FatOK estimates the fitness state for adults from the four points of view:
- Health (waist-to-height ratio) [1,2]
- Traditional body mass index (the estimation of "thickness or thinness" by weight and height) 
- US Army Body Fat Standard (the estimation of the body fat percentage by the height and the circumferences of waist, neck and may be hips) 
- Fitness  (FatOk uses circumference method  to estimate the body fat percentage)
Body Mass Index "thickness or thinness" estimations are shown in Weight entry of FatOk, after you enter the weight and press Add.
All other fitness estimation are shown in Body Fat (%) entry after you enter the circumferences and press Add
FatOk works in close cooperation with the Apple iPhone Health app. In particular it stores the weight, height and body fat percentage in the common Health App database and you may watch the plot of your weight and body fat percentage with time in the Health App.
Therefore at the first start FatOK ask your permissions to read and write in the common Health App database the height, weight and the body fat percentage. It also asks the permission to read the date of birth to calculate age and the sex because in general the estimations are different for men and women and depend on the age.
Before you begin, check that you entered the date of birth and sex (male or female) in Health app. Health App does not allow other applications change this values.
When you run FatOk first time, start from entering the height if it was not entered in Health App because the estimations depend on the height.
WARNING: You should be aware the FatOK gives only the estimations (see the formal medical disclaimer). This estimations are approximately correct for the majority of people but not for all. In case of doubt always seek the advice of your physician or other qualified health provider.
You also should be very careful when you continue to loose your weight in the "healthy" interval. The point is that though the weight of most healthy persons indeed is in the "healthy" interval, it does not mean that each of them can reach any point in the "healthy" interval without the damage for her / his health. The same is true for the waist also.
As for the rate of the probability, a 2010 study that followed 11,000 subjects for up to eight years concluded that waist-to-height ratio is a much better measure of the risk of heart attack, stroke or death than the more widely used body mass index.[6,1]. Also a 2010 systematic review of published studies concluded that waist-to-height ratio may be advantageous because it avoids the need for age-, sex- and ethnic-specific boundary values[6, 7]
In general FatOK estimations of the fitness are approximately valid for the adults of the age 21 years old or older. US Military Body Fat Standard and body fat percentage estimations approximately valid for the adults of the age 17 years and older.
How to measure the body circumferences
The information and images below are from The Army Body Composition Program 
Neck measurement (for men and women)
Measure Soldier's neck circumference at a point just below the larynx (Adam's apple and perpendicular to the long axis of the neck). The Soldier should look straight ahead during the measurement, with shoulders down (not hunched). Round the neck measurement up to nearest half inch.
For men (Figure B-3)
Measure the Soldier's abdominal circumference to nearest half inch (through the navel (belly button)). Take measurements at the end of Soldier's normal relaxed exhalation. Round down to nearest half inch
For women (Figure B-4)
Waist (abdomen) measurement. Measure Soldier's natural waist circumference against the skin at the point of minimal abdominal circumference. Soldier's arms must be at the sides. Take measurements at the end of Soldier's normal relaxed exhalation. Round the natural waist measurement down to nearest half inch
Measure Soldier's hip circumference while facing Soldier's right side by placing the tape around the hips so that it passes over the greatest protrusion of the gluteal muscles (buttocks) as viewed from the side. Ensure tape is level and
parallel to floor. Round hip measurement down to nearest half inch.
- Schneider; et al. (2010). "The predictive value of different measures of obesity for incident cardiovascular events and mortality.". The Journal of Clinical Endocrinology & Metabolism 95 (4): 1777-1785
- Ashwell, M., Gunn, P. & Gibson, S., 2012. "Waist-to-height ratio is a better screening tool than waist circumference and BMI for adult cardiometabolic risk factors: systematic review and meta-analysis." Obesity Reviews, 13, pp.275-286.
- "BMI Classification". Global Database on Body Mass Index. World Health Organization. 2006.
- Army Regulation 600-9. Personnel-General. "The Army Body Composition Program." Headquarters Department of the Army Washington, DC 28 June 2013. (Unclassified)
- Natalie Digate Muth. "What are the guidelines for percentage of body fat loss?" December 2, 2009. http://www.acefitness.org/acefit/healthy-living-article/60/112/what-are-the-guidelines-for-percentage-of-body-fat
- Browning Lucy M, et al. (2010). "A systematic review of waist-to-height ratio as a screening tool for the prediction of cardiovascular disease and diabetes: 0.5 could be a suitable global boundary value.". Nutrition research reviews 23 (02): 247-69
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