Your doctor is advising you very generally, recommending that every food item you eat be a low fat one (3g per 100g/ml). This will not guarantee weight loss, nor is it particularly healthy for you. Whilst you do need to not exceed your daily recommended fat intake (about 20% of your daily intake), there are plenty of foods that are high in good fats that are exceptionally healthy when eaten in moderation. Avocados, raw nuts, raw olive oil, coconut oil and natural nut butters are good examples. These products would far exceed the 3g per 100g recommendation, but can still be eaten in moderation so you are not exceeding your recommended daily amount. Calories and kilojoules are far easier to keep track of – the average woman should take in about 1500 calories/ 6300 kilojoules a day, but this is dependent on your activity levels and should be accurately calculated by a dietician.