"It's a mistake to make too strong a connection between health and the amount of body fat you have."
"The connection between body fat and health outcome has defied a simple answer" says Steven B. Heymsfield, M.D., co-director of the body-composition laboratory at St. Luke's-Roosevelt Hospital in New York. "That's why there is not a standard for health based on body fat."

A 2007 report, issued by the American Institute for Cancer Prevention and the World Cancer Research Fund says there is a link between weight and cancer risk. It recommends keeping Body Mass Index between 21 and 23. See Also:
When It Comes to Avoiding Cancer, Weight Matters at U.S.News
Obesity Research Center at St. Luke's-Roosevelt Hospital.

Body Fat / Body Composition Measurements

• Hydrostatic (underwater) weighing: +/- 1.5% Most accurate. Based on the fact that bone and muscle are more dense in water than fat tissue. Less accurate below 15% bodyfat where skinfold may be more accurate.
• Standardized Skinfold (SKF): +/- 3% fairly accurate if administered by a trained professional.
  Based on the fact that approximately 50% of your total body fat is beneath the skin.
  , Spread Sheet for several calculations using SKF
• Bioelectric impedance (BIA): +/- 3% Fat-free tissue is a better conductor than fatty tissues. It measures phase shift in an applied alternating current around 50 kHz. The phase shift is caused by capacitive effects in body tissues.
  Another source explains it as "extracellular water (ECW) or fluid found outside the body cells, in combination with electrolytes, is the main conductor of the body. Fat, which has a low water content, has a high resistance and lower reactance."
  BIA Tends to overestimate body fat in very lean individuals and underestimate with obese individuals.
  No national standard for the devices has been developed; calculation is based on each company's proprietary formula.
  (See explanation at rjlsystems and Basic Principles of Bioimpedance Testing )
  
• Bod Pod (Air Displacement): +/- 3% It involves sitting in a comfortable egg-shaped chamber while computer sensors determine the amount of air displaced by the body. Same principal as Hydrostatic weighing.
• Dual Energy X-ray Absorptiometry (DEXA): DEXA takes bone mineral content into consideration when estimating body fat and muscle, therefore it is considered to be more accurate and valid than a two component model of body composition such as underwater weighing.
• Anthropometric ( height, weight and girth) measurements - Not very accurate.
• Near-infrared interactance: +/- 4% - developed by USDA for measuring animal body comp. - may not work as well as skinfold.

My tests with a sharper Image ME 802 Handheld bodyfat analyzer show it varied +/- 3% (19% - 22% body fat) depending on how hydrated I was. Lower readings in the morning and higher in the afternoon after consuming liquids.
A more sophisticated handheld BIA gave a reading of 17.4% bodyfat.
The scale BIA at Brookstone gave a reading of 14.2%. The Tanita scale reported 15.5-17%.
Skinfold estimates were 6.8-8.0 which is consistent with the note above that BIA overestimates fat in lean individuals; However the difference seems to be excessive.

You get more internal body fat as you age, so most calculations compensate. My Tanita scale reported 30% higher body fat for age 60 vs age 30. The skin fold test formulas showed about 80% higher at age 60.

Guidelines for Bioelectrical Impedence Tests

• Abstain from eating and drinking within 4 hours of the test
• Avoid exercising within 12 hours of the test
• Void (urinate) completely prior to testing
• Do not drink alcohol within 48 hours of the test
• Avoid taking diuretics prior to testing unless instructed by your physician

Consumer Reports January 2004 issue reported that: "The best body-fat scale we tested (Taylor 5553) was within 5% of our laboratory standard for only about 80% of the people who tried it."

See Also:
Skinfold Body Fat Measurement
National Institute for Fitness and Sport (NIFS), Gerogia St. U., BisSpace Body Composition Analyzer - Method comparison, Wright St. Univ. Med. School,   Body Composition Assesment at MedBeraad, Body-Fat Calipers at BodyTrends.com

Measuring Body Fat:
Calculator at DrSears.com (The Zone)
Body-Fat Estimator
Spread Sheet for several calculations using Standardized Skinfold (SKF)

Tanita scale recommendations - Healthy range: (male)
Age: 20-39   8-20%
     40-59  11-22%
     60-79  13-25%
 

The estimated minimal level of body fat compatible with health is 5% for males and 12% for females (31); however, optimal body fat percentages for an individual athlete may be much higher than these minimums and should be determined on an individual basis.
Source: Lohman TG. Basic Concepts in body composition assessment. Advances in Body Composition Assessment. Champaign, Ill: Human Kinetics; 1992:109-118.

Extremely low body fat, in fact, can be detrimental to health and fitness. For most women, 11 percent body fat or less is medically unsafe. For most men, 5 percent or less is medically unsafe, according to the Biosyn Technical Manual, by Barry Sears.

Body-fat percentage tests are among the most popular voluntary exams at health fairs and gyms, said Judi Ulrey, owner of Fitness Consulting in Costa Mesa, Calif., which conducts fitness tests.

People should be less concerned about the amount of fat and more concerned about its type and location, Gaesser said. Recent studies indicate that some body fat is good, and some is not. >{? "Most of our fat - somewhere between 70 and 80 percent for men, and more for women - is subcutaneous; that is, right beneath the skin," Gaesser said. That's the good body fat.

The bad stuff is the deep body fat, also called intra-abdominal or visceral fat, most of it in the abdomen around our internal organs, he said. A lot of deep body fat can give a person an apple shape, and it "seems to be associated with all health problems on which fat seems to have any bearing.

What's more, 80 percent of the way a person's body looks - including fat percentage - is genetically determined, Joan Hackett, exercise physiologist and director of fitness programs at Shiley Sports and Health Center in La Jolla, CA, said. "Athletes are born, not made," she said.
Source: The Skinny On Body Fat -- Many Health Experts Say It's Overemphasized As A Yardstick For Fitness | Seattle Times Newspaper

Athletes:
According to "The Zone"

You need fat for an energy store, protection of internal organs, and as a component of nerves and cell membranes. Elite athletes will have lower levels (Gymnasts, wrestlers - 4%; Body builders-5%; Cross-country skiers, triatheletes-8%; Raquetball-9%; basketball, soccer, swimmers-10%; distance runners, football defensive backs-11%)

Diet and Body says:

"Male athletes with the lowest estimates of body fat (less than 6%) include middle-distance and long-distance runners and bodybuilders, whereas male basketball players, cyclists, gymnasts, sprinters, jumpers, triathletes, and wrestlers average between 6% to 15% body fat *. Male athletes involved in power sports such as football, rugby, and ice and field hockey have slightly more variable body fat levels (6% to 19%).

Some studies show swimmers perform better with a higher % body fat than runners.

According to Baby Boomers.com Fitness experts recommend acceptable body fat ranges of 18 to 20 percent for women under age 40, 13 to 16 percent for men under age 40, 23 to 27 percent for women ages 40 to 60+, and 19 to 20 percent for men ages 40 to 60+.

See Body Composition at U. Mass.
Body-Fat at: BodyTrends
Body Composition Methods: Comparisons and Interpretation | NIH
Body-Fat Estimator
Genetic Influences on the Response of Body Fat and Fat Distribution to Positive and Negative Energy Balances at Journal of Nutrition.
Body Composition and Body Fat
The Skinny On Body Fat -- Many Health Experts Say It's Overemphasized As A Yardstick For Fitness | Seattle Times Newspaper
"Big Fat Lies: The Truth about Your Weight and Your Health", Glenn Gaesser, associate professor of exercise physiology at the University of Virginia