There are certain “persistent” pollutants that tend to accumulate in any living thing, whether animal or plant. The higher up the food chain, the greater the accumulation. Take swordfish for example. Swordfish is known to have high levels of mercury, a toxic metal.
The problem starts when small bait fish eat organic material that contains mercury. They metabolize the organic material but retain the mercury. Small predatory fish then eat the bait fish, thus inheriting their accumulated mercury. The more bait fish they eat, the more mercury they accumulate. Finally a swordfish comes along and eats a bunch of small predatory fish, and picks up a load of mercury. So a big swordfish can gather a lot of mercury over time. But the swordfish is not the end of the chain. You are! You eat the swordfish, and now all that accumulated mercury is stored in “your” body.
Swordfish is only one example out of hundreds. Here’s the point. You’re at the top of the entire food chain on earth. Therefore, you unknowingly accumulate heavy metals and chemical pollutants, which are proven to be detrimental to your health. If you consume animal foods, you need dietary supplements to help you handle the pollutants they contain.