Starting at what is commonly called middle age, operations of the human body become more vulnerable to daily wear and tear. There is a general decline in physical, and possibly mental, functioning. In the Western countries, the length of life often extends into the 70s. However, the upward limit of the life span can be as high as 120 years.

During the latter half of life, an individual is more prone to problems with the various functions of the body, and to a number of chronic or fatal diseases. The cardiovascular, digestive, excretory, nervous, reproductive, and urinary systems are particularly affected. The most common diseases of aging include:

• Alzheimer’s
• Arthritis
• Cancer
• Diabetes
• Depression, and
• Heart disease.

Human beings reach a peak of growth and development during their mid 20s. Aging is the normal transition time after that flurry of activity. Although there are quite a few age-related changes that tax the body, disability is not necessarily a part of aging.

Health and lifestyle factors, together with the genetic makeup of the individual, determine the response to these changes. Body functions that are most often affected by age include:

• Hearing, which declines especially in relation to the highest pitched tones.
• The proportion of fat to muscle, which may increase by as much as 30%. Typically, the total padding of body fat directly under the skin thins out and accumulates around the stomach. The ability to excrete fats is impaired, and therefore the storage of fats increases, including cholesterol and fat-soluble nutrients.
• The amount of water in the body, which decreases, reducing the body’s ability to absorb water-soluble nutrients. Also, there is less saliva and other lubricating fluids.
• Liver and kidney activities, which become less efficient, thus affecting the elimination of wastes.
• The ease of digestion, which is decreased, resulting in a reduction in stomach acid production.
• Muscle strength and coordination, which lessens, with an accompanying loss of mobility, agility, and flexibility.
• Sexual hormones and sexual function, which both decline.
• Sensations of taste and smell, which decrease.
• Cardiovascular and respiratory systems, with changes leading to decreased oxygen and nutrients throughout the body.
• Nervous system, which experiences changes that result in less efficient nerve impulse transmission, reflexes that are not as sharp, and diminished memory and learning.
• Bone strength and density, which decrease.
• Hormone levels, which gradually decline. The thyroid and sexual hormones are particularly affected.
• Visual abilities, which decline. Age-related changes may lead to diseases such as macular degeneration.
• A compromised ability to produce vitamin D from sunlight.
• Protein formation, which is reduced, leading to shrinkage in muscle mass and decreased bone formation, possibly contributing to osteoporosis.