Fat-conscious spiders teach us diet sense

If you want to achieve a balanced diet, look no further than spiders.

No, you don't have to eat them. But Auckland University biologist David Raubenheimer and colleagues in Denmark, Britain and Israel have found that spiders and beetles choose their foods to maintain a healthy diet better than many humans do.

Their experiments have overturned the conventional wisdom that carnivores such as spiders eat as much as they can catch. They have found that spiders actually pick and choose which flies and other insects they eat to ensure that they get the right balance of proteins and fats.

Dr Raubenheimer and Sydney University colleague Steve Simpson have also tested humans and found that we also unconsciously choose a balance of proteins, fats and carbohydrates from a buffet.

Specifically, we seem to be biologically "hard-wired" to eat at least a certain amount of protein, found in foods such as meat and dairy products, and usually not much more than that amount.

But if we have to get that protein from high-carb modern processed foods, we are willing to eat much more fat and carbohydrates than we need. Unlike spiders, we lack a strong "override" control to stop us over-indulging on fat and carbs.

South African-born Dr Raubenheimer, 44, became a leader in the new field of "nutritional ecology" in 16 years at Oxford University in England. He is a keen, lean tramper and diver, and brought his young family to New Zealand last year to enjoy our outdoor lifestyle.

In his spider experiments, reported in the American journal Science on January 7, he fed two groups of wolf spiders with flies that had been raised on special diets to make one group rich in fats and the other rich in proteins.

He then gave all the spiders either fat or lean flies. The spiders that had previously eaten fat-rich flies and were then given more ate fewer of them than did the spiders that had previously eaten lean flies. But when they were given lean flies, the first group of spiders ate more of them.

In effect, the spiders picked their next meal to balance their last one.

"They have nutritional wisdom. They know what to select that is good for them."

Dr Raubenheimer believes that the problem with humans is that we evolved to rely mainly on protein-rich lean meat at a time, before the invention of farming, when foods rich in carbohydrates and fat were rare. Our hunter-gatherer ancestors got about 37 per cent of their energy from protein, compared with about 15 per cent today.

"We have an evolutionary history of feast or famine. We have evolved a thrifty metabolism that is very, very efficient at storing energy, and has a strong predilection for selecting foods that are rich in energy - foods that enable us in rich times to stock up on fat and thereby survive the lean times.

"At the same time, the amount of energy we spend in exercise is very low compared with what our hunter-gatherer ancestors would have spent, just to find and capture food.

"An office worker might typically spend 2000 kilocalories per day, whereas hunter-gatherers typically spend as much as 3000 kilocalories."

In this new world, with sedentary jobs and sweet and fatty foods all around us, our hard-wired predilection for fat and carbohydrate has become bad for us.

But Drs Raubenheimer and Simpson believe that our strict appetite for protein may help us to overcome the problem through high-protein diets, such as the Atkins and "Zone" diets, which give us the protein we need without too much fat and carbohydrate.

"We tend to feel satisfied when we have eaten enough protein and we don't tend to eat more beyond that. If you increase the proportion of protein, we eat less food overall," says Dr Raubenheimer.

"For long-term maintenance of a healthy lifestyle, we also need exercise and good amounts of fruit and vegetables, from which we get fibre, vitamins and minerals."

Spider wisdom

* Spiders that have eaten plenty of fats seek their next meal from prey that is low in fats and high in protein.

* Humans also unconsciously select a balance of proteins, carbs and fats from a buffet.

* We lack the spider's "override" that stops it eating too much fat and carbs, because our ancestors needed to store fat for lean times.

* But we do have a good ability to eat the right amount of protein, so a high-protein diet may help us to eat less and still feel full.