system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What are the benefits of eating consumers (primary, secondary, tertiary) vs producers?

In my biology class, my professor makes it sound like humans would be way better off just eating plants because 90% of energy is lost in each chain, so 90% is lost to primary consumers, 90% again to secondary, and 90% again to tertiary, and the end result is we get next to 0 benefit eating consumers as opposed to producers. On top of that, toxins concentrate as they move up the food chain.

So is there any truth to this, or is my professor just a hippie vegan using pseudoscience to push an agenda? Would humans really be better off just eating plants?
gpt
Your professor is right - as a general rule it is better to eat as far "down" the food chain as you can because you lose energy each time you move up a run.  Eating vegetables gives you 100% of the energy from the vegetable stock, but eating the animal that ate the vegetables gives you less because the animal had to expend energy just being alive that doesn't translate to the muscle mass you consume.

That said, there are still benefits eating consumers vs. producers.  It is actually somewhat difficult for humans to get a complete diet eating vegetables alone.  It takes a lot of careful watching of the diet for someone to be a healthy vegan - very much possible, but not something you can do without putting thought into it.  Eating meat, in contrast, gives us access to a complete diet without much thought.  It is also (in our current society at least) much easier and cheaper to be an omnivore than a pure vegan.  If you are someone that doesn't have time to shop and cook your own meals, you are much more likely to find complete nutrition from restaurants by eating both plants and animals vs. eating just plants alone - most restaurants don't provide vegan meals at all, and most of the ones that do do not take the time to ensure the nutritional completeness of that meal.

So, in short, your professor isn't wrong but they are also not factoring in the effort involved for the individual consumer.  We would be better off just eating plants, but it would require greater nutritional education and a deeper investment in providing access to  complete plant-based nutrition.