Obese Divers

By Dr. Sawatzky

Why does the body make fat? As babies, we all need fat as a ready source of energy, for thermal protection, and as a "source of heat". Children and small adults have a lot of skin for the size of their bodies. The amount of heat we produce at rest is largely a function of our weight and the amount of heat we loose is a function of the surface area of our body, or the "amount of skin we have". Therefore, children and small adults have a higher "surface area to weight ratio" than larger people and they will loose proportionally more heat through their skin than larger people. Babies "need" the fat to insulate them from this unusual heat loss. They also have limited ability to generate heat by exercise and often have little control over their food intake. Because of these two factors, "nature" has provided babies with a unique way to generate more heat. They have something called "brown fat". Brown fat generates a lot of heat for its mass and thereby helps babies keep warm. As we grow and mature, we gain the ability to generate heat by exercise, we have more control over our environment (we can wear more cloths or seek shelter) and we tend to have control over our food intake. This is a good thing because, as we mature we lose our "brown fat" and that mechanism of heat generation.

In adults, fat is still a ready source of energy and an insulator. When we eat, our body converts the energy in the food into chemical forms of energy that the body can use. If the body does not require the energy at that moment, it is stored. Some energy is stored in the liver and the muscles as a complex carbohydrate called glycogen. The rest is stored as fat. When a muscle is working, the actin and myosin filaments use ATP to provide the energy to contract. The ATP is reduced to ADP by the filaments and the ADP is converted back to ATP in the mitochondria (chemical factories) in the cells. The mitochondria use glucose (sugar) to power this conversion. The glucose comes from the blood and the liver. When more energy is needed than can be provided by the available glucose, the mitochondria start to use the glycogen stored in the muscles (glycogen is broken down into glucose molecules). When the glycogen is depleted, the mitochondria are forced to use fat as an energy source. The chemical reactions to obtain energy from fat are much slower than those used for glucose and glycogen. Therefore, when the muscles run out of glycogen, the amount of work that they can do is greatly reduced. Marathon runners frequently will run out of glycogen at about the 20 mile mark. They call it "hitting the wall". They are running along at a fast pace, feeling okay, and all of a sudden they feel terrible and they can only continue running at a much slower pace. Distance runners and other endurance athletes will alter their diet and training before a competition to increase the amount of glycogen stored in their muscles (carbohydrate loading). They will also drink "sugar" during exercise to try and spare their glycogen in an attempt to make it last until the end of the competition. In reality, the biochemistry of exercise is much more complex, but this basic understanding meets our needs.

Now that we know why the body makes fat, why does the body sometimes make excessive amounts of fat? This one is very easy to answer. Obese people eat more calories than they burn, period. Yes I know, all obese people think that they have a "glandular problem". In reality, less than 1% of obese individuals have a biochemical problem that is responsible for their obesity. The most common is "hypothyroidism". The thyroid gland produces a hormone (thyroxin) that controls the resting metabolic activity of the body (comparable to the amount of gas your car burns when it is stopped at a red light). If you have too little thyroxin, you will always be tired, you will have very poor cold tolerance, and you will quickly gain weight, even though you exercise and eat a reasonable amount of food. A simple blood test will determine if you have this problem. If your thyroid is not producing enough thyroxin, you will have to take a pill every day to replace it. For the other 99% of obese people, you simply have to eat fewer calories and burn more (exercise). I am also aware of all the recent press on the "fat" gene, etc. I do not believe it is a major factor. North Americans are rapidly getting fatter, grossly so. We have not changed our "genetics". What we have done is reduce the amount of activity in our lives and increased the number of calories, especially simple sugars, in our diets. The result, predictably, is a rapid rise in obesity. Enough on fat per se, let's now turn to the effect of obesity on diving.

Let's start with the advantages of obesity in diving. Fat is a great insulator and it definitely keeps the diver warmer. In Canada, the water tends to be a bit on the cold side and a bit of extra insulation is a good thing. However, before you all decide to intentionally gain 25 pounds, recent research has shown that muscle is also an excellent insulator when you are resting and an excellent source of heat when you are exercising. Therefore, you would be far better off to gain 25 pounds of muscle than 25 pounds of fat!

A second advantage of obesity is that all fat people are strong. They have to "exercise" every time they stand up, walk, climb stairs, etc. They will have strong muscles to enable them to move all that excess weight around, and they will have strong bones. Strong muscles are needed in diving to enable us to move tanks, weight belts, and other gear, not to mention getting in and out of the water. Nevertheless, you would be far better off getting the extra muscle by lifting weights. Finally, although it is not an advantage, obese divers have fewer problems than obese people in many other sports because in diving, the person does not have to support the excess weight whilethey are in the water.

The disadvantages of obesity in diving are many. The obese person needs a larger wet or dry suit and if it is made of neoprene, the suit will have more buoyancy. Fat is lighter than water and the obese person will need to wear more lead to make themselves neutral in the water. Both of these problems mean that the obese diver will usually have to wear a MUCH heavier weight belt than the non-obese diver.

The obese person usually has a lower level of fitness and when diving this means that they will use more air. Therefore, the obese diver will need a larger tank or they will be doing very short dives. The larger suit, the heavier weight belt, the larger tank, and the excess weight of the diver means that the obese diver will havesignificant difficulties getting in and out of the water. If they have a problem, it will also be very difficult for other divers to help them.

Obese divers have an increased cross-sectional area. This means that they will have more "drag" when they try to move through the water. They will have to work harder to swim at the same speed as their thinner companions. Fat stores five times as much nitrogen as water, when saturated. You might think that this would greatly increase the risk of DCS in the obese diver. However, fat tends to have a relatively poor blood supply and on most recreational dives, fat will take up relatively little nitrogen. Therefore, the only times fat as a storage area for nitrogen should matter would be when the diver was doing multiple dives per day for several days, and on altitude or saturation dives. Many dives in a short period of time would allow the fat to store enough nitrogen to increase the risk of DCS.

Altitude diving is a problem because the fat will be saturated at the lower altitude and will require more than 24 hours to 'off gas" when the diver goes to altitude. However, the obese diver usually is more susceptible to DCS than the non-obese diver because they tend to be less fit and therefore they work harder during the dive and take up more nitrogen. The obese diver is much more susceptible to hyperthermia (overheating) than the non-obese diver. In Canada this is a definite problem in warm weather when the water is still cold. The diver will have to wear significant thermal protection for the dive and they will tend to become extremely hot before the dive. It can also be a problem when the diver misjudges the water temperature and wears more protection than they really require. Then if they work hard during the dive, they can overheat. 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 very bad thing in divers. The only advantages are increased insulation and strength. The insulation can be purchased (dry suit and warmer underwear) and the strength can be obtained in the gym. The only concession I will make is that 10 to 15 pounds (5 - 7 Kg) of excess weight in an otherwise extremely fit diver is not a bad thing. So now is the time to live up to those New Years resolutions. Get a membership in a good gym and use it. Also change your diet so that you are eating more "healthy" food containing fewer total calories. You should aim at losing a maximum of one pound per week.