DEAR DR. BLONZ: What makes a vitamin water-soluble? I know vitamins A, D, E and K are not water-soluble, and that B and C are. Does this relate to the fact that we do not store water-soluble vitamins in our body? -- H.T., Phoenix
DEAR H.T.: First, some history about what we now call vitamins. The work of Louis Pasteur (1822-1895) and Robert Koch (1843-1910) revealed that germs were responsible for diseases. Scientists then began working to identify the bugs behind every illness. Sanitation was found to be a key, and the “germ theory” of disease ruled the day. Some ailments, however, were found to persist even when sanitation was under control. The concept that what we ate, or failed to eat, was involved was not widely embraced, but as time went on, scientists began to investigate the role of diet.
The usual way to establish the essentiality of a nutrient is through an investigation of what happens when it is absent. Because vitamins are present (and needed by the body) in relatively small amounts, it was not until researchers had an ability to purify foods that they could know what, precisely, was being fed. Answers began to emerge when experiments started using purified diets that only contained protein, fat and carbohydrate. These nutrients, by themselves, did not support life; young animals failed to grow, and mature animals failed to maintain their body weight. It became clear that there were other essential substances.
As analytical procedures progressed, the different essential micronutrients were discovered. The first of these nutrients contained the element nitrogen, in the form of an amino group. It was assumed that all micronutrients would have a similar structure, and this new group was referred to as “vital amines,” or “vitamines” -- a word coined in 1911 by a Polish scientist named Casimir Funk. It was later determined that not all of these substances were built in the same way, but the name -- shortened to “vitamins” -- had already become part of scientific jargon.
An adult is about 60% water by weight. Water gets consumed, serves as the medium for biochemical reactions, then serves as the vehicle to help eliminate metabolic byproducts through the kidneys. By contrast, the body conserves fat by virtue of its role as the body’s concentrated source of metabolic energy.
When the vitamin discoveries began, it became convenient to classify these substances by whether they dissolved in water or fat. This classification was not based on whether the vitamins were stored; that was not known at the time. This system, however, has remained. Due to the constant turnover of water through the body, water-soluble vitamins are not effectively stored to any appreciable degree. Fat-soluble vitamins tend to get distributed in body fat, so they hang around for a while.
The fat-soluble vitamins include A, D, E and K, as you mentioned. The water-soluble vitamins include thiamine (B1), riboflavin (B2), niacin (B3), pantothenic acid (B5), pyridoxine (B6), ascorbic acid (C), vitamin B12, folate, choline and biotin.
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