The simple act of recording all purchases of food and especially drink was itself enough to reduce consumption - "I could buy a chocolate bar in the vending machine, but then I'd have to note it in the accounts".
I've found that being deliberate about any of the aspects tends to lead to an improvement, and oftentimes you're deliberate about all/a lot of it. A huge shift in both health & wealth comes from switching to home-cooked meals, and then paying attention to what you put in those will help you save money and eat well.