My contemplation of the world situation led down the line of "suppose there's just no food at all. How long can I survive, and how do I recover when food becomes available again?" We know that people in harsh circumstances can survive for days, or weeks, without eating. It's not pleasant, but many survive. Maybe they're stranded by a car wreck, migrating across barren terrain, or drifting in a life-raft after a ship sank... it happens. They go hungry, then they're rescued.

So, what happens next? I'll just quote Wikipedia:

Refeeding syndrome is a metabolic disturbance that occurs as a result of reinstitution of nutrition in people and animals who are starved, severely malnourished, or metabolically stressed because of severe illness. When too much food or liquid nutrition supplement is eaten during the initial four to seven days following a malnutrition event, the production of glycogen, fat and protein in cells may cause low serum (blood) concentrations of potassium, magnesium and phosphate.[2][3] Cardiac, pulmonary and neurological symptoms can be signs of refeeding syndrome. The low serum minerals, if severe enough, can be fatal.

And, what to do about it? Wikipedia, again:

..., if phosphate drops to below 0.65 mmol/L (2.0 mg/dL) from a previously normal level within three days of starting enteral or parenteral nutrition, caloric intake should be reduced to 480 kcals per day for at least two days while electrolytes are replaced.[3] Daily doses of thiamine, vitamin B complex (strong) and a multivitamin and mineral preparation are strongly recommended. Blood biochemistry should be monitored regularly until it is stable. Although clinical trials are lacking in patients other than those admitted to intensive care, it is commonly recommended that energy intake should remain lower than that normally required for the first 3–5 days of treatment of refeeding syndrome for all patients.[5]

Now, I doubt that we're likely to have the ability to monitor blood biochemistry in a difficult situation. For now, I think it's sufficient to put a note in my emergency medical reference (along with the recipe for oral rehydration solution): "Just because you have it, don't eat it all! Resume eating gradually. 480 kcal per day is about 1/4 lb (140 g) of carbohydrates and protein (and less of fat). Have dry electrolytes and vitamins in reserve."