By Dr. David Sawatzky
As I write this column it is the middle of the Christmas holiday season. No matter what your beliefs, virtually all of us celebrate the end of the old year and the beginning of the New, and that celebration usually involves eating excessive amounts of food. So, what topic could be more timely than obesity and diving!
Is excessive “personal insulation” a good thing, a bad thing, or does it not really matter in diving? The answer is “all three” but you will have to read on to learn why.
Every person has some fat. The amount of fat in our bodies varies from a few percent in the “excessively skinny” to a very high percentage in people who weight several hundred kilograms. The heaviest person on record weighed 635 kg (1,400 pounds).
There are several methods to try and determine how much of our bodies are fat. You can measure the thickness of a fold of skin at several specific locations (as the amount of fat under the skin increases, the skin fold gets thicker). You can send an electrical signal through the body and measure the resistance (resistance increases as the percentage of fat increases). Some new bathroom scales will do this measurement. Most of our bodies are water so you can also weigh a person when they are completely underwater to get an estimate of their percentage body fat.
All of these methods have limitations. Some people tend to store more fat under their skin than others. The electrical resistance of the body changes depending on diet, electrolyte concentrations, state of hydration, etc. Underwater weighing is the most precise, but it is difficult to do accurately and also varies with the amount of gas in the intestines, lungs, etc.
The most commonly used estimate of fatness is the BMI (body mass indicator). It is simply your weight in kilograms (kgs) divided by your height in meters, twice. For example, I currently weight 75 kgs and I am 1.73 meters tall. Therefore my BMI is 25.1 (75/1.73/1.73). For comparison, the world’s heaviest person had a BMI of 186. The highest BMI I have seen in a patient in recent years is 54.
BMI is a good measurement of “fatness” in a large group of people but it is relatively inaccurate in an individual. A tall person will have a higher BMI than a shorter person with the same percentage body fat. A very muscular person will have a higher BMI than a person with very little muscle but the same percentage body fat.
However, BMI is very easy to measure and it does give a general indication of “fatness”. In extremely general terms a BMI of less than 20 indicates the person may be too thin. A BMI of 20-25 is considered normal and 25-30 is usually overweight. A BMI of 30-35 is moderately obese (class I), a BMI of 35-40 is severely obese (class II) and a person with a BMI over 40 is considered to be very severely obese (class III).
Personally I prefer the “jelly roll” test. Find the thickest fold of skin on your abdomen, usually near the umbilicus (belly button). In a man it should be less than 2.5 cm (1 inch) and in a woman less than 3.75 cm (1.5 inches). This does not translate into a meaningful percentage body fat, but it will definitely get you to a “reasonably healthy” weight.
So why do we have fat in the first place? Fat is very important. It is a high density, long-term store of energy. When our muscles perform work (or any other tissue in the body does pretty much anything) a molecule of Adenosine-5’-triphosphate (ATP) is changed into a molecule of Adenosine diphosphate (ADP) and energy is released to perform a function. ADP is changed back into ATP in the mitochondria of the cells. The energy to do this is provided by sugar and other nutrients that are delivered to the cells by the blood.
When the energy requirements of a cell are large, ATP is used up faster than it can be replaced by this method. Most cells also contain glycogen. Glycogen is also used to produce ATP. If the energy requirements of the cell remain high enough for long enough, all of the glycogen in the cell will be used up.
Blood also delivers fat to cells and the mitochondria can use fat to convert ADP back into ATP. The bad thing about this process is that it is relatively slow. Therefore, when a person runs out of glycogen in cells the amount of work they can do is dramatically reduced. Endurance athletes call this “hitting the wall”.
A distance runner may experience this after running more than 30 km (20 miles) or so. They try to delay this by loading with carbohydrates before the race (done correctly this increases the amount of glycogen in the cells) and by consuming sugar during the race. Running slower
also works but is not usually desirable in a race! If a runner depletes their glycogen they will be able to continue running but only at a MUCH slower pace.
