What about the case of desperate poverty, where a whole family goes hungry?
Starvation is not fasting and it is the duty of others to help out the distressed. There are many teaching stories from the Vedic Puranas about the "poor righteous" - usually Brahmins in the story - who get helped out in desperate trouble. But, starvation in this day and age is a disgrace to our common humanity. What to do if we fall through the safety net? I've had that experience myself and it is not easy at all. An American friend of mine flirted with the reality of having no money and no food to eat for years. He would get down to his last can of food regularly. These situations, even if they are karmic, are not easy, and lead people to desperate acts in order to save their kids from this kind of fate. Involuntary fasting is a whole different thing, a sad thing, something that should make us all hang our heads in shame. And it happens regularly.
Only when you have both lived in the west and a poorer country do you come to realise just how much the well-off have, and how much the poor have. In my business persona, I've often taken long taxi trips in different countries and then stopped off to see the taxi drivers families. In the west we take it for granted a house has clean walls, heating, carpets, furniture, a well stacked fridge and so forth. This in a global scale is a rarity. So: another thing, why not dedicate your fast to the benefit of world hunger?
Why should this have any effect? Isn't it naive to think so?
If you believe the world is simply what you see in front of you, Then I agree. But, as the saying goes, when you can see the back of your own eyeball, tell me what you then think of this issue. The hidden element in life becomes self evident through careful sadhana. We do not just eat, live, excrete and die. We swim in a wider sea.