Robyn Williams: Today we kick off with one of those reminders that those in charge can so often miss out on the obvious, like when is a calorie not a calorie? The prompt comes from Richard Wrangham, Professor of Anthropology at Harvard.

Richard Wrangham: About 15 years ago my colleagues and I, as biological anthropologists, started realising that there was something very different about human and non-human diets in the sense of food processing being so universally important in humans, and, in the form of cooking at least, being completely unimportant in the diets of any other species except possibly some of the domesticated species.

And as non-nutritionists we then wanted to find out what was significant about cooking in human diets. We were amazed to discover that there was very little work indeed that summarised the importance of cooking for providing energy since, as animal biologists, we expected that energy would be the principal nutrient that was really limiting and important from the point of view of food choices and food preferences among creatures living in the wild. We wanted to find out what we could, and that led to a series of experiments in my lab that Rachel Carmody will describe.

Meanwhile, what we discovered is that if you go to the food labels and you go to the food tables the, as it were, official conclusion is that there is no difference between cooked food and raw food in the number of calories it provides. And what we in fact find is that there are all sorts of evidence that there is a big difference. So we wanted to convene a symposium in which we would explore the differences between raw and cooked food to some extent, but more generally the failures of the Atwater system in understanding the significance of caloric differences in foods.

Rachel Carmody: My role in the symposium is to sort of integrate these ideas. I'm looking at the energetic consequences of food processing. And the data is derived from a series of experiments that my collaborators and I have conducted over the last several years, the first of which was something we published a couple of years ago in PNAS where we fed mice diets of beef or sweet potato that had been processed in different ways.

And we were curious to know, is a calorie a calorie? And if that's the case then Atwater would have predicted that regardless of how these pieces of beef or sweet potato were processed, mice should gain weight the same when you account for differences in food intake and activity. But what we observed is that the mice who were eating the processed diet were fatter. They grew heavier than those that were eating the unprocessed items, and this is after controlling for differences in food intake and activity.

And so this led us to challenge the idea of the Atwater convention, and what we did was we calculated out the effective additional energy that was extracted from these processed diets, and we found that although Atwater predicts no difference with processing, cooking increased energy gain from beef by about 15%, and cooking increased energy gain from the sweet potato diet by 39%. And these differentials are not currently being reflected, either in the scientific literature or in the nutrition labels that are being presented to consumers.

So what we've begun to do on the back of this kind of study is to go in and say, well, if the Atwater system misestimated these effects, then it must be that there are things missing from the Atwater system. So we've gone in, we've started to probe the effects of food processing on bioavailability and on diet-induced thermogenesis, but essentially the short message is that by improving our estimates of bioavailability and our estimates of diet induced thermogenesis I think we could bring the Atwater system more in line with energy gains that are observed in the human body.

Robyn Williams: Rachel Carmody from Harvard. And the Atwater system, as you'll have inferred, is an index of energy and food, how much grunt in your grub. Going back to the 19th century, that system. But as Richard Wrangham says, it seems incredible that it took anthropologists studying cavemen to show up this anomaly.

Let's go back to what made you look at this question of calories in the first place. You were examining the way fire affected human beings way back. Was that the inspiration?

Richard Wrangham: Well, the inspiration actually was the fact that as a primatologist I studied chimpanzee feeding behaviour and I was thinking about the fact that chimpanzees represent a pretty good model for the kind of ape that would have given rise to the hominin lineage that gave rise to humans, and I often used to try and eat the chimpanzee foods, and I discovered that this was entirely inappropriate. I couldn't possibly survive, even for a few days, on chimpanzee foods.

And then it occurred to me that what was really important about human foods is that we cooked them, and that's what led me to try and find out what it is that's different between cooked verses uncooked foods, and this astonishing realisation that if you look in the official food tables there is no difference between the amount of energy you get from cooked and raw.

