No, the word 'healthful' is not a noun, it is an adjective, a word used to describe a noun as good for the health of the body or mind (healthful exercise, healthful relaxation).

The noun form for the adjective healthful is healthfulness. A related noun form is health.

You don't. There really isn’t any scientific debate about the healthfulness of a purely plant-based diet. Informed professionals no longer maintain that it’s necessary for humans to consume animal flesh or secretions. Mainstream professional organizations acknowledge that a sound vegan diet is perfectly adequate for human health, including:

• the Academy of Nutrition and Dietetics
• the American Diabetes Association
• the American Heart Association
• the British Dietetic Association
• the British Nutrition Foundation
• Dietitians’ Association of Australia
• Dietitians of Canada
• the Heart and Stroke Foundation
• the Mayo Clinic
• the Cleveland Clinic
• New York Presbyterian Hospital
• the British National Health Service
• the National Institutes of Health
• Kaiser Permanente

In addition, many researchers and physicians believe that plant-based diets have significant health benefits over diets containing animal products, including:

• T. Colin Campbell, PhD
• John A. McDougall, MD
• Caldwell Esselstyn, MD
• Neal Barnard, MD
• Michael Greger, MD
• Milton Mills, MD
• Kim Williams, MD
• Joel Fuhrman, MD
• Dean Ornish, MD
• Nathan Pritikin