A panel of international scientists have discussed, at a meeting for American Association for the Advancement of Science in Boston, “Why a Calorie Is Not a Calorie and Why It Matters for Human Diets.”
The researchers report that energy consumed from the human diet is consistently under estimated because features of the digestive process are ignored, including the activity of gut microbes, bioavailability and the metabolic cost of food digestion. The scientists note that this could lead to errors of up to 30 per cent. One of the scientists on the panel, Prof Richard Wrangham from Harvard University, states that it is time for a high-level panel to consider how best to improve the quality of information provided to the public about the real energy value of their foods noting that the current Atwater System is out of date. High fibre foods such as muesli contain more calories than stated, as the current system does not take into account calories from fibre. An average bowl of bran cereal contains on average 20 extra calories, while muesli contains 12 more. British nutritionist Geoffrey Livesey who was also on the panel states that generally the system uses calories conversion factoring which means a gram of protein or carbohydrate provides four calories, while a gram of fat provides nine calories. This works for highly digestible foods but assumes that fibre has no energy value to the body. He notes that fibre used to be mostly cellulose which was difficult to digest and would pass straight though the body however now fibre also includes pectins and soluble fibre which are broken down in the large intestine into compounds that provide energy for the body. He states this is equivalent to around two calories per gram of fibre. Wrangham states: “currently the standard method for determining dietary energy value (the Atwater system) systematically over-estimates the energy gain that is derived from relatively unprocessed foods.” Consumers could reduce their calorie intake by consuming raw instead of cooked foods as they are less digestible and take more energy to break down. Klaus Englyst from Southampton talks about “Bioavailability of carbohydrates.” He notes that “the ability to characterise dietary carbohydrates in detail and taking into consideration their bioavailability attributes, such as the rate and extent of starch digestibility, provides tools with which to better understand the impact of diet on metabolism and health.” Rachel N. Carmody from Harvard University, one of the panellists says that energy values often reported in scientific literature and on nutrition labels suggest that food processing has little caloric effect. However recent controlled experiments in her laboratory using animal models have found that processing by thermal and/or non-thermal means contributes importantly to energy harvest from plant and animal foods.