Using respirometry and electromyography, Van Casteren, Adam et al. (2022) investigated the energetics and evolutionary importance of human mastication. 


The net energy gain from food will change if the energetic cost of mammalian mastication changes. Mammalian mastication is a process combining simultaneous food comminution and lubrication. The associated metabolic costs of chewing various objects are unknown at this time, despite the fact that understanding the energetic efficiency of masticatory effort is crucial to understanding the evolution of the human masticatory system. Here, the researchers show that chewing by human participants represents a significant energy drain by using respirometry and electromyography of the masseter muscle. 

The efficient acquisition, processing, and consumption of food was essential to our ancestors’ survival, and modifications to their masticatory system were crucial to the evolution of our own species. For all endo-thermic creatures, including mammals that maintain a high and relatively constant temperature, the effectiveness of the feeding system is extremely important. One of the primary forces driving the evolution of mammalian mastication has been the need to optimize feeding in order to extract the most energy possible from food sources without wasting it on processing costs. This has resulted in significant morphological innovations in mammalian teeth, jaws, pharynges, and masticatory muscles. 

By repeatedly closing the jaws and pressing the teeth’s working surfaces into food particles, mastication is a kinematically complex process that involves both vertical and lateral movements of the mandible. This causes the food particles to be reduced to a tiny fraction of their original size. Chewing was first developed 260 million years ago and is present in many different animal groups. However, the exact occlusion and lateral movements that make mammalian mastication stand out as an unique evolutionary novelty have sparked dietary diversification and are likely to have aided in the spread of mammals around the world. 

The process’s efficiency is determined by the amount of energy required to break down food particles from their ingested size to what is swallowed. Adaptive modifications in masticatory morphology are thought, at least in part, to yield reductions in the work needed to create a given food particle size, which is hypothesized to have driven the evolution of variably complicated tooth morphologies and masticatory kinematics. Mammals must have the capacity to chew efficiently, breaking down food into small pieces with the least amount of effort and within a reasonable amount of time, depending on the specific selection pressures acting on the individual, when consuming foods where nutrients are not readily available, such as in many plant-based resources. 

Therefore, mastication in hominoids involves a variety of muscles that are used to varied degrees when breaking down food. There has been a significant amount of research on the bite forces and kinematic movements that are produced during mastication, however there has been little to no research into the energetics of mastication. This error is surprising given that the evolution of numerous musculoskeletal systems in humans has frequently been attributed to changes in energetics, particularly when such systems are linked to distinctly human characteristics like bipedalism. However, most of a hominin’s total energy expenditure is often accounted for by major, energy-intensive systems like locomotion or digesting (TEE). 

Goal of the study 

There has been little focus to whether the evolution of this reduced form has translated into meaningful differences in TEE, despite the fact that the modern human masticatory system is highly derived when compared to that of our extinct or extant relatives, with modern humans having smaller and more gracile dentofacial features. It is impossible to know how much natural selection has energetically optimized the human chewing mechanism without measurements of the metabolic cost of chewing, which makes evolutionary predictions about how energy has shaped the masticatory system of modern humans somewhat uncertain. The goal of this study is to ascertain whether employing the masticatory system has metabolic costs that are both quantifiable and significant in contemporary people. The study investigates the metabolic expenditures of chewing in humans and evaluates the potential effects of a relative change in the physical characteristics of the objects being masticated. 

We investigated the notion that changes in gum base mechanical qualities result in significant differences in the energetic cost of chewing by performing prolonged bouts of mastication on these gum bases. 

The study also assessed the amount of the energy cost of chewing as a fraction of human TEE and how this related to masseter muscle activation. The researchers can give context for how the energetic costs of chewing may relate to evolutionary changes in hominin masticatory systems over the evolution of humans by responding to these concerns. 


Controlling confounding metabolic costs, such as those related to digestion and the stimulus response from smell, taste, and familiarity, was important to separate the metabolic costs particularly connected with chewing and not the overall metabolic costs of feeding. This was done by having participants chew flavored and flavorless gum bases with various mechanical qualities (soft and stiff). 

