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There are a number of factors that can affect calcium absorption, which include a healthy diet and sufficient amounts of vitamin D. High amounts of sodium, caffeine, and saturated fats, on the other hand, can inhibit calcium absorption. The amount of calcium that is consumed, age, and low estrogen levels can also affect how calcium is absorbed.
Vitamin D seems to play a large role in the amount of calcium that is absorbed into the body. It helps to increase the production of calcium-binding proteins within the digestive tract, which aid in calcium absorption. In addition, vitamin D is also helpful in reabsorbing calcium within the kidneys. It can be obtained naturally from sunlight exposure as well as from vitamin D-fortified foods. Other nutrients that help with calcium absorption include vitamin C and magnesium.
Estrogen, which is a female hormone, is helpful in absorbing calcium as well. The production of this hormone generally decreases with age and therefore can often result in low levels. As a result, the amount of calcium that gets absorbed decreases.
A moderate dose of healthy fats is also important as fat gets converted into cholesterol, which is essential for the production of vitamin D. Excess amounts of fats, however, can have adverse effects. They tend to contribute to an acidic environment within the digestive tract, which can inhibit the absorption. Consuming plenty of alkaline-forming foods can help to maintain a healthy environment within the digestive tract. These include vegetables, nuts and seeds, and most fruits.
Certain acids in foods also tend to interfere with calcium absorption. Leafy greens such as collards and spinach contain oxalic acid, which can inhibit calcium absorption, especially when consumed with dairy. Many nuts and seeds, as well as beans, contain phytic acid, which also binds to calcium and reduces the amount that is absorbed. These foods, however, might not hinder the process too much because many of them also contain amino acids and silicon, both of which help to increase the absorption of calcium. Other calcium-binding substances are caffeine, sodium, and the stress hormone, known as cortisol.
Calcium absorption naturally decreases with age and thus the encouragement for a higher calcium intake. The doses, however, should be kept at minimal dosages, as the higher the dose, the less is absorbed into the body. Therefore, it is best to take a calcium supplement several times per day for maximum calcium absorption.
I don't know if this would be considered a factor affecting "absorption" of calcium or not, but remember that how much and what kind of exercise you get are also going to affect how your body uses calcium.
If you spend all day sitting on the couch (or even swimming or other activities in which your bones don't have to fight gravity), your body won't put a lot of energy into maintaining your bone density. Why should it? You're showing it every day that you don't need strong bones.
Weight-bearing exercises (anything in which you are *on your feet*) and weight training, on the other hand, tell your body to make strong bones.
If you have kids, part of helping them develop a healthy lifestyle is making sure that they get this kind of exercise; childhood and adolescence are crucial periods for bone development and what they eat and do during those years will have a lifelong impact.
Something else to be aware is that animal protein also interferes with calcium absorption in the body. I'm not a vegan or vegetarian myself, but I've been trying to get more protein from vegetable sources as evidence is increasing that it might be more beneficial in some ways than plant protein.
USDA calcium recommendations are quite high to compensate for the amount of animal protein in the typical American diet; people who eat much less (or no) animal protein may not need as much.
There's a lot of emphasis on dairy as a source of calcium, but among vegans (who consume no animal products of any kind, including dairy), calcium deficiency is very rare. That indicates that they must be on to something - they probably eat more of those good plant sources of calcium, like dark leafy greens and broccoli, and they don't have plant proteins interfering with absorption.