Energy value of foods is most often allocated by using the Atwater general factor system, in which the main food components – protein, fat, and carbohydrate – have a single energy factor, regardless of the food in which they are found or how they are processed. Carbohydrates and proteins are considered to contain four calories per gram, fat nine calories per gram, and the system also includes a value of seven calories per gram of alcohol.
However, this latest research, conducted by Rachel Carmody, a student in the Department of Human Evolutionary Biology at Harvard's Graduate School of Arts and Sciences, found that cooked food delivered more energy than raw.
Carmody said that given all the effort people put into processing food in different ways – grinding, slicing and pounding, for example – we don’t understand what effect these efforts have on the energy we extract from food.
“It is astonishing, since energy gain is the primary reason we eat,” she said.
She fed two groups of mice diets that consisted of either meat or sweet potatoes prepared in one of four ways – cooked and whole, cooked and pounded, raw and whole, or raw and pounded – and tracked changes in body mass and how much the mice used an exercise wheel. She said the results clearly showed mice gained more energy from the cooked foods than raw, and led her to question the use of the Atwater system for energy labeling.
Carmody said: “The system is based on principles that don’t reflect actual energy availability. First, the human gastrointestinal tract includes a whole host of bacteria, and those bacteria metabolize some of our food for their own benefit. Atwater doesn’t discriminate between food that is digested by the human versus the bacteria. Second, it doesn’t account for the energy spent digesting food, which can be substantial. In both cases, processing increases the energy accrued to the human. Such evidence suggests that food labels do not properly account for the effects of food processing.”
Human brain development
Carmody said that her findings could also call into question the commonly held belief that meat consumption in itself was the main contributor to ancestral human brain development, suggesting that cooking the food that was already available – such as meat and tubers – may have had a significant role in increasing energy availability.
Carmody's research could also inform how food scientists tackle obesity and malnutrition. She points out that recommendations to deal with either problem tend to treat the body as “an efficient digestion machine.”
“In fact, it’s not – but we now see that its efficiency is affected by food processing, particularly cooking,” she said.