Almost every month, a new piece of research appears linking diets high in processed “junk” foods with obesity and poor health. It isn’t yet clear if they have a cause-and-effect relationship. But deeper research is starting to appear from trials that compare diets which are based on either extremely processed foods or wholefoods (天然食品), yet are carefully matched for nutrients in all other ways.

The links need investigating (调查) as a matter of urgency. If these processed foods really do carry essential health risks, it could mean that official advice about healthy eating has been aiming at the wrong target for decades. In almost all high-income countries, nutrition guidelines say the key to healthy eating is avoiding too much fat, salt and sugar.

While many types of processed food contain significant quantities of these not-well-thought-of ingredients (成分), not all do, and there are wholefoods that are also high in some of them. Red meat and some dairy products come with their share of fat, for instance. It is still unclear if it is better to switch to “healthier” low-fat versions of processed foods, or to cook from scratch, whatever the ingredients.

Equally not clear is what action governments should be taking. Some campaigners are now calling for higher taxes on factory-made foods. That would be controversial, however, because these foods make up to 60 percent of people’s diets in countries such as the UK and the US. Additionally, any price increase is likely to hit lower-income households hardest, many of which consume more of such products because processed foods can be cheaper than making meals from their original ingredients, and the cost difference is even greater if you take into account the time taken to cook from scratch.

Rather than taxation, a most practical approach may be for schools to start teaching pupils how to make quick and simple home-cooked meals. This approach would take many years to bear fruit, as the encroachment (侵蚀) of processed foods into Western cuisine took place over decades. It isn’t going to be completely changed overnight.

Rats that eat high levels of a natural sugar known as fructose(果糖) seem to age faster than other rats-and the same could be true for people who eat too much sweet junk food, Israeli researchers said Monday.

Fructose, found naturally in honey and fruit, is used widely in foods ranging from soft drinks to yogurt. But while its sweet taste is popular, the sugar could cause wrinkles, the researchers said. Dr. Moshe Werman and Boaz Levi of the Technion-Israel Institute of Technology fed large amounts of fructose to laboratory rats. Writing in the Journal of Nutrition, they said the fructose-fed rats showed changes in the collagen(胶原蛋白) of their skin and bones. Collagen basically holds the body together. The loss of collagen is what causes loose skin and deep wrinkles in older people.

The process affected, Werman’s team said, is known as cross-linking(交叉相连) . “Too much cross-linking makes the skin stiff, and these are the conditions that encourage wrinkled skin,” Werman said in a statement. He said the same could be true of people, although this has not been shown. “Americans are eating more and more processed foods such as baked goods, canned fruits, jams and dairy products that contain fructose,” Werman said. Other studies have shown that taking in high fructose may cause diabetes(糖尿病).

The rats Werman worked with were fed much more than the average adult person might eat in a day, which is standard in such experiments. The rats were fed 12.5 grams of fructose per kg of weight every day for a year.

You are what you eat, or so the saying goes. But Richard Wrangham, of Harvard University, believes that this is true in a deeper sense than the one implied by the old saying. It is not just you who are what you eat, but the entire human species. And with Homo sapiens(智人), what makes the species unique in Dr Wrangham’s opinion is that its food is so often cooked.

Cooking is a human universal. No society is without it. No one other than a few crazy enthusiasts tries to survive on raw food alone. And the consumption of a cooked meal in the evening, usually in the company of family and friends, is normal in every known society. Moreover, without cooking, the human brain (which consumes 20-25% of the body’s energy) could not keep running. Dr Wrangham thus believes that cooking and humanity are contemporary.

In fact, as he outlined to the American Association for the Advancement of Science (AAAS), in Chicago, he thinks that cooking and other forms of preparing food are humanity’s “killer app”: The evolution made people such unusual animals.

Humans became human with the emergence 1.8m years ago of a species called Homo erectus(直立人). This had a skeleton much like modern man’s—a big, brain-filled skull and a narrow pelvis and rib cage, which imply a small belly and thus a small gut. Up till now, the explanation for this shift from the smaller skulls and wider pelvises of man’s ancestors has been a shift from a vegetable-based diet to a meat-based one. Meat has more calories than vegetable, the theory went. A smaller gut could therefore support a larger brain.

Dr Wrangham disagrees. When you do the sums, he argues, raw meat is still insufficient to bridge the gap. He points out that even modern “raw foodists”, members of a back-to-nature social movement, struggle to maintain their weight. Pre-agricultural manfeeding on raw food would have starved.

Start cooking, however, and things change dramatically. Cooking changes food in three important ways. It breaks starch molecules(淀粉分子) into more digestible pieces. It changes the nature of protein molecules, so that their amino-acid chains unfold and digestive enzymes(酶) can attack them more easily. And heat physically softens food. That makes it easier to digest, so even though the stuff is no more calorific, the body uses fewer calories dealing with it.

In support of his theory, Dr Wrangham, who is an anthropologist, has searched other fields and come up with an impressive range of material. Cooking increases the share of food digested in the stomach and small intestine(肠子), where it can be absorbed, from 50% to 95%. Previous studies had suggested raw food was digested equally well as cooked food because they looked at human waste as being the end product. These, however, have been exposed to the digestive mercies of bacteria in the large intestine, and any remaining goodies have been removed from them that way.

Another telling experiment, conducted on rats, did not rely on cooking. Rather the experimenters made food into small balls and make them softer. Rats fed on the softer balls weighed 30% more after 26 weeks than those fed the same weight of standard ones. The difference was because of the lower cost of digestion. Indeed, Dr Wrangham suspects the main cause of the modern obesity (肥胖) is not overeating but the rise of processed foods. These are softer, because that is what people prefer.