By Dr. David Sawatzky
A couple of years ago I started writing a column on healthy eating. I believed it to be relevant to diving but there was very little research to clarify that relationship. Recently, several interesting articles have been published on this topic so it is time to review what we know.
The food that we eat is comprised of several components. One component we absorb into our bodies and use completely. Another we absorb, use partially and eliminate the remainder. The third component we do not absorb, but simply allow to pass through our system unchanged; this is the fibre found in fruits, vegetables, whole grains and legumes. Fibre holds water in the stool, which keeps the stool soft, reduces transit time through the gut, helps maintain a healthy weight and lowers the risk of diabetes and heart disease. In general, western diets do not contain enough fibre.
Those parts of the food that we absorb into our bodies can be divided into three groups: carbohydrate, fat and protein. Our bodies have the ability to process carbohydrates extremely efficiently and the only waste products are carbon dioxide (which we breathe out) and water (which we need anyway). Carbohydrates are simple sugars such as glucose and fructose and complex sugars like the starch in vegetables, grains and legumes.
Simple sugars are absorbed into the blood rapidly. As the level of sugar in the blood rises, the body releases insulin from the pancreas into the blood. Insulin increases the uptake and utilization of sugar by the cells and reduces the level of sugar in the blood. If we have only eaten simple sugars (like a sweet), the excess sugar in the blood will be cleared long before the insulin. When the simple sugars have been absorbed from the gut and used by the cells, the remaining insulin will force the level of sugar in the blood to drop to a very low level. We will sense this low level as hunger and eat more, basically eating constantly throughout the day. The end result of a diet that is high in simple sugars is gross obesity (typical North American diet).
Complex carbohydrates (starches) are basically simple sugars all connected together into very large molecules. These molecules are absorbed into the blood and carried to the liver where they are metabolized by cutting off one simple sugar at a time. In general these simple sugars are slowly released into the blood and used by the other cells in the body. We do not get the spike in blood sugar seen after eating a sweet and, more importantly, we do not experience abnormally low blood sugar 20 minutes later. Complex carbohydrates are the ideal food but if you eat too many you will still end up obese, as they contain significant calories!
Within reason, fats are required by babies and young children, largely to build brain cells. Adults require less fat in their diet. Fats can be divided into ‘good’ fat and ‘bad’ fat. Good fat (unsaturated), especially polyunsaturated fats, can actually lower your cholesterol. Bad fats (saturated and trans fat) raise cholesterol levels and increase the risk of heart disease and stroke. Fat also contains two to three times as many calories per unit weight as carbohydrates and proteins do. In general, adults should reduce the amount of fat in their diets, particularly saturated and trans fats.
The good side of eating fat is that it takes a long time to digest and leaves you feeling ‘full’ or ‘sated’ for several hours. A diet high in fat will contain too many calories but you will not eat constantly, since you feel sated much of the time. As a result you will tend to be overweight but not grossly obese. An example of this is the traditional high fat British diet, which resulted in many people being ‘dumpy’ (10-15 kg overweight). Many Brits have now adopted a high simple sugar North American diet and are becoming grossly obese.
Processed food frequently has a lot of fat (usually saturated and trans), sugar (simple) and salt added to enhance the flavor. Fortunately, over the last several years, the trans fat content of many prepared foods has declined. Nevertheless, we should all try to reduce the amount of processed food in our diets.
Protein is obtained primarily by eating meat, but there are many other sources. Milk, cheese, yogurt, eggs (up to one per day) and legumes are excellent sources of protein. Milk should be skimmed to reduce the fat content. Most legumes derive 20-30% of their calories from protein and soybeans are 35-40%. Seafood is usually low in fat and the fat it contains is usually good fat. White poultry contains a bit less fat than dark. The skin should be avoided as it is loaded with saturated fats. Lean beef has only slightly more saturated fat than skinless chicken breast. Proteins are made up of amino acids and our bodies need amino acids to build muscles. Therefore, growing children need to eat protein.
When we exercise we break down the muscle fibres. Our bodies need amino acids to repair the damage and to make the fibres stronger. The more exercise we do, the more protein we need in our diet. Individuals who spend two to three hours lifting weights every day may need to increase the amount of protein in their diets but the average adult in North America eats three to four TIMES as much protein as their bodies need. Even people who do a lot of exercise do not usually need to increase the amount of protein in their diets, and they definitely do not need protein supplements.
If we eat more protein than our bodies require, we metabolize it to produce energy. Unfortunately, protein is not an efficient source of energy. As our bodies break it down we obtain some energy but we are left with a large amount of waste material, most importantly nitrogen. We have to eliminate this excess nitrogen and we do so by converting it to uric acid and excreting it in our urine.
Our kidneys can only concentrate uric acid so much; we require a fair amount of water just to get rid of the uric acid. Most of us do not drink enough water anyway (two litres per day is recommended) and a high protein diet simply increases the risk of dehydration. Your urine should always be clear or a very light yellow color. Dark colored urine almost always means that you are dehydrated.
At the same time, excess hydration before diving can increase your risk of immersion pulmonary edema. Excess hydration also creates potential problems for drysuit divers (need more be said?) The optimal course seems to be the middle ground of being well – but not over – hydrated before diving.
Of interest, some desert animals have diets that are primarily carbohydrates and their kidneys are able to concentrate urine more than ours. As a result they obtain more water from the metabolism of the carbohydrates in their diets than they need to eliminate the waste nitrogen from the protein. They NEVER have to drink water!
So how does what we eat relate to diving?
Many factors influence our risk of developing decompression sickness (DCS) after diving. A recent study looked at the percentage of calories eaten as fat in the diet and the levels of cholesterol in the blood. Researchers exposed 56 divers to dives in a chamber to 100 and 200 feet (30 and 60m). Twenty nine of the 56 subjects had bubbles in their blood (detected by Doppler) after the dives, indicating decompression stress. Those subjects who had a high fat diet had significantly higher levels of cholesterol in their blood AND developed more intravascular bubbles after the dive, indicating that they had a higher risk of developing DCS. This small study suggests that a high fat diet may increase the risk of developing DCS after diving.
Many divers are obese. Excess body fat often indicates a reduced level of physical fitness (increased risk of DCS). It also provides extra insulation and avoids hypothermia (reduced risk of DCS). For many years I have suggested that the ideal cold water diver would be fit, and a bit overweight.
Two recent studies looked at percentage body fat and the development of intravascular bubbles after diving. Researchers also looked at age, level of fitness (as measured by maximal oxygen consumption or VO2max) and body mass index (BMI). They found that for middle aged and older divers, body fat and BMI had NO effect on bubble scores. They did, however, find that age and VO2max affected the scores. Older and less fit divers are more likely to develop intravascular bubbles after diving, and have a higher risk of DCS. These studies support my guess that a good cold water diver would be fit and ‘solid’. We cannot do much about age, but we can definitely all increase our level of fitness.
A lot more work is required in this field but it seems as if a high fat diet, associated with elevated levels of cholesterol in our bodies, increases the risk of decompression sickness. Obesity does not seem to be a separate risk factor but lack of physical fitness definitely is.
The bottom line is that a healthy diet and lifestyle will almost certainly enhance our diving experience and reduce the risk of developing decompression sickness.
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