Barricade:

First, my approach will be "Don't ever shop at Whole Foods, because their corporate mission is to promote veganism." 

Their "Scientific Advisory Board" is entirely composed of old male vegans, they use the deliberately misleading ANDI scale to promote vegan foods, they heavily promote the vegan "Engine 2" diet -- even for kids, which is dangerous and irresponsible -- and they prohibit their employees from giving information on any other diet!  I buy most of my meat by the side from local ranchers, and I buy my produce elsewhere.  (Costco has a surprising selection of organic produce, FYI.)

That being said, my approach to a debate depends on who else is listening. 

The person I'm debating is never going to change their mind, because otherwise they wouldn't have the religious conviction necessary to confront me!  I'm only concerned about the effect on anyone who might be listening.

In the case of being confronted by a random vegan at Whole Foods or any other public place, you'll maybe have a couple random people overhearing you -- so it's not worth arguing and I'd just be looking to shut them down quickly, e.g. "What makes you think it's appropriate to accost random strangers and insult their food choices?"  If they still insist, "I eat this way because I've found it's the healthiest way for me to eat.  Good day."  Then leave.

If it's an Internet argument among people who might be convinced, you can point out:

1. Chimpanzees have tiny brains and haven't left the tropical forests in six million years.  Eating more meat than chimps is, quite literally, what made us human: it allowed us to survive outside the forests and support the calorie requirements of our big human brains (3-4x the size of a chimp's).  (See my series on "Big Brains Require An Explanation".)  This is not a matter of debate in the scientific community: it is an established fact.

So if you're happy living naked in the dirt in a tropical forest with no telephone or medical care -- without even shelter, fire or sharp rocks! -- by all means advocate chimpanzees as your dietary model.

2. Fruit is seasonal.  Anyone who doesn't understand that is ignorant of basic reality and has no standing to lecture anyone on diet.  Q: Where do all the fresh fruit and fresh veggies in the store come from for much of the year?  A: Either greenhouses or the Southern Hemisphere. 

The reason chimps eat meat is because there's no fruit around in the dry season -- even in tropical forests, let alone any climate with winter!  Sheesh.  Again, veg*ans are often completely ignorant of how food actually grows.

3. Vegetarians are sources of the most incredible falsehoods, like "Nothing can be burned for energy without first being converted to carbs".  Anyone who says that has no standing to lecture anyone on diet, because they're ignorant of basic, uncontroversial freshman biology.  Beta-oxidation produces ~80-90% of your energy at rest if you're healthy and haven't eaten recently.  Also see 2. fruit is seasonal, and the article series in 1.

Again, you can't reason with these people, because their arguments are based on crazy falsehoods with no basis in reality.

4. He's slaughtering innocent animals by existing: agriculture kills all the animals that used to live where crops are grown.  Just because you don't see them die doesn't mean you didn't kill them!

Next, the water figures commonly bandied about by veg*ans are wild exaggerations, based on the incorrect idea that even industrially-raised animals spend their entire life in a feedlot eating grains.  (They only spend the last few months of their lives there, getting fat...all cattle grow up on grass, because they have to!  Grains aren't a natural diet for cattle any more than they are for us.) 

Then, any animal raised on pasture returns all the "wasted" water it drinks to the earth in the form of urine and feces -- e.g. fertilizer.  Animals are a necessary part of this cycle, without which the dead plants simply build up.  Think about it: if grazing destroyed the land, the Serengeti and the Great Plains would have blown away into dust tens of millions of years ago.  The Dust Bowl was created by agriculture -- destroying soil that took millions of years to create.

More info here, and in Lierre Keith's The Vegetarian Myth.

5. These people talk about "juice fasts" a lot.  PROTIP: it isn't a "fast" if you're drinking liquid sugar.

Read Lierre Keith's The Vegetarian Myth for a long exploration of these issues.

Again, though, I don't bother contradicting them unless there's an audience in danger of being convinced.  Otherwise I just shut them down as mentioned above.

JS

"Every single thing the guy puts in his body is pure fruit….The guy clearly has very little muscle mass but he's super passionate about his model of eating.  I see a fire in his eyes when he speaks of eating a pure fruit diet."

Of course...he's on a constant sugar high!

And of course he's got no muscle mass...he's eating no protein.

Most of those people crash and burn after anywhere from a few months to a few years unless they supplement or secretly cheat...the nutrient deficiencies catch up.  They also usually lose teeth from all the acidic, sugary fruit.  But just as there are a few people with the genetic gifts to run a four-minute mile or deadlift 3x their bodyweight, there are a few people with the genetic gifts to survive on that sort of diet. 

As far as your own diet, are you trying to gain mass?  Make more stews.  Jamie at Chaos and Pain has a great series on stew going right now...you'll find it much easier to get plenty of meat down the hatch, and you'll be able to start with cheap cuts like chuck roast instead of expensive steaks. Plus you can make huge batches, freeze them, and defrost as needed = less prep time, more eating. And veggies taste much better in stew than they do on their own.

Finally, re: the Japanese, diet isn't a thing where you have to get everything right or you die young.  The Japanese did a lot of things right: no wheat, lots of fish and shellfish, lots of seaweed (iodine and minerals), and lots of mouth-to-tailfin eating.  (Note that the much-vaunted Okinawan diet is high in fatty pork, contrary to vegetarian propaganda.)  So the fact that lots of carbs from white rice isn't optimal was balanced by the nutrient density of the rest of their diet.

I, too, feel far more kinship to the hunters of the past: none of the agrarian utopias ever resonated with me, because many of my relatives were farmers and I know that it's basically drudgery from sunrise to sunset.  Imagining Head-Smashed-In still gives me chills.

Thank you for your work!  Keep making more gnolls.

Paul:

I've always found fruitarianism to be very childlike: "I want to eat nothing but dessert for the rest of my life -- and I'm going to construct an elaborate series of rationalizations for the fact that I love sugar highs and never want to come down."

JS

OdysseusUnbound:

Welcome to the fire!  Do stick around.  And read Imagining Head-Smashed-In, if you haven't already.

JS