The ageing of societies around the world is progressing quickly. In particular, East Asian countries such as Japan, China and South Korea have shown a rapid ageing since 2000. Moreover, birthrates have been declining at the same time as populations have been ageing. In Japan, 2065 will be a time in which 1.3 people aged 15-65 years will support the lives of one person aged 65 years and over.¹ In such an era, a number of healthy and independent older people will be essential, meaning that the achievement of a healthy and long-lived society is indispensable. The dental profession must consider what it can help to achieve healthy longevity.

In recent years, there have been many reports on oral problems affecting general health. Many studies have been published on oral uncleanness and aspiration pneumonia,² periodontal disease and diabetes,³ the relationship among periodontal disease, oral function and dementia,^{4, 5} tooth loss and nutrition⁶ and more. In 2014, the research group at the National Center for Geriatrics and Gerontology presented the pioneering concept of a progression of systemic dysfunction through oral dysfunction.⁷ In the conceptual diagram of this report, the deterioration of oral function was coined "Oral Frailty." The term referred only to the oral cavity with mild symptoms such as decreased tongue motor function, slight coughing and spillage during meals, and an increase in the number of foods that could not be chewed. To recover from "Oral Frailty," it is important not only to maintain oral hygiene for preventing periodontitis and root caries but also to restore and maintain oral function with appropriate prostheses and eating habits. Tanaka et al reported that older people who showed signs of "Oral Frailty" were more than twice as likely to need long-term care or die than those who did not,⁸ which indicated that eliminating oral frailty and maintaining good oral function could help to delay the occurrence of frailty and dependency.

In 2016, the Japanese Society of Gerodontology published a position paper on "Oral Hypofunction" outlining conditions in which dental care intervention was advised to prevent the transition to whole-body frailty via deterioration of the oral cavity. Among the seven proposed criteria,⁹ "Poor oral hygiene" and "Oral dryness" reflected the environment of oral function. "Reduced occlusal force," "Decreased tongue-lip motor function" and "Decreased tongue pressure" pertained to the elements of oral function itself. The "Decreased masticatory function" and "Deterioration of swallowing function" related to integrated oral function. If three of those items were below standard values, the individual was considered having "Oral Hypofunction." Whereas "Oral Frailty" is a disease concept, and "Oral Hypofunction" is a condition that triggers the intervention of dentists. Two years afterwards, the "Oral Hypofunction" was adopted into the Japanese health insurance system, thus establishing a system that allowed all dental clinics to examine, diagnose and manage "Oral Hypofunction." The establishment of this condition is to help preemptively provide treatment so that the oral status of relatively healthy older people who can visit dental clinics does not get worse. Kugimiya et al¹⁰ reported that the rate of “Oral Hypofunction” in rural Japan was 43.6%. The adoption of "Oral Hypofunction" into health insurance system may allow dental care intervention in many older people and prevent the deterioration of oral function.

In Japan, the research on "Oral Frailty" and "Oral Hypofunction" has gradually increased. The association between "Oral Hypofunction" and frailty, sarcopenia and MCI^{11, 12} as well as recovery from "Oral Hypofunction" by exercise including a chewy diet¹³ have been reported, but the body of evidence on "Oral Hypofunction" is insufficient. Importantly, there remains a lack of both critical and supportive evidence on the concept of "Oral Hypofunction" itself. In particular, what kind of interventions and treatments can reverse "Oral Hypofunction," and can it be screened before occurring? "Oral Hypofunction" is intended for older people who live independently. However, can a similar concept be constructed for temporarily hospitalised patients or institutional residents?

Also, many dentists do not yet understand the condition of "Oral Hypofunction." I believe that this concept is important for protecting oral health, which may have a significant influence on the health of the whole body. For maintaining health and an independent life, I hope that many more people will become aware of this concept. But, how can the information of "Oral Hypofunction" be disseminated? Based on the above expectations, this Special Issue on "Oral Hypofunction" was planned, with many papers already having been submitted on this topic. As one of the advocates of "Oral Hypofunction," I am very pleased with the response of dental professionals and hope that this will lead to further scientific advancements on "Oral Frailty" and "Oral Hypofunction." It is my firm brief that the day will come when dentistry can significantly contribute to people's health and happiness.