No fat and low fat diets rarely work because your body is acting against you. Because fats are essential if your body isn’t getting any fat in the diet it starts to conserve the fat supplies it has in the body. Inevitably this is not going to lead to weight gain. Taking in a proper ratio and healthy fat actually help aid in weight loss. Healthy fats like olive oil will stimulate bile flow to improve digestion and break down fats properly. Good fats also contain HDL cholesterol (the good cholesterol) that circulates through the body and helps get rid of plaque deposits on blood vessels.
Your body also depends on vitamins and minerals to survive. Fat soluble vitamins use fats and oils in your food to get transported through the body to its destination, or to be stored. Without these good fats to carry and store certain vitamins you are leading yourself to a possible nutritional deficiency.
Remember to keep fats in your diet in a good ratio. Fat is a long term source of energy so it can be used in higher ratios for certain athletes. Also fats are necessary for brain development in babies and children.