According to the theory of biological aging, dna in body cells is gradually damaged through spontaneous or externally caused mutations.

The Cross-Linkage Theory or also referred to as the glycosylation theory of aging was discovered or proposed by Johan Bjorksten in the 1940s. According to this theory, the aggregation of cross-linked proteins can damage cells and tissues this slowing down the bodily processes that eventually results to aging. In recent studies, cross-linking is associated with age-related changes in the studied proteins. Furthermore, this theory stresses out that the binding of glucose to proteins can cause various problems. Once the said binding occurs, the protein becomes impaired which leads to its performance inefficiency. Living a longer life would also mean increasing the possibility of oxygen-glucose meeting and protein. Some of the known cross-linking disorders include senile cataract and the appearance of tough, leathery, yellow skin.