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I remember nutritionist explained me that taking kefir does not make much sense for calcium since its lactose is converted into the acid and that acid washes out as much calcium as milk gives you. I wonder about such acid-rich foods as lemonads, fruits and particularly lemons. You like them because fruits are associated with health. But what is their effect on calcium loss in the skeleton?

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Arguments that suggest that there exists a rather trivial vulnerability in the human body that would be relevant even if you stick to a healthy lifestyle, should a priori be considered to be unlikely. Animals have survived for hundreds of millions of years in the wild without needing to consult expert dietitians to make sure calcium absorption is not impaired. Now, there is actually some hard evidence that this particular argument is bunk. As pointed out here:

A popular concept in nutrition and lay literature is that of the role of a diet high in acid or protein in the pathogenesis of osteoporosis. A diet rich in fruit and vegetable intake is thought to enhance bone health as the result of its greater potassium and lower “acidic” content than a diet rich in animal protein and sodium. Consequently, there have been a number of studies of diet manipulation to enhance potassium and “alkaline” content of the diet to improve bone density or other parameters of bone health. Although acid loading or an acidic diet featuring a high protein intake may be associated with an increase in calciuria, the evidence supporting a role of these variables in the development of osteoporosis is not consistent. Similarly, intervention studies with a more alkaline diet or use of supplements of potassium citrate or bicarbonate have not consistently shown a bone health benefit. In the elderly, inadequate protein intake is a greater problem for bone health than protein excess.

The core mistake made when such arguments are presented, is that the human body is pictured like a simple machine (e.g. a car) that is subject to wear and tear and then one argues that when subject to certain conditions, the body will degenerate. But the human body is not like a simple machine that we can make, it's a machine that is constantly maintaining itself. Your bones are living tissue that are constantly breaking down and are being rebuild. They should not be compared to a concrete wall of a building, it's far more appropriate to compare it to a building site where a lot of building work is constantly going on.

When the system is perturbed, e.g. due to changes in the diet, less calcium is being absorbed, many feedback mechanisms at many different levels start to act to prevent this perturbation from causing problems. E.g. lower calcium levels in the blood will cause the kidneys to remove phosphate from the blood. The phosphate in the blood is in chemical equilibrium with the phosphate bound to the bones, if phosphate is removed then the blood gets undersaturated with phosphate and phosphate from the bones is released onto the blood which then also releases calcium from the bones into the blood.

At the same time the kidneys release calcitriol in the blood which ends up in the cells of the intestines, there they switch on a gene that codes for an enzyme that increases the calcium absorption from food. This is just one of the many layers of feedback systems that regulate calcium metabolism.

To get into trouble you have perturb the system so much that the feedback mechanism can no longer compensate for that, e.g. if you get way to little calcium from your diet. Or if your vitamin D levels are much too low then the body cannot make cacitriol, which is derived from vitamin D.

Now, in the flawed reasoning where you picture the human body as a simple system like a building, there are no feedback mechanisms. A wall of a building does not repair itself. The human body, in contrast, does repair itself but not perfectly, otherwise we would live forever. What we can do is make the self-repair mechanisms more effective, or at least make sure they don't degrade very fast as we age.

An unhealthy diet that causes arteriosclerosis is not just bad for the heart, arteries throughout the body will be clogged. This will make the transport of nutrients to all cells in your body to get impaired, including those in your bones, and that then compromises the self-repair capacity. This is why eating broccoli is better for your bones than eating a cheeseburger.

Exercise will cause the self-repair mechanisms to be boosted to keep up with the increased need for repairs and as explained above, the more active the self-repair mechanisms are the better your body including your bones will be maintained.

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Finding an ideal balance of acidic and alkaline foods keeps your body’s pH neutral. Low pH is acidic, while high pH is alkaline. Neutral pH is considered to be at pH 7.0.High acidic food intake can adversely affect your bones by causing them to lose calcium to your body.

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