It has already been said that the calcium content of plants varies. In general dark green leafy vegetables have more calcium than other plants. The United States Department of Agriculture book
Composition of Foods
lists, with other items, the calcium content of all foods in milligrams per 100 grams edible portion. This makes it easy to figure percentage composition. Each 100 milligrams is 0.1%. Turnip greens are often said to be one of the best vegetable calcium sources. They are 0.246% calcium raw (246 milligrams per 100 grams), and the percentage goes down to 0.184% if they are cooked in a small amount of water for a short time and drained, More water and longer cooking further reduce the calcium content. One of the highest percentages of calcium in a land plant is found in lambsquarters,
Chenopodium album
, a very edible and very common weed. Its presence is said to indicate good fertile soil. It may be that only in fertile, well balanced soil can this plant accumulate its 0.309% calcium, which is almost three times the 0.118% calcium in whole fluid milk. Green amaranth, another common garden weed, has 0.267% calcium. It is used as a vegetable in China and the seeds of a related plant were used as a grain by the Aztecs.
A study of the
Composition of Foods
makes it clear that the highest calcium contents for whole foods are in foods from the sea. Kelp seaweed has the highest percentage listed for any whole plant food. It is 1.093% calcium, with other seaweeds ranging from 0.885 to 0.296%.
As we move from bacteria to plants to animals the weight percentage of calcium increases. In all organisms oxygen, carbon, hydrogen and nitrogen make up over 95% of the weight. Calcium, phosphorus, potassium and sulfur are the next most common elements in living things.
Biology of Plants
gives the percentages of fresh weight composed of the four major elements plus phosphorus and sulfur, for humans, alfalfa and bacteria. This tells us how much room is left for calcium, potassium and all the other elements found in living things. The six listed, CHNOPS by their chemical symbols, make up 97.90% of humans, 99,60% of alfalfa and 99.72% of bacteria. Calcium makes up 1.5% of the average human body.
Animals accumulate calcium from the foods they eat. Herbivores such as cows must get all their calcium from pasture, hay and grains they eat. Carnivores or omnivores can get their calcium from animals or, in the latter case, also from plants. A 150 pound person has about 2.25 pounds of calcium in the body of which 99 is present in the bones and teeth which leaves about one third of an ounce of calcium in all other locations in the body. The edible portions of most meats contain about 0.01 calcium. This makes many vegetables better sources of calcium than meats, and may be why it takes an herbivore like a cow, which consumes prodigious amounts of vegetation to concentrate enough calcium to produce milk in quantity over the long term. It also explains why carrnvores such as dogs chew on bones. In all animals, except for periods of growth, the calcium taken in food and drink equals the calcium excreted in feces, urine and sweat and in any milk produced. A balance is maintained in a healthy animal.
Of the calcium ingested in foods by humans,
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usually as little as 20-30% is absorbed and it can be as low as 10% Calcium is primarily absorbed from the food into the body in the duodenum (the first part of the small intestine) in an acid medium. As with the parent rock and the limestone in the soil, acids are necessary to free calcium from its sequestered state. In general, the absorption of calcium is reduced in the lower part of the intestinal tract as the food content becomes more alkaline (higher pH). There is evidence, however, that some calcium can be absorbed in the colon. The calcium in the fibrous plant cell wall only becomes available when the cell wall structure is digested by microbial fermentation in the colon. This fermentation changes calcium into an available form. It also suggests a connection to the digestive process of cows and other ruminants where microbial fermentation in the rumen, an antechamber of the stomach, is an essential early step in the digestive process. Ruminant animals, goats, sheep, camels as well as cows, provide most of the world’s milk.
Calcium is absorbed by active transport, which requires energy, and by passive diffusion. It is absorbed only if it is present in a water soluble form in the intestine and hasn’t been precipated by something else in the food. Whatever is not absorbed leaves with the feces.
There are six factors which can increase calcium absorption. Vitamin D, the sunshine vitamin, stimulates calcium absorption in the intestines. Lactose, or milk sugar, enhances the absorption of calcium in people with normal lactase, an enzyme which splits lactose into galactose and glucose for digestion. It is thought that lactose functions either by forming a sugar-calcium complex which can be transported across intestinal mucosa or by preventing the precipation of insoluble calcium complex as the contents of the intestinal tract change from acid to alkaline. This obviously relates to the importance of milk as a source of usuable calcium. However, in humans with lactase deficiency, lactose inhibits calcium absorption.
A high intake of protein causes a greater amount of calcium to be absorbed than with a low intake of protein. Certain a ino acids act upon intestinal pH and upon the formation of the soluble complex with calcium facilitates calcium absorption. Also, moderate amounts of fat increase transit time through the digestive tract which makes for more time for “mineral” absorption. (Studies of the diet of Cro-Magnon people indicate that these Stone Age folks ate three times the protein, half the fat and twice the fiber and calcium as present people do. The researcher noted that our diet has evolved much faster than our genes have.)
