Yes, that is a subjunctive used in relation to a conditional.
If you want to go on a diet...
That sets up the condition, albeit with minimal detail and you have to construct that from other phrases/clauses.
a dietitian ... would tell you...
Is the hypothetical (hence subjunctive) event that one expects to occur in relation to that condition.
In fact is just sugar, really, set up to indicate this sentence's relationship with prior text. The modifying effect of one of the first things is hopefully clear, as is that of in Sri Lanka.
The overall effect is to state that, should one tell a dietitian in Sri Lanka that one wanted to go on a diet, the expected result is that they will tell you to moderate the use of coconut milk.
However, it's a little different to the most natural way I would express it. It uses want rather than wanted, and if this were a purely hypothetical situation I would expect the latter. The effect, to me, of using the simple present in the conditional is to create a greater sense of dialogue between the text and the reader; you used in the specific rather than the general sense. It thus feels like it is actually talking about the possibility that the reader does, right now, want to go on a diet.