The best nutrient to prevent bleeding of gums, loosening of teeth, and poor wound healing is ascorbic acid, also known as vitamin C. Ascorbic acid plays a crucial role in maintaining healthy gums and teeth by promoting the production of collagen, a protein that helps in wound healing and maintaining the structural integrity of gums and teeth. It also supports the immune system, which is important for preventing and fighting gum infections.

Folic acid, also known as vitamin B9, is important for overall health, but it does not specifically prevent bleeding of gums, loosening of teeth, or poor wound healing.

Iodine is essential for thyroid function and metabolism, but it does not have a direct impact on gum health or wound healing.

Zinc is involved in various biological processes, including wound healing, but it is not the best nutrient to prevent bleeding of gums, loosening of teeth, or poor wound healing.

In conclusion, ascorbic acid, or vitamin C, is the best nutrient for preventing gum bleeding, tooth loosening, and poor wound healing, due to its role in collagen production and immune system support.