You might be underestimating how nutrient-rich a steady diet of salmon can be.

According to a ranger in Denali I spoke to: It depends on where the bears live in relation to the food. Coastal grizzlies are much larger because they have plentiful fat and protein sources like salmon. Inland grizzlies that live near glacial streams don’t have that food source and subsist on larger quantities of berries and the like and are noticeably smaller than their coastal counterparts.

So the food sources directly impact the sizes of the animals within the same species. The grizzly I saw in Denali appeared no larger than well-fed black bears I have encountered in the Sierra Nevada range. I say appeared, because it very well may have been larger. But it certainly didn’t look like the ones near the coast.

Kind of an answer, but I’m not a bear biologist.

I think comparing our diets to bears does not make sense at all … our systems are completely different.

A bear can eat massive amounts of food in a short period of time, gain lots of weight and then fast for several months and go to sleep and do nothing.

If we tried the same thing, we’d probably go into cardiac arrest, or some serious intestinal problems if we ate too much too fast in a short amount of time … and then we’d ruin our kidneys and liver and probably die of starvation if we went on fast after.

Depends on how much you ate, but similar to us, bears can be omnivores (though they tend to eat more meat type stuff): They’ll eat whatever they can, which can definitely include a lot of berries and fish, but they also eat nuts, roots, insects, honey, carrion, etc.

Bears tend to eat larger portions.

If you ate berries and salmon in the quantities bears do, you would absolutely not be very slender. That’s a diet of protein, fat, and sugars.