Can our bodies recycle nutrients? Which nutrients can we recycle?

Question: "Since our body can go 20 days with water alone without danger, it seems that nutrient depletion is going at a very slow rate. I am sure that some nutrients do go faster than others. I would like to know which ones we really need to get more often and what frequency, and quantity. A more important question would be which ones are necessary for keeping our more important organs healthy... It seems so complex, animals bodies are so complex, but it seems that all wild animals find easily their ideal diets while we are still wondering and counting calories."
You raise excellent questions! I'm so glad to hear something other than "Where do you get protein?" for a change!
Protein, incidentally, is something we can recycle a lot. So is vitamin B12. However, our ability to recycle minerals, proteins, vitamins, etc, is hampered by toxins. So if you live in a standard chemical-filled home in America, wear conventional clothing, ride in vechicles, spend time in cities, etc, then you're exposed to a lot of toxins constantly.
To deal with those toxins you need a lot of antioxidants, which is why so many people get so much benefit from adding antioxidant-rich foods to their diet (such as blueberries and goji berries).
If you go out into nature far away from cell phone towers, traffic, drywall, paint, cigarettes, and so on. If you sleep under the stars wrapped in an organic-cotton sleeping bag... You'll find your need for nutrients diminish dramatically.
When I travel by airplane I pack a huge sack full of apples, oranges, celery, pomegranates, kiwis and so on, and I eat the entire time. If I stop eating for more than two hours I start to feel sick, exhausted, drained, and dizzy. That is because airplane travel is so toxic to our bodies. That is the main reason why people need so much time to recover. The time difference thing really doesn't affect people nearly as much as being in oxygen-deprived areas and exposure to jet fuel fumes.
The other factor in recycling nutrients is what your body is used to. While the human body can thrive on only 25 grams of protein per day, you'll seriously hurt yourself if you dropped from 110 grams to 25 grams out of the blue. It takes your body time to adjust. Also, the more the protein comes from leafy greens, the better your body will be at recycling the protein and building muscle with it.
As for what organs are most important and what nutrients fuel those organs best... There really isn't an answer. Every organ is inter-dependent. For example, your kidneys make your bone marrow. Without healthy kidneys, your bones will degrade.
Your liver is responsible for what toxins make it to your colon versus your kidneys versus being stored in your fat. Without a healthy liver, your kidneys can be compromised, which in turn will compromise your bones and teeth. If your heart isn't circulating blood well, then you may not get nutrients to your extremities very well, resulting in problems in your feet, hands and vision.
If your stomach isn't producing enough acid, then food may go into your intestines not digested, causing bacteria and worms to eat the food instead, which can lead to severe flora imbalances which affect which nutrients you absorb, which can lead to not getting enough B-vitamins, which will make you feel depressed.
You see how low stomach acid can cause depression and how a weak liver can lead to cavities in the teeth? It may seem far-out, but it is true. In fact, cavities themselves are so stressful on the body that they can cause adrenal fatigue, leading to thyroid problems which lead to excess weight. It all goes in circles because every organ depends on every other organ. Nutrients work the same way — they all depend on one another.
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