The core issue with a vegan diet has to be the lack of easily digestible protein. This issue can be compounded by a reliance on commercial, packaged ‘vegan’ products.
However, a vegan diet is probably fine in a state of low stress. Say you’re a healthy adult who’s getting plenty of sun, and doesn’t have to deal with most stresses of modern life (high PUFAs, EMF pollution, darkness etc.). Then a vegan diet focusing on digestible foods (meaning: no raw vegetables or unsprouted grains) could potentially be doable. Such a diet would necessarily include cooked potatoes and tubers, coconut oil and milk, ripe fruits and juices, mushrooms, some cooked vegetables, rice, some sourdough bread, spices, honey etc.
The key remains the thyroid and how you’re able to produce energy to mitigate the stress of living. Protein is highly important in a state of illness or stress, but less so in a well functioning organism. Say your pulse is constantly above 80 and your body temperature is above 98 upon waking up. Then your thyroid is functioning efficiently.
The obvious problem is that very few of us live in a state of low-stress, especially after our 20s. High intake of protein in the form of gelatinous cuts of meats, offal, high quality milk and cheeses has been shown to be very protective against stressful situations (being especially recommended for soldiers in combat situations). Modern life activate the fight or flight response in most of us on a regular basis. Obviously fixing your environment and removing the stressors can be an invaluable step, but chronic exposure to several stressors, including mere thoughts (Why Zebras don’t get ulcers, Robert Sapolsky) can lead to chronic state of stress and a degradation of bodily functions, all of which is made obvious by a reduced rate of metabolism.
In Paul Pitchford’s Healing with Whole Foods, which is largely focused on the foundations of a fully vegan diet, the value of high quality protein such as goat milk, honey or even meat is recognized as invaluable in states of illness. Supplementing with thyroid hormone, (Cynomel and Cynoplus), focusing on limiting PUFAs and limiting stress can probably allow a vegan diet to be successful. But we cannot ignore the value of high-quality animal products in time of need. Children, in particular would benefit greatly from a higher protein content and of course breastfeeding. This would give them a great start on life.