30 lbs in 30 days is not healthy. You didn't gain 30 lbs in 30 days, right?
40 lbs in 3 months is not realistic, but not ideal. If you don't care about health then it's easily attainable. But keep in mind you will stress your body and probably gain back some (if not all) of the weight. If you are truly looking to lose weight and become healthy, you need to lose 1-2 lbs per week. That means 40 lbs should take roughly 4-5 months.
Only you can choose what is best for your lifestyle. But from a health standpoint, it's been proven that 1-2 lbs per week is the healthiest. Eating well and exercising is the healthiest way to achieve your goals. I've never heard of the shift diet. But I disagree with any diet that promotes 1,000 calories per day. Unless a doctor prescribes it, no one should eat less than 1,500 calories a day.
Good luck with whatever you decide is best for you!
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