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An aging body has changed in many ways, and not just in those obvious to visual inspection. The typical old body is identifiably different from the typical middle-aged body at the level of cells, genes and biochemistry: biochemical processes, gene expression, levels of molecular damage, cellular behaviors, cellular populations, and so on. Some of these differences are clearly causally linked - a wide range of age-related changes can often be shown to be caused by a lesser number of underlying changes. For example, damage to mitochondria leads to oxidization of low-density lipoproteins (LDL), which in turn leads to detrimental changes involved in atherosclerosis, which is the principal cause of coronary heart disease and other forms of cardiovascular disease.
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