Consider B3 deficiency – the diversion of more and more tryptophan to make more B3 is guaranteed or death will result. This diversion leaves less tryptophan for picolinic acid synthesis (long term result may be poorer absorption of dietary zinc). This diversion of tryptophan leaves less tryptophan for serotonin and melatonin synthesis.
B3 deficiency tends toward deficiencies in tryptophan, serotonin, melatonin, and zinc and each of these deficiencies has long tentacles of its own. More and more problems downstream and more and more deficiencies.
Melatonin deficiency –> poorer sleep and all of its consequences, from eating more and less exercise, poorer moods, and tendency toward depression.
With serotonin deficiency – also with a tendency toward depression, the person unconsciously does a lot of harmful things that are rewarded and reinforced whenever moods improve or serotonin levels rise and moods accordingly. Example – overconsumption of sugary treats.
Same for zinc deficiency – the decrease in taste acuity has a person over-salting and over-sugaring his foods. Makes diabetes more likely if he is not diabetic and diabetes worse if he already is diabetic. If he is a salt-sensitive person, his edema and blood pressure will increase. If he is already on thiazide diuretics for hypertension, his zinc deficiency should increase and his taste acuity decrease accordingly, in a vicious cycle.
Will taking B3 immediately reverse these things? Of course not – it takes a long time to correct these problems, because each of these deficiencies has tentacles of its own. A long, long time, and people do not have the patience. There is only one genuine solution and it is to eat right and supplement appropriately. Eating right takes a lot of effort, and people, even people with means, do not make sufficient effort.
Now consider this: as bad as B3 deficiency is or could become, imagine also a general protein deficiency. That means almost certainly a tryptophan deficiency. But imagine a specific tryptophan deficiency. How much worse would niacin, serotonin, melatonin, and zinc deficiencies become? Much worse. Imagine a local deficiency if you cannot imagine a systemic deficiency. Imagine how bad things can get in a particular locale where multiple deficiencies coexist.