By the time you're diagnosed with Y it's way too late to do anything.

OK, that's probably true. It is also true in the case of deficiencies arising from a more conventional diet however. How frequently do micronutrient deficiencies occur under regular diets? How is the chance of timely detection and intervention affected by controlled intake in conjunction with deliberate ongoing monitoring versus an unmonitored ad libitum diet?

Let's consider two contrasting propositions regarding diet and nutrition: A) Consuming a varied diet of naturally occurring, unprocessed foods prevents micronutrient deficiencies. B) Deliberately engineered supplementation prevents micronutrient deficiencies.

Before reading on, take a moment to consider how much confidence you have in proposition A versus B.

Since you introduced the example of iodine deficiency, let's consider it in more depth. The Wikipedia page on iodine deficiency indicates: "According to World Health Organization, in 2007, nearly 2 billion individuals had insufficient iodine intake ..."

Furthermore, this deficiency appears to be common even in wealthy industrialized countries where a wide variety of food is readily available: "In a study of the United Kingdom published in 2011, almost 70% of test subjects were found to be iodine deficient."

The article proceeds to explain that iodine deficiency is addressed by deliberately and artificially introducing supplemental iodine into the food supply.

I'm shocked to learn just how widespread iodine deficiency is among people eating a "normal" diet (I would have guessed less than 10% prevalence). It seems like traditional diets do a startlingly poor job of avoiding this particular deficiency. I'm updating my beliefs in favor of proposition B over A in light of this data.