The Case for a Plant Based Diet
Firstly, we know that plant-based diets can be at least adequate, because there is no essential nutrient (which means, a nutrient essential for survival) found only animal food products that cannot be found from vegan sources. Essential nutrients include things like vitamins, minerals, a source of sufficient calories, essential fatty acids, etc. Vegan food and supplement sources include things like plants (grains, beans and legumes, fruits, vegetables, nuts and seeds), fungi (mushrooms, nutritional yeast), bacteria (which produce Vitamin B12), or inorganic materials (inorganic means “nonliving”, like minerals, if, just suppose, they were taken directly from the ground).
Note: Simply getting enough vitamins and minerals to survive doesn’t mean this necessarily produces a diet you would thrive on. Plants may contain other things, including fiber, water content, and phytonutrients (basically a catch-all term for any potentially beneficial compounds found in plants), that, while not essential for short-term survival, can be very good for you.
Traditionally, research into vegetarianism focused mainly on potential nutritional deficiencies, but in recent years, the pendulum has swung the other way, and studies are confirming the health benefits of meat-free eating. Nowadays, plant-based eating is recognized as not only nutritionally sufficient but also as a way to reduce the risk for many chronic illnesses.
In fact, it’s possible some of the leading causes of premature death in the Western diet could be addressed (or there is growing evidence that they can be) through healthy plant-based diets. One such piece of evidence for why, is the link between saturated fat intake, and increased risk of heart attack (the single leading cause of death); typical non-vegan diets that include meat, dairy, and eggs tend to be higher in saturated fat, whereas plant foods tend to be lower in it.
However, this does not mean every plant-based diet is the same. We know that diets centered around whole plant foods (without the fiber removed and without large amounts amounts of sugar, salt, and fat added) tend to be healthier than diets containing those things. https://www.hsph.harvard.edu/news/hsph-in-the-news/not-all-plant-based-diets-are-healthy/
For example, trans fats are also very bad for you, and are found in high levels in deep fried foods (including vegan ones), and should be avoided.
If you want the “healthiest diet possible” the best thing to do is follow not only a plant-based diet, but a whole foods plant-based diet (sometimes abbreviated WFPB) that emphasizes whole plant foods and reduces or eliminates added fat, refined carbohydrates (like sugar) and salt. This can include paying attention to specific cooking methods (boiling, stewing or steaming, instead of baking, roasting, or deep frying) and problematic ingredients (some margarines made from vegetable oils might technically be vegan, but if they are hydrogenated to include high levels of trans fats, they are not good for you).
For a basic introduction to vegan nutrition that includes substitutions and in what proportion to aim to eat them, you can look here, and for a more comprehensive one, there’s http://www.veganhealth.org/