Malnutrition after 3 weeks? I mean, pack 21 multivitamins.

I had ridges in my nails after an exceptionally bad bout of flu. It's not obscure or complex.

She’ll probably do that on her next 3 week trip.

Indeed. Given average western diet, 3 weeks of malnutrition can only do good.

How is malnutrition ever good?

"Mal", bad, in the latin

Losing weight is probably worth the tradeoff for most Americans.

Sure, losing weight is of benefit. But malnutrition is defined as a "serious deficiency" in the intake of energy and nutrients. Healthy weight loss is not malnutrition, and you can quite definitely be missing essential vitamins and be malnourished without losing weight.

Heck, some of the worst western diets are both poor in vitamins/minerals and also still rich in calories.

Not just americans. With very few exclusions, we are all overweight, even people who do look great. The food we consuming is mostly crap.

So it's a benefit almost for everyone.