Losing weight gradually is more likely to be lasting and is gentler on the body than crash dieting. About 0.5 to 1 kg per week generally reflects fat loss rather than mainly water and muscle, and it is achievable through a modest daily calorie reduction combined with more activity.
Faster loss can happen at the start, especially when reducing carbohydrates (water weight), but very rapid ongoing loss raises the risk of muscle loss, gallstones, nutrient deficiencies and fatigue. Keeping protein adequate and including strength activity helps protect muscle while losing fat.
Very low-calorie diets or rapid medically supervised weight loss are sometimes used for specific reasons, but they should be done under professional guidance. Seek medical advice before starting if you have diabetes, heart disease, an eating disorder history, or take medicines that need dose adjustment as you lose weight.