Catholic rules for fasting are rather simple – one can have three meals; and as long as the two lesser meals combined do not equal the mount of the main meal, a person is observing the 'fasting regulations'.
It doesn't take much ingenuity to figure out that the only real problem might be 'in-between' snacks. A person could actually eat ll they wanted for breakfast and lunch – and as long as they consumed more  at the other meal, they would still be technically 'fasting'.

Dieting would be more challenging!

Better idea would be to figure out some new 'virtue' one would like to acquire, and for 40 days set about acquiring it and making it habitual.