You can survive on it, but you won't thrive
Measured on the things that actually predict health outcomes, vegans do well. Where a gap is real it comes down to creatine and long-chain omega-3, both of which supplement cheaply and neither of which has to come from an animal. The harder problem is that "optimal" is doing a great deal of work in this argument and is almost never defined.
Nobody serious claims a vegan diet kills you. The question is whether it is the best available, and several compounds concentrated in animal foods, creatine and carnosine and taurine and long-chain omega-3 among them, do sit lower in vegans. If you are trying to perform at your limit rather than merely avoid deficiency, removing the richest sources of those looks like accepting a handicap for reasons that have nothing to do with performance.
This is the better version of the health argument, and it deserves better than the answer to the weaker one. Whether a vegan diet is adequate is settled; the dietetic associations of the UK, the United States and Canada all say it is. This argument concedes that and asks a different question, so quoting those positions back is not a reply.
Start with where the gap is real. Creatine is the clearest case: the body makes about a gram a day from amino acids, dietary creatine comes almost entirely from meat, and vegetarians do carry lower muscle stores. That is a genuine difference. It is also the most easily closed one in nutrition. Supplemental creatine is synthesised rather than extracted from animals, it costs a few pence a day, and several trials find vegetarians gain more from it than meat eaters precisely because they start lower. Long-chain omega-3 is similar: algae oil supplies EPA and DHA directly, which is where the fish get it from in the first place. Carnosine and taurine do sit lower, and no clear performance consequence has been established for either.
Then the word doing the work. Optimal for what? Peak strength, endurance, longevity and cancer risk do not all point in the same direction, and the diet that maximises one rarely maximises the others. A claim to be optimising has to say what is being optimised, and this one almost never does. Without that it is not really a claim about diets at all.
There is also the question of who the standard is applied to. Someone eating meat and not supplementing creatine is not at optimal either, and very few people making this argument are operating anywhere near a performance ceiling. A standard invoked against one diet and not the other is not being used as a standard.
And it is worth noticing when this argument arrives. If it was the claim from the start, it is a fair one and the answer above is the answer. If it appears only after the question of whether a vegan diet is adequate has been conceded, the goalposts have moved, and the thing to do is say so and then answer it anyway.
- A vegan diet is adequate, but not optimal
- You can be healthy on it, but never optimal
- You can't reach peak performance without animal foods
- Vegans can be healthy, but they can't be at their best
Different wording, same argument. The response above answers all of them.
Raising the standard of proof once the original standard has been met, so that no amount of evidence is ever sufficient.
How it applies here: Arrives once the question of whether a vegan diet is adequate has been settled, replacing it with a standard nobody was defending and which almost nobody on either side actually meets.
Rejecting a workable option because it falls short of a perfect one. Also called the perfect solution fallacy. The relevant comparison is with the realistic alternative, not with perfection.
How it applies here: Treats anything short of theoretically optimal as a failure, a standard that would rule out virtually every diet anyone actually eats, including the one being recommended instead.