QUOTE from Dr Fung (emphasis mine):
MANY PEOPLE WORRY that fasting may cause malnutrition, but this concern is misplaced. The body’s fat stores are, for most of us, quite ample for our bodies’ needs. Even studies of prolonged fasting have found no evidence of malnutrition or micronutrient deficiency. Potassium levels may decrease slightly, but even two months of continuous fasting did not decrease levels below normal, even without the use of supplements. Note that this duration of fasting is far longer than is generally recommended without medical supervision.
Magnesium, calcium, and phosphorus levels during fasting are stable—presumably, because of the large stores of these minerals in the bones. Ninety-nine percent of the body’s calcium and phosphorus is stored in the bones. A multivitamin supplement will provide the recommended daily allowance of micronutrients. In one case, a therapeutic fast of 382 days was maintained with only a multivitamin, with no harmful effect on the subject’s health. Actually, this man maintained that he felt terrific during the entire period. There were no episodes of hypoglycemia, as blood sugars were maintained within the normal range. The only concern may be a slight elevation in uric acid, which has been described in fasting. 
I get muscle cramps. What can I do? LOW MAGNESIUM LEVELS, particularly common in diabetics, may cause muscle cramps. You may take an over-the-counter magnesium supplement. You may also soak in Epsom salts, which are magnesium salts. Add a cup to a warm bath and soak in it for half an hour. The magnesium will be absorbed through your skin. 
NOTE (my commentary)
Our bodies are created and adapted to deal with times of little or no food in the environment. What is natural for humans created in a time prior to refrigerators, factory farms and meatpackers is to get up and look for food every day. Fresh food. Unprocessed food, found in a natural state. And so our bodies are ready to cope with periods of no food so that we can get up and get out and go find food.
Even studies of prolonged fasting have found no evidence of malnutrition or micronutrient deficiency.
In our age of abundance and too much insulin, most human beings are carrying extra weight. Our bodies are very well prepared for the next famine. At 3500 calories per pound, my current level of obesity – 85 pounds – works out to 248 days at 1200 calories per day. My body is ready to cope with days of no food. Over 8 months!
As Dr. Fung points out, a multivitamin is probably sufficient. And if there are specific problems related to micronutrients, like cramps, they can be addressed as they arise. Fasting websites, particularly on Facebook, reflect an obsession with regard to electrolytes and a fear that insufficiency will cause a broad variety of negative symptoms. Such persons take a wide variety of supplements in order to prevent those negative symptoms … but, generally, based on the comments above by Dr Fung, a standard multivitamin is sufficient. It’s also possible that the “nocebo effect” – negative expectations – accounts for the negative symptoms and the “placebo effect” – positive expectations – is responsible for the positive results from taking additional electrolytes and minerals.
MY STRATEGY: I take a multivitamin; I believe it is sufficient.
What gets my attention?
Do I understand the need or problem?
Do I understand the potential solution?
Do I understand how to apply that strategy?
What questions do I have for the experts? What might be the answers?
Who needs to hear this?
What do I do next?
SOURCE – Footnotes:
 Quote from The Obesity Code: Unlocking the Secrets of Weight Loss by Dr. Jason Fung; Greystone Books (March 3, 2016). Page 241-242.
 Ibid, 252.
Photo Fasting-a-glass-of-water-on-an-empty-plate, via Wikimedia Commons
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Key: Fung-07 Last Revision: 03-11-2021