What is the most important nutritional concern?

The most important concern is getting reliable information. Nutrition is a new science but it includes a lot of very old folk knowledge. Researchers occasionally change their minds about what they know, and folk knowledge might be good or bad. For example, the official opinion about butter has reversed several times in the last few decades. In the 60s they recommended to mix vegetable oil with butter to make it healthier. Another problem is that doctors have no training at all in the field, and they don't trust anything they didn't learn before they got their degree. So the field is flooded with information that is not certain, and people who don't know any of it, good or bad.