The Cross-Linking Theory of Aging is also referred to as the Glycosylation Theory of Aging. In this theory it is the binding of glucose (simple sugars) to protein, (a process that occurs under the presence of oxygen) that causes various problems.
Once this binding has occurred the protein becomes impaired and is unable to perform as efficiently. Living a longer life is going to lead to the increased possibility of oxygen meeting glucose and protein and known cross-linking disorders include senile cataract and the appearance of tough, leathery and yellow skin.
Indeed, you can see cross-linking in action now. Simply cut an apple in half and watch the oxygen in the air react with the glucose in the apple as it turns yellow and brown and eventually becomes tough.
Diabetes is often viewed as a form of accelerated aging and the age related imbalance of insulin and glucose tolerance leads to numerous problems; these have been called Syndrome X. In fact, diabetics have 2-3 times the numbers of cross-linked proteins when compared to their healthy counterparts.
The cross-linking of proteins may also be responsible for cardiac enlargement and the hardening of collagen, which may then lead to the increased susceptibility of a cardiac arrest.
Cross linked proteins have also been implicated in renal disorders.
It is also theorized that sugars binding to DNA may cause damage that leads to malformed cells and thus cancer.
The modern diet is of course a very sweet one and we are bombarded with simple sugars from soft drinks and processed foods etc. One obvious example to reduce the risk of cross-linking is to reduce sugar (and also simple carbohydrates) in ones diet. The positive ramifications to health of such a change are massive and far-reaching; they are even greater than those available from quitting smoking!