When you start a low carb diet, you immediately lose a considerable amount of water weight. It's why people consider Atkins to be a scam (as they would immediately lose 5-10 pounds only to gain it back just as fast when they stopped dieting).

However, I have some personal experience eating low carb, and the rate of weight loss, for me at least, was around 2 lbs/week, as predicted by my food/calorie tracking app (MyFitnessPal).

It's loss of water weight. Low carb dieters shed initial water weight very quickly.

If I understand correctly, it's because some carbs are stored in the liver compounded with water as glycogen. When you start low carbing, the glycogen starts being used and the extra water is released (via your kidneys).

I started carb reduction without reading much about it and experienced the classic effects: quick weight loss, dehydration, muscle cramping, sudden loss of appetite. Maintaining hydration and electrolyte balance is important when adjusting your body towards using less carbs.

Well, about twenty implies not much better than one sig fig, so I'm not willing to go more precise than around a pound/day.

A pound of fat is a mere 3500 calories. Why can he only be losing pure 100% fat? As a farmer rule of thumb a pound of raw cow is only about 1000 calories. I looked it up on google and its actually closer to 1300. Anyway the point is that your average people-of-walmart dieter probably biologically internally resembles a cow more than, say, a tank of olive oil, so I'd expect a delta-one-pound for a human to be much closer than 1300 or so input calories. If on the other hand you are a tank of biodiesel wondering how many calories of olive oil to add to weight one more pound, then sure it'll take 3500, but us mammals are more "a bit over a thousand cal per pound" in overall bulk average. Not this 3500 foolishness.

Finally maybe TMI but at the fattest weight I was ever at, given my height, a basal metabolic rate calculator estimated me at around 2500 calories just laying around doing nothing, like in a coma. But I like long distance hikes and stuff like that and even just housework burns more than the catatonic basal estimate. So I can easily see 3500 cal going out per day, cutting intake back to a "mere" 2200 calories per day coming in, net big mammal carcass equivalent of 1300 cal per pound, ta da, loss of a pound a day still eating a chezeburger for dinner.

Its quite achievable.

It's definitely not fat, when people with horrible diets eat less sodium they lose a surprising amount of water weight. I've seen estimates an extra 400mg of sodium being retained leading to an extra kilogram of water.

For the very overweight this is very likely; running a substantial daily caloric deficit just by eating healthy and drinking water will cause large early losses. This isn't sustainable, however, and really just kickstarts the process. As someone nears a breakpoint (different for everyone) they'll fall into the one pound per week rule.

I lost seven pounds in the first week when I started changing my diet. It was primarily by significantly reducing my sugar intake (snacks, soda, and sheer easy calories). I was nowhere near this guy's weight, but I hit a high of 197 and freaked out and began changing my lifestyle.

It's clearly possible that he lost 20 pounds.

Depends where you start from. If guy starts from 300 lbs, it's not a biggie to lose that much "water weight". I started from ~170 lbs, lost ~5 lbs in first month, but it took me five more months to lose 20 more lbs.