Dr. Dan lays out the theory: Optimal Foraging Strategy.
When you look at cost/benefit analysis, a major investigative technique in optimal foraging theory, it becomes obvious that meat is by far the more efficient choice. Optimal foraging theory is a powerful new theory which states that animals forage in a highly efficient manner so that they maximise energy intake and minimise energy expenditure. For example, a crow trying to crack open a nut will fly just high enough so that it cracks the nut but no higher. it could fly 100’s of meters into the air and drop it but this would be a waste of energy and would not be ‘optimal’ foraging. Optimal foraging theory has led to a much greater understanding of many ecological and evolutionary concepts (adaptation, energy flow, competition). And so it is true for our paleolithic ancestors as well.
Want a practical example? Observe various plant foraging or grazing animals. Pretty much, they're eating all the time. Gorillas are a prime example. To sustain those massive, strong bodies on the fibrous, very low density food they eat, they pretty much have to be chewing every waking hour. Then you have the ruminators. When they're not grazing, they're chewing their cud. Now, contrast that with predators like wolves and big cats. Most of their day is spent lying around. (Dogs too)
Here's another practical example of this. While not foraging, these guys clearly understood that if they ate only protein and fat, then they could significantly reduce the weight they would have to carry on a trek that took over a month.
A trio of Canadian adventurers said Friday they have set a new record for fastest trek across Antarctica to the South Pole, after suffering through whiteout conditions, temperatures as low as minus 40 — and a steady diet of deep-fried bacon and butter.
Ray Zahab said he and his teammates completed the 700-mile journey from Hercules Inlet on Antarctica's Ronne Ice Shelf to the South Pole in 33 days, 23 hours and 30 minutes, finally arriving Thursday. […]
…They kept themselves fueled with a 7,000-calorie-a-day diet of deep-fried bacon, cheese and huge chunks of butter.
All very high fat choices; fat is 9 calories per gram vs. 4 for both protein & carbs. Fat metabolism is ideal for such a trek requiring continual steady exertion over a long period of time in adverse conditions. At 7,000 calories per day for three men, that's 21,000 calories. Assuming they had enough for 40 days, that's 840,000 calories. Given that bacon (depending on composition) has some protein, as well does cheese (butter is negligible), let's say the diet is 80% fat, 19% protein, and 1% carbs (the cheese). So, that's 672,000 calories from fat, 159,600 calories from protein, and 8,400 from carbohydrate. Let's discount water for this, but keep in mind that high carb diets have more water in the food, as well as make you to retain several pounds due to the glycogen stores. So, in terms of the poundage, there's 453.5-ish grams in a pound. The fat on that 700 mile trek in minus 40 from the outset would be 165 pounds. Protein weight would be 88 pounds, and carbs would be 4.5 pounds, for a total weight of 257.5 pounds.
Now, suppose the diet was instead high carbohydrate, as many of these trekkers do (I see them eating baked potatoes on TV shows a lot). Let's say it's 50% carbs, 30% protein, and 20% fat. Total weight? 412 pounds, a 155 pound advantage — one they didn't only have to carry between the three of them, but had to eat over time! So, each guy would have had to consume an additional 52 pounds of food to get the same energy. The result of that extra weight and food they'd have to consume would up their caloric requirements as well, or, they would have to lengthen the time of the trek (which could also increase caloric requirement, etc.).
Vegetarianism may be fine and dandy, but those people must be seriously math challenged.
Oh; by the way, though they don't say what the previous record holders ate, they did beat the previous record by about 6.5 days.