'Fast Eating, Swallowing Whole, and Insufficient Chewing' Found to be Independent Risk Factors for Adult Obesity and Metabolic Syndrome: A Comprehensive Explanation of Multiple Adverse Effects Including Decreased Diet-Induced Thermogenesis (DIT)

Various systematic reviews have confirmed that in the risk of developing obesity and lifestyle-related diseases in adults, not only the mere amount of calorie intake but also “insufficient chewing during meals (low chewing count) and fast eating” themselves contribute as independent and powerful risk factors. It is said that there exists a mechanism where a decline in chewing ability and the habit of eating fast cause a relative decrease in energy expenditure (decrease in DIT) and deterioration of digestive dynamics, ultimately linking directly to the weight gain process.

A comprehensive systematic review regarding the “association of decreased masticatory function and obesity in adults,” which forms the basis of this content, can be confirmed in the following literature.

Association of mastication and factors affecting masticatory function with obesity in adults: a systematic review | Archives of Public Health https://doi.org/10.1186/s13690-018-0261-2

3 Major Adverse Metabolic Effects Caused by Fast Eating and Insufficient Chewing

In modern diets, reliance on softly processed foods (soft diets) and liquid foods has advanced, causing a drastic decrease in the number of chews per meal. By integrating data from multiple epidemiological surveys and physiological experiments, it has been reported that fast eating caused by insufficient chewing aggravates obesity and metabolic abnormalities (metabolic syndrome) broadly through the following three pathways.

1. Occurrence of Chronic “Overeating (Calorie Overload)” due to Delayed Stimulation of the Satiety Center

The mechanical stimulation possessed by the act of chewing itself activates the histamine nervous system in the hypothalamus of the brain, inducing a strong sense of fullness (satiety) early on. However, in the case of “fast eating” or “swallowing whole” where one does not chew well, due to the lack of physical chewing counts, the meal is finished before the gastrointestinal tract is satisfied. As a result, it is pointed out that a large amount of calories is consumed before sufficient feedback signals are sent to the brain’s satiety center, and total calorie intake tends to increase automatically.

2. Slowdown in Energy Expenditure due to Relative Decrease in Diet-Induced Thermogenesis (DIT)

In clinical studies targeting healthy subjects, it has been found that even with foods having identical calories and nutrients, depending on the presence or absence of chewing behavior during intake, a clear difference occurs in the amount of heat generated in the body after a meal (Diet-Induced Thermogenesis). When swallowed without chewing, postprandial DIT significantly decreases compared to when swallowed with chewing. It is said that the accumulation of differences in these small daily energy expenditures over months to years directly leads to the accumulation of body fat as a result.

3. Induction of “Blood Sugar Spikes (Postprandial Hyperglycemia)” due to Influx of Undigested Matter

Large food boluses (a state where the particle size is large) swallowed without being chewed sufficiently have their contact area with digestive enzymes in the digestive tract physically limited. Due to this, irregular delays and rapid influxes of sugar tend to occur in the timing of digestion and absorption, and as a result, it is reported that the risk of inviting a rapid rise in postprandial blood sugar levels (blood sugar spikes) increases. Blood sugar spikes cause excessive insulin secretion, becoming the “trigger for obesity” that stores excess sugar as neutral fat in cells.

The Correlation Between Chewing Function and Metabolic Syndrome Indicated by the “Suita Study”

As epidemiological evidence backing up the aforementioned mechanisms, data from the “Suita Study,” a large-scale survey targeting about 600 middle-aged and older men, is cited.

In this survey, when individuals’ chewing ability (calculated from functional tooth units, etc.) and the risk of developing metabolic syndrome were analyzed, the group with low chewing ability showed an overt bias towards adverse events like the following compared to the high group.

• The morbidity risk of overall metabolic syndrome is independently and significantly high
• The worsening trend of each metabolic-related component, such as “hypertension,” “hypertriglyceridemia (high neutral fat),” and “fasting hyperglycemia,” is prominent.

These results suggest that the continuation of a “cannot chew / do not chew” state due to the loss of teeth from aging or daily soft eating habits functions not merely as a localized problem in the oral cavity, but as the gateway to a pathology leading to critical systemic lifestyle-related diseases.

Approaches Toward Prevention

It is concluded that in order to sever the vicious cycle toward obesity and metabolic syndrome caused by fast eating and insufficient chewing, it is essential not only to rely on restricting the calories of nutritional components but to intervene in the “way of eating” itself.

Specifically, approaches recommended are intentionally increasing the number of chews per bite and striving not to swallow until the food is completely liquified, or forcibly ensuring the chewing count by intentionally incorporating elastic foods and dietary fiber (vegetables, seaweeds, etc.) into meals. Furthermore, “planned chewing of gum” utilizing work hours and breaks is also highly anticipated to be utilized in future clinical settings as a preventive measure supporting back-end metabolic optimization through increased DIT and lowered stress hormones.