In order to be able to digest our food better, we chew it up before swallowing it. But the chewing process itself also costs energy. A new study has now measured the energy consumption during chewing for the first time. Accordingly, the activity of the masticatory muscles increases our basal metabolic rate of energy by up to 15 percent, depending on how solid the food is. While these costs are negligible for modern humans, they may well have played a role for our early human ancestors, who did not yet cook food.
An important step in the digestive process is the chewing of food. The food is already broken down in the mouth in such a way that the nutrients it contains are more readily available for the further digestion steps. Although the use of the jaw muscles also costs energy, chewing has become established as a more energy-efficient alternative to swallowing larger pieces in the course of evolution. But how much energy does chewing actually cost?
Chewing gum for science
A team led by Adam van Casteren from the University of Manchester investigated this question. To do this, the researchers asked 21 subjects to chew gum in the laboratory. To ensure that the results were distorted as little as possible by any digestive processes stimulated by chewing, the researchers opted for tasteless and odorless chewing gum in two different degrees of firmness. They also instructed the subjects not to eat anything from the evening before the experiment. In the laboratory, they first measured the subjects' basal metabolic rate for 45 minutes while they lay relaxed on a couch. Depending on body weight and gender, this averaged 4.27 kilojoules per minute, i.e. 1.02 kilocalories, in the test group.
Next, the researchers had their subjects chew hard or soft chewing gum for 15 minutes and measured how the basal metabolic rate changed. The result: "Chewing either gum led to a significant increase in energy expenditure compared to the basal metabolic rate, with the harder gum requiring more energy than the softer one," the researchers report. When chewing the soft chewing gum, the basal metabolic rate increased by around ten percent to 4.69 kilojoules per minute (1.12 kilocalories), while chewing hard chewing gum by as much as 15 percent to 4.91 kilojoules per minute (1.17 kilocalories).
Relevant in human evolution
However, anyone who hopes to become slim by chewing has to be disappointed by the researchers: "Although chewing one of our test substrates causes a significantly higher energy rate compared to the basal metabolic rate, the daily costs of chewing are relatively low, even with the longest chewing times reported in humans and are well below one percent of the basal metabolic rate.” According to earlier studies, today’s people chew for hardly more than half an hour a day. “In addition, as modern humans we eat cooked food that we have previously worked on with tools. So we don't chew as much as our relatives and our early ancestors did,” says van Casteren.
Modern apes and probably our early human ancestors, on the other hand, probably spent much more time chewing. Studies indicate daily chewing times of around 4.5 hours for chimpanzees and even 6.6 hours for orangutans. "Such chewing habits are probably more representative of how much chewing work early humans had to do," the researchers write. In addition, the available food was probably significantly harder than both chewing gums used in the experiment, which may have increased the energy costs of chewing. More effective chewing and emerging technical possibilities to make food easier to chew and digest could therefore have played a decisive role in human evolution.
Source: Adam van Casteren (University of Manchester, UK) et al., Science Advances; doi: 10.1126/sciadv.abn8351