By NINA PLANCK

This particular calamity — at least the third such conviction of vegan parents in four years — may be largely due to ignorance. But it should prompt frank discussion about nutrition.

I was once a vegan. But well before I became pregnant, I concluded that a vegan pregnancy was irresponsible. You cannot create and nourish a robust baby merely on foods from plants.

Indigenous cuisines offer clues about what humans, naturally omnivorous, need to survive, reproduce and grow: traditional vegetarian diets, as in India, invariably include dairy and eggs for complete protein, essential fats and vitamins. There are no vegan societies for a simple reason: a vegan diet is not adequate in the long run.

Protein deficiency is one danger of a vegan diet for babies. Nutritionists used to speak of proteins as “first class” (from meat, fish, eggs and milk) and “second class” (from plants), but today this is considered denigrating to vegetarians.

The fact remains, though, that humans prefer animal proteins and fats to cereals and tubers, because they contain all the essential amino acids needed for life in the right ratio. This is not true of plant proteins, which are inferior in quantity and quality — even soy.

A vegan diet may lack vitamin B12, found only in animal foods; usable vitamins A and D, found in meat, fish, eggs and butter; and necessary minerals like calcium and zinc. When babies are deprived of all these nutrients, they will suffer from retarded growth, rickets and nerve damage.

Responsible vegan parents know that breast milk is ideal. It contains many necessary components, including cholesterol (which babies use to make nerve cells) and countless immune and growth factors. When breastfeeding isn’t possible, soy milk and fruit juice, even in seemingly sufficient quantities, are not safe substitutes for a quality infant formula.

Yet even a breast-fed baby is at risk. Studies show that vegan breast milk lacks enough docosahexaenoic acid, or DHA, the omega-3 fat found in fatty fish. It is difficult to overstate the importance of DHA, vital as it is for eye and brain development.

A vegan diet is equally dangerous for weaned babies and toddlers, who need plenty of protein and calcium. Too often, vegans turn to soy, which actually inhibits growth and reduces absorption of protein and minerals. That’s why health officials in Britain, Canada and other countries express caution about soy for babies. (Not here, though — perhaps because our farm policy is so soy-friendly.)

Historically, diet honored tradition: we ate the foods that our mothers, and their mothers, ate. Now, your neighbor or sibling may be a meat-eater or vegetarian, may ferment his foods or eat them raw. This fragmentation of the American menu reflects admirable diversity and tolerance, but food is more important than fashion. Though it’s not politically correct to say so, all diets are not created equal.

An adult who was well-nourished in utero and in infancy may choose to get by on a vegan diet, but babies are built from protein, calcium, cholesterol and fish oil. Children fed only plants will not get the precious things they need to live and grow.

Nina Planck is the author of “Real Food: What to Eat and Why.”