Diseases and disorders of aging: an hypothesis.

The definition of two classes of age-associated diseases and disorders (age-dependent and age-related) is offered to provide an utilitarian hypothesis with preventive, therapeutic, and research implications. Age-dependent diseases and disorders are defined as those whose pathogenesis appears to involve the normal aging of the host. Mortality and morbidity from age-dependent diseases and disorders (e.g. coronary artery disease and Alzheimer's disease) increase exponentially. Age-related diseases and disorders, on the other hand, have a temporal relationship to the host but are not necessarily related to aging processes. They occur at a specific age and then decline in frequency or continue at less than an exponential rate of increase (e.g. multiple sclerosis and amyotrophic lateral sclerosis). A model is presented for this hypothesis which relates aging processes to age-dependent diseases and disorders.