Hereditary Fructose Intolerance

Most of us can hardly wait to finish our dinner so that we can plunge into the dessert and satisfy our craving for sweets. We threaten to withhold dessert from our children until they polish off every last vestige of vegetables on their plate. But for about 1 in 20,000 people, this lust for sweets is a completely foreign concept, just the thought of sweetness can make them ill. They suffer from a disease known as hereditary fructose intolerance. When we use the word sugar in everyday language, we generally refer to the stuff we put into our coffee or tea. A chemist, however, would describe this extract of the sugar beet or sugar cane as “sucrose,” since in a scientific context the term “sugar” has a different meaning. It refers to any one of a number of water soluble substances which have a certain commonality in molecular structure and which serve as the body’s prime source of energy. Sucrose can be referred to as “a sugar.”

To complicate things further, sucrose is actually composed of two smaller sugars, namely glucose and fructose, joined together by a chemical bond. In the digestive tract sucrose is broken down into glucose and fructose and both of these compounds can be used by the body as a source of fuel. To put it simply, our bodies burn simple sugars for energy, just as a car burns gasoline. In both cases the major products of the combustion process are water and carbon dioxide. But the conversion of simple sugars into carbon dioxide and water by the body is a complicated business requiring many steps and biological catalysts known as enzymes. The unfortunate people suffering from hereditary fructose intolerance lack one of the enzymes (phosphofructoaldolase) needed to break down fructose. The result is the accumulation of a fructose derivative (fructose-1-phosphate) in the liver which throws the body’s whole energy production system out of whack. In a healthy person, fructose is converted to glucose which the body uses as its main source of energy. Excess glucose is converted to glycogen which can be reconverted to glucose whenever needed. But in hereditary fructose intolerance, glycogen cannot be converted to glucose with the result that blood glucose levels fall. This condition is known as hypoglycemia. Basically, what this means, is that for the affected people, nausea, sweating, tremors, confusion, vomiting, abdominal pain, seizures and even coma can follow the ingestion of any food which contains fructose. A diagnosis of hereditary fructose intolerance is confirmed by a liver biopsy and an examination of the tissue to reveal that that the critical enzyme is missing. Anyone suffering from this condition should carry around glucose tablets to use in case of a hypoglycemic attack if they inadvertantly consume fructose.

Unfortunately it sometimes takes a long time before a proper diagnosis is made. The medical literature records the case of a young Englishman who had a history of infrequent seizures over a ten year period and went through the usual battery of tests aimed at detecting brain abnormalities such as epilepsy. The doctors could find nothing wrong. The only information the patient was able to provide was that he had always disliked sweets, but his physicians were unable to relate this observation to his medical problems. The mystery was finally solved by the gentleman himself. He happened to read an article in the newspaper about hereditary fructose intolerance and recognized the symptoms as his own. Upon reflection, he realized that his attacks of weakness and seizures always followed the ingestion of sugar. Genetic testing revealed that he did indeed suffer from hereditary fructose intolerance. The concept of the required treatment is relatively simple. Avoid fructose! But it is not so simple to put this notion into practice. Since table sugar, or sucrose, breaks down in the body to glucose and fructose, it must be curtailed. Fruits and many vegetables contain fructose and therefore have to be avoided. Even the sweetener sorbitol has to be shunned because it is converted in the body to fructose. Rhubarb, tangerines, grapefruit and tomatoes are low in fructose and do not cause a disturbance. Glucose is fine as a sweetener, and in fact, as mentioned, hypoglycemic seizures characteristic of fructose intolerance can be treated with glucose. So, an understanding of the chemistry of fructose metabolism has led to at least at partial solution to the problem of fructose intolerance. But it is unfortunate that now, when modern research is demonstrating the beneficial effects of eating fruits and vegetables on cancer and heart disease, the sufferers of hereditary fructose intolerance have to be counseled to stay away from these foods. One man’s treat is another one’s poison! Can the fructose intolerant people take solice in anything? Why yes! Since they avoid sugar, they do not get cavities.

 

Joe Schwarcz

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