An old Russian proverb says you should "Eat your breakfast alone, eat

Russian proverb - "Eat your breakfast alone, eat your lunch with your friend, give your dinner to your enemy."

Dietitians (new) - "missing dinner twice a week actually contributes not only to the patients' weight loss, but also to their general health"
Dietitians advocate missing dinner twice a week instead of never having it (As per Russian proverb).

Assumption of Dietitians?
A. While the Russian proverb argues that dinner is a problematic meal, it does not promote forsaking it altogether.
Seems Perfect as the Dietitians are advocating that missing dinner twice a week actually contributes to the patients' weight loss and their general health

B. Eating dinner with enemies is a sign of reconciliation, which may improve one's health.
The Russian proverb doesn't literally mean that one should eat dinner with enemies. So this option can't be an assumption of Dietitians.
C. While eating solids is to be encouraged at breakfast and is permissible during lunch, Dinner should contain only fluids.
nowhere in the argument type of food is discussed for each meals. this option is out of context.
D. The Russian proverb states that one should give his dinner to his enemy, so that one never has dinner.
If this statement is an assumption then Dietitians would not advocate to miss dinner twice a week.
E. Russian metabolism works differently from western metabolism, and therefore while for the Russian the evening meal is merely problematic, the westerner should actually do without it.
This option is a rephrased version of option D.

Now, lets apply 'Negation Test' on the main clause of Option A - "it (Russian proverb) does not promote forsaking it altogether"
Neg Statement - Russian proverb does promote forsaking dinner altogether - In this case the argument/conclusion of Dietitians fails altogether as the Dietitians are advocating to miss dinner couple of times only.
Therefore option A is correct Answer