What a calorie deficit means

How large it can be

The UK's NHS names about 600 kcal less per day.^[1] That is a general guide. Your needs are only an estimate, so your deficit is too. See How much energy do I need?.

Which diet fits

In a large trial, 811 adults with overweight followed four plans with different amounts of fat, protein and carbohydrate. After 2 years, weight barely differed between the plans. Hunger and satisfaction were similar too.^[2]

So no single nutrient was the key to weight. That speaks for choosing the form you can keep up in everyday life. What you lose is a different question: for keeping muscle, there is a lot to be said for more protein.

Why the numbers are not a straight line

When your weight falls, your expenditure changes too. So the same deficit does not bring the same result week after week.^[3] See Why the app gives no target date.

More is not better

Diets of 800 kcal a day or less count as very low-calorie and, according to a task force of the US National Institutes of Health, belong under medical supervision.^[4] See When to get medical advice.

How the app handles this

With the goal “lose weight” you can choose a deficit. The app shows a target range and keeps to fixed lower limits. For large deficits it points to the usual values from guidelines. In some cases it suggests no deficit at all, for example with a low BMI or after certain answers to the safety questions.