A fast change on the scale is not always a fast change in body fat. Fluid, stored carbohydrate and digestive contents can affect short-term weight. The cause and method matter more than treating a single weekly number as universally safe or unsafe.
Be cautious with severe restriction
Very-low-calorie plans can make it difficult to meet nutritional needs and may require medical supervision. Rapid weight loss can also increase gallstone risk. A plan should account for health conditions, medicines, activity and the ability to maintain adequate nutrition.
Choose a repeatable routine
Instead of compensating for a large meal with a crash diet, return to regular meals and a manageable activity pattern. Review changes over time and seek help if a plan leads to dizziness, weakness, persistent hunger or distress around food.
Unintentional loss is different
If weight is falling without trying, especially with other symptoms, discuss it with a healthcare professional. Do not assume it is a positive result or try to explain it with an app’s calorie estimate.
Source: NIDDK: dieting and gallstones.
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