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The thing is that you have to burn calories to lose weight. The more intake of calories, the more work out is required to burn that much calories.
So, eat rice and potatoes (must be really tasty!) but work out more. And lastly never go for the treadmill !! It is a dangerous thing and is found to bring about a heart attack to otherwise healthy youngsters even. Jogging for a longer time (no running around the ground as it is too like treadmill) and weight exercise.
Having said all that, I don't think rice and potatoes should be kept so close to heart!!! :)