Twenty-one human subjects, aged between 18 and 45, were enlisted: 15 girls and 6 males. Each volunteer first self-assessed their physical and oral health, and they were only asked to continue if they found no problems. If a prospective participant underwent major dental surgery within the previous 12 months, they were requested not to move on. Every participant in the trial provided written informed consent and the Medical Ethical Review Committee of Maastricht University approved the study (METC number 2017-0182). 

To enable proper data collection for evaluating the energetics of mastication, the digestive system’s movements had to be restricted. Prior to the experiment, we asked the participants to fast so that they would be in a post-absorptive state for BMR measurements. They were allowed to take any necessary medications up until midnight before the trial but were told to only drink water during that time. Just before the experiment began, their cooperation was verified. 

An alternate chewing substrate that was both flavorless and odorless had to be used in place of food. 

Metabolic measurements 

The indirect calorimetry technique was used to measure metabolic rates. Utilizing a ventilated hood system (Omnical, Maastricht University, Maastricht, Netherlands), which encloses a subject’s entire head in a Perspex hood and measures respiratory gases from samples of air that flow through the hood, oxygen consumption (V.O2) and carbon dioxide production (V.CO2) were continuously measured. Fresh air is continuously pumped into the hood at a rate of about 80 liters per minute. With a typical resolution of 0.001%, the device measures the whole airflow through the participant’s face and calculates the gas concentrations of O2 and CO2 for air that is inhaled and exhaled. (Dilution of flow with open-circuit respirometry) 


The findings are the first to show that chewing requires a significant amount of energy and that the stiffness of the substrate significantly affects the metabolic cost of mastication. When chewing the softer gum, individuals’ energy expenditure increased on average by 10.2% above BMR, and when chewing the stiffer gum, it increased by 15.1% above BMR. 

Fig. 2 shows the average respirometry data for both chewing conditions and BMR. Humans’ energy expenditure is influenced by chewing, with studies consistently showing a significant increase [F(2, 28) = 290.4, *P 0.0001] in energy expenditure compared to BMR (blue circles). When masticating on the stiff (purple triangles) gum base, more energy is expended than when masticating on more compliant substrates (soft, orange squares). Boxes stand in for the 25th and 75th quartiles, while dashed lines indicate means and solid lines represent medians. 

The data demonstrates that changes in metabolic energy are correlated with variations in the work being performed by the masseter muscle if the EMG mean voltage is used to measure amplitudes as a proxy for mechanical power. By framing the findings in terms of TEE and daily chewing periods, the results can be put into context. Human chewing times have been extensively studied, and a dataset of chewing times for 26 groups of adult modern people was created by Organ et al. The mean chewing time per day is 35.3 minutes, whereas the lowest observed chew time per day is 7.2 minutes and the highest is 75.7 minutes. Additionally, generalized TEE values for living apes, including humans, can be obtained in the literature. Therefore, it is possible to predict the percentage of daily EE that an average human male (TEE = 11,385 kJ) or female (TEE = 9163 kJ) would consume while chewing either of our substrates over the course of a full day of feeding using our respirometry data in conjunction with published chewing times and values. 

The estimated daily cost of chewing each gum based on our respirometry data is shown in Figure 5. We can calculate the daily chewing costs of our two gums by combining respirometry data, chewing times, and TEE values from the literature. It should be noted that the daily energetic cost of chewing is remarkably low for both substrates and both sexes. Boxes denote the 25th and 75th quartiles, while solid and dashed lines, respectively, represent medians and means. 

Even though chewing any of our experimental substrates does result in an energy rate that is noticeably higher than BMR, the daily cost of chewing—even for the longest chewing times observed in humans—is still quite modest, coming in at well under 1% of TEE. The use of tools, food processing, cooking, and agriculture has freed modern humans from protracted daily periods of mastication. 

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Van Casteren, Adam et al. (2022) The cost of chewing: The energetics and evolutionary significance of mastication in humans 2022 Science Advances 8 33 doi:10.1126/sciadv.abn8351