I was told the same thing - if you want it - eat it.
ANYTHING is better than nothing, and if you have cravings etc, then your body needs it - be it salt, sugar, fat, chocolate, a certain fruit etc.
We can worry about any weight gain once we are healthy(and to be honest Ihaven't gained ANY weight while on chemo - the amounts I ate were so small and the frequency of my eating wasn't great either, but I forced myself to eat at least twice a day.
And believe it or not - when nauseous eating small frequent amounts can really actually help with stopping the nausea, it worked for me - I ate a tiny bit every 2 hours the 2 days after chemo when nauseous (1st cycle and last 3 cycles - last 3 from neulasta), it really helped - it might only have been a piece of cheese or a couple of crackers or a piece of toast or 1/2 a cheeseburger, or a handful or popcorn chicken,or even a slice of cake but it was something and it really helped (until I went to sleep -then I didn't of course eat :o)