Diet: Cutting through the confusion

Professor David Raubenheimer, a co-author of the latest scientific paper on the low-protein/high-carb diet, admits it's no wonder most of us are confused. And he thinks he knows why.

"It's because we're always trying to pin the blame on one nutrient," he says.

South African-born Raubenheimer, who lived in New Zealand from 2004 to 2012, has just published another paper with Sydney University colleague Stephen Simpson proposing a completely new approach to nutrition drawn from the field they both come from - evolution and ecology.

Raubenheimer has studied animals from insects to elephants and found that they have all evolved to regulate their own diets according to what they find in their environments. If their environment is short of protein, they will seek it out; and if it is short of carbohydrates they will look for more carbs.

But studies with humans have found that our dominant dietary drive is to eat just the right amount of protein. We feel hungry if we don't have enough of it, but we also stop eating it when we've had enough.

But if our diet is low in protein, we'll eat a lot of whatever we can find until we get enough protein. And that means we tend to eat more than we need of the other two broad food types - carbohydrates and fats.

Raubenheimer believes nutritional science has been fixated on blaming single nutrients because of its great success in pinning some diseases to specific nutritional gaps early in the science's history.

"Drop vitamin C, you will get scurvy. Add vitamin C, you won't get scurvy. A whole range of diseases respond like that. It was a golden era," he says.

Nutritionists naturally tried the same approach when the problem of excessive food consumption first arose in the 1950s.

First they blamed fat, then carbohydrates, and specifically sugar. Food companies responded with "low-fat", and then "reduced sugar", products. Yet we kept getting fatter.

Raubenheimer says the problem is neither the fat nor the sugar by themselves, it's the imbalance.

"What we have advocated is a balanced diet," he says.

"We are in the phase of mega food industries producing ultra-processed foods such as doughnuts, crisps and ice cream, and we have shown that what is diluting the percentage of protein is the increasing substitution of that category of foods into our diets."

He believes the change is no accident.

"Why are ultra-processed foods eaten in such large quantities that they transform traditional diets and lead to obesity?" he writes.

"The short answer is that they are designed to. The processed ingredients are assembled into products that are precision-engineered to achieve one goal: high sales."

Moreover, fat and sugar are cheaper than protein. Raubenheimer and Simpson show that the prices of foods are directly related to their protein content, so low-protein foods are easier to sell, especially to poorer customers.

Protein has shrunk dramatically in the human diet from 30 to 35 per cent of food intake for our hunter-gatherer ancestors to about 15 per cent in a typical Western diet today.

We eat roughly the same proportion of fat as the hunter-gatherers (30-50 per cent), while the invention of agriculture, not to mention the processed food industry, boosted carbohydrates in our diets from 30-40 per cent to more like 35-55 per cent.

The processed food industry's contribution to these trends seems relatively minor. Protein has dropped from 16.5 per cent of the average American diet in 1971 to 15.7 per cent in 2005-06.

But Raubenheimer says even that decline is significant.

"A decrease of just 1.5 per cent of protein will see a 14 per cent increase in absolute fat and carbohydrate," he says. Our drive for protein compels us to keep eating more of what's available until we get the protein we need.

He says a protein-heavy hunter-gatherer diet is actually no longer what we need, because scientists have found that too much protein accelerates ageing and "the degeneration of supportive coordination".

That didn't matter for the hunter-gatherers because they died early anyway for other reasons such as accidents, infections and starvation. But it matters for us.

"The work also shows why some popular diets that are designed to avoid carbs by increasing protein intake might in the long term be doing tremendous harm - for example the paleo diet," he says.

He endorses the official Australian and NZ advice that a balanced diet should comprise 15 to 25 per cent protein, 20 to 35 per cent fat and 45 to 65 per cent carbohydrates - as long as they are the right kind of carbs.

"If the carbs come from natural food sources that are tied up with fibre, that prevents us from over-eating," he says.

"If it's sugars and simple carbs with very little fibre content, then we will become fat."

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