Robyn Williams: Of course one of the great things that you've talked about before is the ways in which having more energy from this cooked food, going back a million years, a million and a half years, certainly the whole of modern human history, is that we could then use this extra energy for things like growing a big brain, not that that wasn't necessarily the intention, but...

Richard Wrangham: Yes, not the intention at all, exactly, but hugely important. And in the last few years it's emerged ever more strongly that the human brain needs such a high rate of glucose flow that people find that you cannot sustain it on the basis of raw foods. What cooking does is to soften foods and enable you to chew it so quickly that it gets into the body fast. If we had to chew raw food at the rate that would be needed to fuel brains of our body size, we would be chewing for 12 hours a day and it would just be impossible.

Robyn Williams: Why is it that the official guides don't recognise this? They assume a calorie is a calorie is a calorie, to quote Gertrude Stein perhaps, and that there is no effect as you describe it. It seems unbelievable.

Richard Wrangham: One of the big things is that our official system does not take account of the cost of digestion, of the fact that when you eat your food then your body is paying all sorts of costs from the chewing, all the way down to the production of enzymes, the production of acid and the cost of transport across the gut wall. There's simply no account of that taken. And raw foods cost a lot more than processed foods. So why wasn't that done? I don't know enough about the history to really understand that. There was some appreciation of the fact that foods did cost something to digest. I think that a large part of it was that cooked foods tend to be fairly similar in how much they cost and people were not interested in raw foods.

Robyn Williams: Yes, it's interesting you say there's a difference between the sort of bread that you eat. Could you give me that example?

Richard Wrangham: There was a study published a couple of years ago suggesting that the difference between the cost of digestion for a multigrain bread sandwich with cheddar cheese versus white bread sandwich with American processed cheese was something like 64 calories difference as a result of the increased cost of digestion for eating multigrain bread and cheddar. So this suggests that even relatively small differences in processing, not cooked versus raw but multigrain versus white bread, could add up to a great way to slim.

Robyn Williams: So you actually get more calories from the white bread?

Richard Wrangham: Yes, you get more calories from the white bread as a net consequence because even though the total number of calories entering the body is the same, the cost of digesting the multigrain bread is so much greater.

Robyn Williams: How have you been following this up?

Richard Wrangham: Well, the traditional way of looking at the costs of digestion has been with the ideal animal model, which is a python. And Stephen Secor has been doing fantastic work in his lab in Alabama because pythons don't move at all and you just give them anything and put it into their cage and they will eat it, and after that you can measure the results very easily. But no one has measured the cost of digestion in mammals using cooked and raw food. So that's what we've done in our department. Rachel Carmody has led a study in which she has shown that rats are paying a greater cost to digest raw food in the form of raw meat than cooked meat, or raw sweet potato than cooked sweet potato.

Robyn Williams: A big difference?

Richard Wrangham: Yes, differences range from about 10% to almost 40%. And so this suggests that mammals in general would do this. We think that it's going to apply to humans, but that's the next level.

Robyn Williams: A final question. Now that you've sort of stirred the possum, do you expect the food industry to take notice of the fact that the calorie is not a calorie but they have to differentiate and make it plain to customers?

Richard Wrangham: I'm sure there are all sorts of huge reasons for inertia in the food industry, and nobody wants to be the first to make a move here. But I do think that we can add to the growing public interest in getting an increasingly accurate system of measuring calories. After all, there are companies like Weight Watchers that already acknowledge the fact that there is some difference between foods in the cost of digestion and therefore recommend the ones that are less processed. We can add to that by encouraging people to do the research that will quantify the effects better and slowly build a political movement that will lead to some kind of improved system of calorie assessment.

Robyn Williams: Thank you very much.

Richard Wrangham: Thank you.

Robyn Williams: Richard Wrangham is Professor of Anthropology at Harvard, and we had him talk about that chimp diet before and revealed but it's not just unending luscious fruit picked from trees but gritty, chewy, un-luscious stuff you wouldn't give a second glance.