I know what the best supplement is for being fit and it's all natural.
The best supplement is water! Not only is life impossible without water, but so is fitness. Why is water so great?
Water fills you up. Being full means that you are much less likely to snack. Drinking water with your meals means you will eat less food. Even though it takes up space, water is great for you.
Water has 0 calories. Soda pop has calories. Energy drinks have calories. Milk has calories. Drinking these things instead of water isn't granting you any benefits. Water has no carbohydrates (carbs), fats (lipids), or proteins.
Water keeps you hydrated. When you are hydrated, you are less likely to crave food. People often mistake thirst for hunger and end up eating when drinking would satisfy them. Don't let this happen to you; remember to drink plenty of water.
Water is cheap! Well, maybe not that fancy bottled water that costs 3 dollars a bottle, but tap water is. Depending upon where you live, a gallon of water could cost only pennies.