calcium in oranges and fiber

[RRM]

it may be that both take up exactly the same amounts of calcium and phosphorus

The way you say that, it makes me think that supplements aren't that bad. After all, our bodies will absorb only the necessary.
(Unless, of course, supplements forces our body to absorb them involuntarily. I don't know how they work.)

That is true up to a certain extend.
If your intake of nutrients from both supplements and food is about the same magnitude of what your diet alone may provide you, there is no problem at all.
If that intake is way too high, you may have a problem.
Something similar is true for when you suddenly change your diet drastically towards the intake of foods that are extremely high in specific nutrients. Oysters, for example, may be extremely high in zinc, which may result in a too high uptake of zinc if you are not used to eating oysters regularly for a very long time.
And what defines excessive intake depends on the nutrient, of course.

Another issue is is that our body is naturally, designed for digesting foods.
Foods that may contain a lot of water and/or fiber, plus nutrients.
The nutrients are surrounded by lots of other stuff. This allowes the body to take up us much as it wants. (Naturally, the uptake mechanism differs per nutrient)
Supplements, on the other hand, are extremely concentrated.
Because of this extreme nutrient density, it is much harder to keep them out.
You may compare it (in warfare) to a position that the enemy tries to take; if the enemy attacks with a steady trickle of a couple of soldiers at the time, they may stand little chance, but if they overrun it with a large number of soldiers all attacking at the same time, its much harder to resist them.

[Novidez]

That is true up to a certain extend.
If your intake of nutrients from both supplements and food is about the same magnitude of what your diet alone may provide you, there is no problem at all.So, for example,
If that intake is way too high, you may have a problem.
Something similar is true for when you suddenly change your diet drastically towards the intake of foods that are extremely high in specific nutrients. Oysters, for example, may be extremely high in zinc, which may result in a too high uptake of zinc if you are not used to eating oysters regularly for a very long time.
And what defines excessive intake depends on the nutrient, of course.

So, for example, the supplements that were prescribed for me, one of them had 5 mg Zinc in each pill. Supposedly, I should have taken 4 pills per day, which would make 20mg Zinc (only from supplements): 2 pills at lunch and 2 pills before sleep.
Considering that probably my absorption rate may be adapted to medium/low intake of Zinc, this could be somewhat problematic no? Even taking 10mg Zinc before going to bed, picking your analogy, would be like a atomic bomb in my organism wouldn't it?
Of course, I would adapt to it over time. But then again, when finished all the supplements, my body would be completely unbalanced I suppose.

[RRM]

Considering how much zinc you need, taking one pill (5 mg) a day should be fine.
20 mg is clearly too much; it will intitially have pro-oxidative effects, and by the time your body has adjusted, your zinc absorption rate will have gone down considerably. The end effect is that dietary zinc alone may no longer provide you with sufficient zinc, due to the lower absorption rate, thus you will depend on the supplement.
Obviously, the best option is to get your nutrients through diet.

[Novidez]

it will intitially have pro-oxidative effects

What does this mean? Do we have symptoms or something like that?

Obviously, the best option is to get your nutrients through diet.

I completely agree with you!
I had to go again to that doctor because the consultation was scheduled and, as you can imagine, my parents aren't still convinced about this. I can noticed that they are continuously afraid of it.

[RRM]

it will intitially have pro-oxidative effects

What does this mean? Do we have symptoms or something like that?

Pro-oxidative effects are effects that increase 'collateral damage' to cells and their organelles in general.
These effects are not noticed (as symptoms), but they do have adverse effects on the lifespan of cells.

[Emeira]

So, for example, the supplements that were prescribed for me, one of them had 5 mg Zinc in each pill

be careful with zinc, too much can inhibit copper in your body. Zinc : copper ratio is very important.

[Novidez]

be careful with zinc, too much can inhibit copper in your body. Zinc : copper ratio is very important.

Now that you mention that, this supplement also has Copper and Vitamin B9 in it. Maybe that's the reason behind it: to create some balance.

[clem24]

Low calcium can cause low blood pressure. I think more research needs to be done on this area. But then I noticed that taking high dose of calcium causes the same thing. My friend, a middle-aged woman treating osteopenia noticed taking high dosage of calcium causes low blood pressure. Though we are still confused about it is the cause of her Low Bp but checked multiple times using the blood pressure calculator at http://healthiack.com/blood-pressure-calculator and had the same result.

[RRM]

Low calcium can cause low blood pressure.

The blood calcium level is strictly regulated by various hormones.
That is because that is essential for muscle contraction.
Relatively low calcium intakes do not result in low blood calcium levels, unless severely calcium deficient.

I noticed that taking high dose of calcium causes the same thing.

Elevated and lower levels may indeed have the same effect.