A balanced and healthy diet is the key, even

for people who want to lose weight. "Black rice" is all carbohydrates, but our body also requires fats and proteins and vitamins and minerals. For a balanced diet, carbs should make up about 55% of our total calories, fats about 35%, and proteins about 15%. 

And it can do our bodies harm if we lose weight too quickly. Generally, losing one or two pounds per week is a reasonable goal.

By the way, 1 gram of fat = 9 calories (or Kcal).