I think that what you say has to be true at the population level. (The panda provides an obvious counter example, but since the panda is going extinct, that is merely a nit pick. Successful species, such as rats or humans are robust to wide variations in their dietary content.

However, at the micro-level, the truth of this proposition is maintained by ruthless culling. The rat population is riddled with weaklings, who lose out in life due to diet-induced health problems.

As for the human population, we have a different perspective on these matters. I can start from a prior belief that my body is probably robust to wide variations in dietary content, but I need non-zero probabilities on a wide variety of dietary vulnerabilities so that updating will work if events supply evidence.