There's more to it than just energy. As a practical matter, nutrient quality is tremendously important. Some animals, particularly ruminants (cows), are able to convert low-quality plant matter into high-quality easily digestible protein with amino acid ratios that closely match what humans need. It's the bacteria in their guts that perform the conversion. We can certainly get what we need from plant sources but this requires careful meal planning to obtain the right amounts of essential amino acids without exceeding energy balance.

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