Vegans seem to be novices/misinformed, about... Well, everything.

Nutrition
Recently got into a conversation with a plant-based person who was suggesting foods for iron deficiency, specifically for children. Lots of it included things like spinach, black beans, lentils, the usual suspects. Also claimed that iron deficiency in vegans/vegetarians in large population studies normalized and that low iron isn't actually something to be worried about. I tried pointing out that it's something that exvegans often talk about and there are meta-analysis studies that say vegetarians have a higher risk of iron deficiency. They deflected by saying, "the study only references ferritin levels," not understanding that ferritin is *literally* what you measure to determine iron deficiency and anemia. They also doubled down when I pointed out that vegan children died and there are actual articles in the news about it. The response was that vegan parents were just... doing it wrong. They made assumptions that the parents were feeding their kids only fruit juice or something dumb like that. Not that there was anything wrong with the vegan diet itself. There's also the constant misinformation around Vitamin A/beta-carotene conversion, the heme-iron/non-heme iron conversation, ignoring the bioavailability of nutrients of animal foods and plant foods, and multiple nutritional assumptions that are built on lies/inaccuracies.

Ecology
Some of the echo chamber people want a world without death. Having predators die out, herbivores overrun the environment, and letting nature run it's course. Anyone who actually knows anything about ecology understands that it's a balance of predator -> prey -> environment that keeps things in check. If any one of those things get removed, the ecosystem slowly degrades until it becomes unsustainable. This is covered in The Sacred Cow, but it's also just a logical conclusion. If any population becomes too overpopulous or disappears, the cycle breaks since it leads to a chaining cascade of negative effects. It's not as simple as nature "fixing itself" if there are no predators around. I've often learned that you have to think about nature in multiple systems, same as the way people should approach learning about the human body. Focusing on one singular detail to the point that it's detrimental means that you're not keeping the whole system in mind, and there *will* be unintended consequences for focusing on just one detail.

Animal Husbandry
There are some memes out there from vegans that are emotionally manipulative and imply that cows die to give their milk. There's also an oft-cited "statistic" that 90% of corn supposedly goes to animal feed, but the reality is that once the US government started creating biofuel, that ratio has changed over the course of the past few decades. It's ultimately still the same ratio of corn for human consumption, 12%, 45% for biofuel, and then the rest is animal feed. It fluctuates, of course, but the idea that humans can consume the corn stalks and hulls that is turned into animal feed is just misinformation. They also tend to lump in cattle with pigs and chickens, although they have highly different feed requirements. Feeding corn to cattle their whole lives actually makes them sick, so it's only during the finishing stage that they're fed corn. They're otherwise on pasture the first part of their lives, which gets lost in the conversation. Factory farms are often filled with pigs and chickens more so than cattle, but cattle get the brunt of the environmental concern. This also ties into ecology, since ruminants wandering the plains have a net benefit effect on the ecosystem around them. Rotational grazing with ruminants like cows, sheep, and goats can do things like bring back birds, insects, and build soil life, but that aspect gets ignored to continue pushing their narrative.

Anthropology
Scientists have analyzed isotopes from Neanderthals and figured out our previous ancestors were highly carnivorous. There wasn't much of any plant material found in their digestive system. Sure, it doesn't imply that they didn't eat plants *at all* since it's difficult to detect that, but it surely doesn't mean that our ancestors were plant-based. Our brains are 60% fat, and to think that we can grow that without animal protein is ridiculous. It requires too much energy to just fuel on a plant-based diet. Veganism and vegetarianism being associated with a higher risk of mental health issues reaffirms that our brains have intense requirements that plant-based diets don't typically fulfill for most people.

History
Then, there's the history aspect. People often reference Japan being vegetarian for a period of time for "Blue Zone longevity", not understanding that was a period of colonization for the country. Meat was reserved for the ruling class, and people were just trying to get by however they could. Any time that a plant-based diet has been initiated in a country, it's often due to power dynamics that made meat become inaccessible for most people. Starvation leads to people figuring out what they can survive on, and more often than not, vegetarianism has ties to roots like politics and religion rather than anything health-based. Culturally, Japan also tends to place pescetarianism under the same umbrella as vegetarianism, which muddies the waters even more.

All of this to say, a lot of plant-based information is just flat out wrong, missing key pieces of information, or deliberately misleading.