When lesser carbs (lower than 50 grams) are consumed in a day; our body eventually runs out of fuel (blood sugar) it can quickly utilize; which usually takes 3 to 4 days. Then eventually the body starts to break down protein and fat for energy, resulting in weight loss. This process is called ketosis.
https://ipsnews.net/business/2021/12/03/best-health-keto-uk-reviews-burn-fat-for-energy-not-carbs/