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June 22, 2007

Body fat, body composition report, and diving

Yesterday I took my 11-year-old to a doctor's appointment at the Kaiser HMO facility in South Sacramento. On the way in I noticed that the Health Education room, sort of a small library with pamphlets and instructional diagrams and such, offered free body fat tests. I thought I'd stop by if the appointment didn't take too long. It didn't.

The body fat test is done by a machine. It looked like a cross between a scale and a treadmill. You stood on it, it measured your weight, and then you entered your height, age, gender, whether you were clothed or not (who wouldn't in a public facility?), and what body type you were, the choices being athletic, normal, and sedentary. You then held on to two metal handle bars so the machine could send a low intensity electrical current through the body. The process is called bioelectronic impedance and uses the different electrical resistance of different tissues, bone, muscle and fat to determine your body composition. It takes but an instant, and then you can print out the results. So what did I get?

Well, I am six foot tall, weighed 159 pounds with clothes and gym shoes, age 56, male, and I optimistically described myself as "athletic." The body comp scale printout said I had 16.9% body fat, 83.1% fat-free mass, and 64.5% total body water. It also stated that my target weight range was 144.4 to 152.1 pounds, and that my daily caloric need was 2,331 calories, based on my "moderate activity level" -- which was defined as participating in an exercise program three days a week for 20-30 minutes. I run a fairly intense course three times a week, so I felt that counted as moderate activity.

I always thought that at about 155 pounds at 6 foot I was fairly slender, and so the printout's suggested target weight of 144 to 152 surprised me. The answer to that puzzle was that I am really a "normal" body type, and not a muscular "athletic" one. According to the body composition report, athletic types have between 6 and 13% body fat, normal body types between 14 and 19%, and sedentary ones over 20%. Had I entered "normal" in response to the body type, my target weight would probably have been just around where I am.

To be honest, I was surprised that my body fat was smack in the middle of the normal range. I thought it'd be less. So I poked around a bit to see what it all means. Here is some information I found:

NOAA has body composition standards both for males and females. At a height of 6 feet, the maximum weight for a male would be 201 pounds. Maximum body fat percentage depends on age. For men under 30 it's 23%, under 40 25%, and over 40 27%. Females are allowed 33, 35, and 37%, respectively. Scuba-doc.com states that "Total body fat of less the 22% in males, and less than 28% in females is desirable" and that "trained males however average 7-10% body fat." So I am apparently not very well trained. Dr. Jolie Bookspan's "The 36 Most Common Diving Physiology Myths" is interesting reading. Bookspan conducted post-doctoral work in saturation decompression and altitude and is certainly an expert. She praises fat as a major protection against cold and also points out that women's higher body fat percentage does not necessarily mean more body fat in pounds as women are generally shorter by several inches and weigh less. Her claim: "It's not yet known whether percentage fat or absolute fat amount is more problematic to decompression issues - if either are important - another area that is still unknown, but prone to myths." However, she points out that "Fat is a slow tissue due to gas solubility. Because of high gas solubility, fat holds much nitrogen and takes time to uptake and offgas it all. This is a property of fat and is true even for fatty areas with the same degree of blood supply as leaner tissue." Activedivers.org says that "As the percentage of body fat increases, so does the risk. Fatty tissue attracts and stores nitrogen more so than other tissue types, and inhibits the off-gas process." "Diving Science by Strauss and Aksenov states that "fat tissues have five times more affinity for nitrogen than lean tissues have."

However, by and large, there really isn't a lot of truly scientific information (readily) available about how body fat affects diving. It obviously has an impact on buoyancy. It has positive properties on heat insulation. Fatter people encounter the usual prejudices, like supposed lack of fitness or problems climbing a ladder to get on board. Wetsuits and BCs generally don't come in larger sizes. Yet, I've seen a lot of fat divers, and some very prominent and accomplished ones.

So perhaps we just don't know all that much about body fat and what it means for diving. I am surprised that according to that machine, I carry 26 pounds of body fat as I certainly can't see it. Then again, I also carry about 100 pounds of water, and that thought is downright weird.

Posted by conradb212 at June 22, 2007 11:15 PM