Take warm water with lemon in the morning, fasting, lose weight?

No! This idea has no scientific basis. Is probably linked to the belief that acidic foods burn fat (which is not true) and that the body uses calories to cool a hot or warm food (which is real but insignificant to lose weight). But the habit does not hurt - unless the person has gastritis! Lemon is rich in vitamin C and is low calorie. And water, warm or cold, should be taken throughout the day.