Final answer:
Extreme calorie-restricted diets are considered unhealthy due to rapid weight loss, nutrient imbalances, potential kidney strain, and the difficulty of maintaining them long-term.
Step-by-step explanation:
Extreme calorie-restricted diets are considered unhealthy for several reasons. Firstly, they often lead to rapid weight loss, which is usually due to loss of water rather than fat. Secondly, these diets are often unbalanced and restrict important nutrients found in foods such as fruits, dairy, or whole grains. Thirdly, extreme calorie restriction can put strain on the kidneys and lead to health issues like kidney stones and gout. Lastly, these diets are difficult to stick with long-term, and once the weight is lost, people often go back to unhealthy eating habits and regain the weight.