The good thing about fat as an energy source is that it contains a phenomenal amount of energy, very roughly 8,000 calories per kilogram (3,500 calories per pound). A person while running burns 500 to 1,000 calories per hour so do the math and realize that the number of calories burned by exercise is relatively small. Bottom line, an obese person who is trying to lose weight is unlikely to be able to exercise enough to have much of an effect on their weight. I will continue this discussion in the next column so let’s return to obesity and diving.
Fat is also a pretty good insulator and helps keep us warm, but of course, it is really more complicated than that. The amount of heat we generate is related to our size. Large people make more heat than small people. The amount of heat we lose is related to our surface area. Large people have a larger surface area than small people BUT the ratio changes. Small people have a much larger surface area compared to their weight than large people. Therefore, babies are at serious risk of hypothermia because they have a very large surface area compared to their weight. This is also true in adults where a small person will cool off faster than a large person, even if their percentage body fat is the same.
Interestingly, resting muscle is just as good an insulator as fat, and working muscle generates a lot of heat. Therefore, if you are interested in staying warm it is far better to go to the gym and gain muscle mass than go to the kitchen and gain pounds of fat.
In general, fat people are strong. Think about it, every time they do anything they have to move their body weight around. Strap a 10 kg (22 pound) weight to your back for a day and see how much more exercise you get just doing everyday things. Obese people who are physically active usually also have strong bones. It should be easier for them to lift and move their dive gear… but…
Obese divers are much more buoyant and therefore will need much more weight to be neutral in the water. Obese divers are usually less fit and therefore tend to use more gas during a dive. This means they need larger tanks (or make shorter dives). As a result, dive gear for an obese diver often weighs much more than dive gear for a fit diver.
The combined weight of the gear and the person can be a real problem for the obese diver trying to get in and out of the water. It can also be a real problem for their dive buddies who are trying to assist them, on both a normal dive and in an emergency.
One small plus is that the obese diver’s weight disappears when they are neutrally buoyant in the water; however, their increased cross-sectional area will increase their drag and increase the work of moving through the water.
When saturated, fat contains four to five times more nitrogen than water. But fat has a relatively poor blood supply. Therefore, an obese standard recreational diver doing routine recreational dives at sea level will not accumulate enough nitrogen in their fat to significantly change their risk of decompression sickness (DCS). However, there are several situations where the excessive fat in an obese diver will increase their risk of developing DCS.
If the obese diver does a lot of diving in a short period of time or dives daily for many days in a row, they will accumulate nitrogen in their fatty tissue. A diver accumulates nitrogen (and helium) much more quickly on deep dives than shallow dives so obese technical and rebreather divers are at increased risk.
If an obese diver ascends and dives at increased altitude, they will start the dive with much more nitrogen in their bodies than the non-obese diver and as a result will be at increased risk of DCS. Of course, divers (obese or otherwise) who are less fit work harder and accumulate more nitrogen in all of their tissues on all dives, thereby increasing their risk of DCS. In addition, the obese diver is at increased risk of ‘hyperthermia’ or over-heating while diving (not usually a problem in Canada).
Finally, the obese diver is far more likely to have cardiovascular disease (heart attack and stroke), endocrine diseases (diabetes) and orthopedic diseases (joint problems) than the non-obese diver.
So what is the bottom line? Obesity is usually a bad thing in divers. The only advantages are increased insulation and strength. Extra insulation can be purchased (dry suit and warmer underwear) and strength can be obtained in the gym. The only concession I will make is that a little excess weight in an otherwise extremely fit diver is a good thing if you are diving in very cold water.
So now is the time to live up to those New Year’s resolutions. Get fitter, build more muscle and, for many of us, lose some weight. In the next column I will give you my best advice on how to lose weight and keep it off. I have lots of patients who are losing weight and I just lost 15-20 pounds by following my own advice.
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