[fredophile]

If someone with low financial literacy asked your advice on how to start a rainy day fund would you tell them to cut expenses and find ways to improve their income or would you just say "start saving". What's the difference between money in, money out and "start saving"?

In both the finance and nutrition examples you have two variables that you can adjust. In both cases they are not independent and changing one may affect the other or may affect other parts of your life that make it unsustainable. Eat less and do more is the correct advice for most people who want to lose weight.