According to recent studies, people whose diet is unusually high in pr

Responding to a pm:

Premises:
People with high protein diet are more likely to suffer from insomnia than people with moderate protein diet.

Conclusion:
It is likely that a diet high in protein is a cause of Insomnia.

We don't need to dwell on "perhaps not the only cause" because unless it is specifically mentioned otherwise, we understand that if A is a cause of B, A may not be the only cause. Good that the conclusion mentions it clearly but it was not really needed.

We need to weaken "a diet high in protein is a cause of Insomnia"

A) Diets that are high in protein are often low in foods that help keep the immune system working effectively.
Out of scope. No idea how immune system affects sleep.

B) People whose diet is unusually high in protein often have other characteristics in common, some of which are likely to cause insomnia.
This says that people with high protein in their diet have some other characteristics too (e.g. too much consumption of raw foods, stressful exercise regime etc). Some of these could cause insomnia. This means that high protein may not be the cause of insomnia at all. Our conclusion says that P is a cause of insomnia. This tells us that people who have P, also have Q, R and S characteristics. Q/R or S could cause insomnia. So we can say that P may not be a cause at all. The cause is replaced by another cause. Q, R and S may not be additional causes but they might replace P since people with P have Q, R and S too. So we don't really know what is causing insomnia. Hence this weakens our conclusion.


C) Reducing the amount of protein in one's diet can be harmful.
Out of scope.

D) People whose diets are unusually low in protein often experience insomnia.
This says that low protein causes insomnia. It doesn't weaken that high protein causes insomnia. It is certainly possible that high or low amounts of something cause the same problem. Perhaps you need a balanced amount to sleep well. This is just another cause of insomnia and hence doesn't negate that high protein causes insomnia.

E) Insomniacs who switched from high-protein diets to more moderate levels of protein intake have reported that the change was so unpleasant that it worsened their insomnia.

Out of scope. This says that "change" worsened insomnia. It doesn't mean that high protein was not causing insomnia.

Answer (B)