As a person ages their chance of developing certain diseases such as hardening of the arteries (atherosclerosis), diabetes, depression, osteoporosis and memory loss (Alzheimer's disease) increases. One of the theories that might explain why age increases the risk of developing these diseases involves a concept called oxidative stress. Oxidative stress is a process that occurs inside the cells in our body. All the cells in our body are constantly undergoing various biochemical reactions, which are important to the cell's life cycle. Byproducts of these biochemical reactions are molecules called free radicals or pro-oxidants. Pro-oxidants can be very damaging to the cell and lead to cell death. Cells also have molecules called antioxidants, which neutralize the pro-oxidants and protect the cell so it can continue functioning normally. Oxidative stress occurs when there is an imbalance in the pro-oxidants as compared with the antioxidants. Other studies have shown that diseases such as hardening of the arteries, depression, osteoporosis and Alzheimer's are associated with elevated levels of free radicals or pro-oxidants inside the cells. It is now possible to measure the levels of pro-oxidants in the cells using blood tests.