Harvard study sheds new light on raw versus cooked food

A downside has emerged to research that shows harnessing fire for cooking is what turned human into relatively intelligent beings.

The evolutionary biology department at Harvard University has produced new evidence in the American Journal of Physical Anthropology that details how fats in foods yield more calories when cooked.

They have also found that cooking makes more energy available in starch and protein.

This research could explain how humans evolved with a combination of bigger body and brain – “a very metabolically expensive tissue,” says Rachel Carmody of the Harvard team – along with a smaller gut and positively bijou set of teeth.

These new physiques required more calories but implied consumption of less food through extracting more energy from the diet. But an exhaustive search for academic research into cooking’s effects on calorie intake drew a blank.

“We thought surely there must be an entire field of science devoted to this,” Dr Carmody says, “but finally we had to accept that this work simply hadn’t been done.”

Maximum carbohydrates
Free sugars, as found in sweets and fruit, are readily digestible but starch is indigestible when raw. Nowadays starch is most commonly associated with items that aren’t eaten raw, such as rice or wheat flour. But it is also present in plenty of salad ingredients. Peas, for instance, are delicious raw.

Starch is made up of long chains of glucose that can’t be metabolised. But cooking reorganises the structure, as it swells with water in a process known as gelatinisation.

Once gelatinised, Dr Carmody says, “our enzymes – primarily salivary amylase in our saliva and pancreatic amylase in our small intestine – can then attack the glucose.”

Different types of starch will provide varying amounts of calories when cooked. “In the research that we’ve done, it looks like you’ll get anywhere between 20 to 40% more calories based on cooking,” Dr Carmody says.

Protein power
Proteins are like balls of wool, consisting of long strings of amino acids, all coiled on to each other. Cooking untangles them, allowing digestive enzymes to cut into them and break them down.

This process is called denaturation. It breaks down the protein into a form that allows it to be absorbed in the small intestine. According to her findings, cooked protein provides around 10 - 20% more energy than raw.

The fat matrix
When it comes to fats (otherwise known as lipids), it’s all about busting through the matrix.

“A lot of nutritionists thought that fat is fat – we absorb all of it anyway, so how could cooking possibly have an impact?”

This led to perplexed nut manufacturers, who found through their own experiments that animals didn’t get as much energy out of raw nuts as the standard nutritional guidelines predicted. These more accurately reflected the calories in roasted nuts.

“If you’re eating a raw nut that contains a lot of lipids,” Dr Carmody says, “a lot of that lipid can’t be accessed because it’s inside cells that are not being broken down.”

Roasting a nut breaks the cells apart. But that’s not all. There’s a secondary matrix, concealing yet more fat.

“Little globules of fats swimming around in cells are themselves covered by a protein layer,” Dr Carmody says. But when the protein is denatured by cooking, that fat is liberated too.

However, despite this double-fat matrix, the difference in calorific content between raw and cooked lipids is less than in carbohydrate and protein. The Harvard team isn’t yet ready to talk numbers but it’s not insignificant.

Energy expenditure
Dr Carmody and her colleagues have also produced unpublished research showing how many more calories are expended in chewing and digesting uncooked food in relation to processed (by pounding) and cooked foods.

“I can tell you for sure that cooking has a big impact, as well as non-thermal processing. They both have a substantial reduction in the amount of energy that we spend digesting the food.”

As a guideline, Dr Carmody points out that, “typically in western society, we spend about 15% of our total energy budget digesting food, which is about the same as what we spend in locomotion – walking around, running, all the activities that we do.”

So eating raw rather than cooked food, she says, is comparable to having gone for that jog.

The dependence on cooked food is borne out by other studies into the effects of a raw food diet.

Even when processing some of their diet, through pounding or blending, strict raw foodists are underweight and 50% of women under 45 have stopped ovulating.

“They were so energy limited that their bodies shut down reproduction and, from an evolutionary biologist’s perspective, this is really mindboggling,” Dr Carmody says.