wholemeal bread, most rice, muesli, pastas, and so on.

Glucose dump

Processing technologies have made meaningless the whole notion of 鈥渃omplex鈥� carbohydrates that are supposedly digested slowly, says Jennie Brand Miller, a nutrition researcher at University of Sydney in Australia. 鈥淏y reducing particle size through milling, and by cooking, we鈥檝e made the carbohydrates more available.鈥� All but the grittiest wholegrain breads dump huge amounts of glucose into the bloodstream within minutes. By contrast, hunter-gatherers got most of their carbohydrates from non-grain unprocessed plants such as fruits and roots which are high in fibre and digested far more slowly.

We, too, should be sticking to foods with low glycaemic index, says Brand Miller. The index measures the surge in blood glucose after eating. So, for instance, although gram for gram grapes contain more glucose than a soft drink, they have a much lower glycaemic index, because you digest them far more slowly. Pasta and most breads and rice have a high glycaemic index.

And according to Brand Miller, it鈥檚 especially important to stop pushing grains, pastas and breads on people of non-European descent who are particularly susceptible to insulin resistance and diabetes (New 杏吧原创, 13 November 1999, p 38). 鈥淚t鈥檚 unethical to tell these groups to eat starch foods that are high glycaemic index,鈥� she says.

Barleys and legumes such as lentils are a better proposition because they have lower glycaemic indexes, but they can worsen our health in other ways, claims Cordain. All grains, including barley, have the sort of omega fatty acid profile that is associated with heart disease. And both legumes and grains contain plenty of proteins called lectins, which in some people may encourage immune cells to attack the body鈥檚 own tissue. The USDA itself acknowledges that consuming grain is a risk factor for coeliac disease, an auto-immune condition of the gut, and some researchers suspect it may help trigger more serious autoimmune diseases such as juvenile onset diabetes and rheumatoid arthritis.

In the end, of course, what we eat in the West at the start of the 21st century will likely be decided by a combination of what tastes good and what is available. It鈥檚 notoriously difficult to get people to follow dietary recommendations, flawed or not. And whereas your average person may be more willing to stick to a high meat diet, if everyone did it would pose its own problems: the Earth simply cannot support 6 billion human beings without relying heavily on grains and severely limiting animal consumption (see 鈥淧rotein at a price鈥�).

Confronted with that Malthusian gloom, Cordain pins his hopes on the future. Perhaps genetic modification could make plants-even cereal grains 鈥� more like animal produce, he suggests, for example by producing higher, heart-protective levels of omega-3 fatty acids. As Cordain sees it, the diet of our hunter-gather forebears merely provided the evolutionary imperative. It鈥檚 up to modern technology to make that diet available to everyone 鈥� not just wealthy yuppies following the latest trend in celebrity diets.