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The acidity of gastric juices, specifically the hydrochloric acid released by the stomach, lowers the pH of the contents of the digestive tract in the small intestine and favors calcium dissociation and hence absorption. The final factor which can increase calcium absorption is the physiological state. The greater the need and the smaller the dieta supply, the more efficiently the body absorbs calcium from the food. Absorption is also increased during periods of growth, but it is decreased after middle age and the response of increased absorption to decreased intake is blunted in elderly persons.
Other factors decrease calcium absorption. Some of these can be inferred from those which increase absorption. These include lack of vitamin D, rapid passing of food through the intestinal tract, alkaline conditions in theintestinal tract which encourage the formation of insoluble and nonabsorbable calcium phosphate, and lastly, some studies suggest that too much fat in the diet may contribute to lower calcium absorption.
Several acids present in food can contribute to lower calcium absorption. Oxalic acid in the digestive tract combines with calcium to form an insoluble compound, calcium oxalate, so the calcium cannot be absorbed. Oxalic acid is present in appreciable amounts in rhubarb, spinach, chard and beet greens. In a similar way, phytic acid combines with calcium to form calcium phytate which is insoluble. Phytic acid is a phosphorus containing compound found principally in the outer husks of cereal grains, especially oatmeal. There is also some evidence that fiber itself, aside from the phytic acid it contains, may decrease calcium absorption from the small intestine.
Exercise stimulates bone cell production. A lack of exercise and lack of weight bearing on the legs cause a decrease in the body’s ability to absorb calcium. Weightlessness, as in space flight, causes bone loss.
Absorption of calcium tends to decrease and its excretion tends to increase with mental stress. People under physical or emotional duress need a higher intake of calcium to maintain proper calcium equilibrium. Some drugs can also decrease calcium absorption.
Some studies suggest that a high ratio of phosphorus to calcium may affect bone or calcium metabolism. Phosphorus consumption under 2.0 grams per day with adequate calcium doesn’t appear to cause ill effects. However, in the teenage growth years, when the need for calcium is high, there may be reason to worry about the replacement of milk in the diet by phosphorus containing cola beverages.
How much calcium should a person take in daily? This will vary, not only because of the above factors, but also with age and for women, with pregnancy, lactation and menopause. The basic recommended daily allowance in this country is 800 milligrams. This is higher than it is in some other parts of the world because of the abundance of food sources of calcium here.
An additional 400 milligrams is recommended for children between 11 and 18 years of age and for pregnant or lactating women. It is often recommended that older women take in 200 milligrams above the basic level.
Once it has been absorbed through the intestinal wall, the calcium ion is transported by the blood to the fluids bathing the tissues of the body, including the bones, and to the cells wherever needed. Most of the calcium is used by the bones, where the calcium is in equilibrium with the calcium in the blood. The average male has about 1200 grams or about 2.6 pounds of calcium and the average female has about 1000 grams or 2.2 pounds. As stated earlier, 99% of this is in the bones and teeth and less than 1% is in the fluids. Of that 1%, about one half is ionized and physiologically active and the other half is non-ionized and physiologically inert, mostly bound to proteins, primarily albumin.
Some of the calcium in the bones is in a form which is rapidly exchangeable and is available when needed to maintain the proper levels of calcium in the blood. It can be stored again when there is excess calcium input. Parathormone and calcitonin are hormones which act to control the flow of calcium into and out of the bones and maintain serum calcium at a normal concentration of 10 milligrams per 100 milliliters of blood serum.
The biomineralization process in vertebrates differs from that in invertebrates. The mineral made by the higher animals as an internal skeleton is hydroxyapatite, Ca5(PO4)3 (OH). It is 40% calcium and 18% phosphorus by weight. Phosphorus makes up about 1.2% of the human body, slightly less than calcium, and a greater percentage of it is in places other than the bones. About 6% is associated with the muscles, especially in the energy storing compound adenosine triphosphate, ADP. In addition phosphorus has more functions than any other “mineral” element in the body. This much wider distribution of phosphorus in the bodies of animals makes meats a good source of this element.
By weight, calcium is present in the body at one half the percentage it is in the earth’s crust. Phosphorus, however is much rarer in the earth’s crust. It is present in the body at ten times the weight percent it is present in the crust. In fact, the bones of vertebrates are the major store of phosphorus in the world. (In
Brave New World
by Aldous Huxley, the bodies of the dead are burned for phosphorus recovery.)
Except for that stored in the bones, the dynamic calcium balance in the body requires that over time all the calcium taken in must be excreted. Most of this is in feces and urine, which through outhouse, septic system or sewage treatment plant reenters the soil or water to be cycled again through plants, animals, geology or agriculture.