Regularly binging on carbonated soft drinks and donuts or biscuits may help speed up weight gain, but it can destroy your health at the same time. The healthiest way to add a few kilos is by gaining a balanced amount of muscle mass and subcutaneous fat - the fat stored just beneath the skin. Muscle mass can be increased through weight training, or by taking recommended supplements. On the other hand, subcutaneous fat can be increased by eating foods that are rich in natural fats and oils such as avocados and salmon. Therefore, it is crucial to eat healthy foods and live an overall healthy lifestyle.