First, distilled water is water that had its impurities removed by distilling. Essentially boiling the water to produce water vapor, gathering the vapor and re-condensing it into a new vessel. So let's start with health implications:
- Hydration is all about water absorption in your body--drinking low salinity water can't make you dehydrated.
- You might be at risk of not replenishing electrolytes your body needs after vigorous exercise--but that risk existed before the water was distilled.
- Your primary source of nutrition should be from the food you eat--this includes vitamins and minerals in addition to the macronutrients your body needs.
- The lack of minerals in your water also comes with a lack of chlorine which is used in the purification process to kill bacteria. Basically, it's a draw here.
- In short: distilled water presents no inherent risk assuming you already use multivitamins.
Second, regarding the claim that it will improve muscle definition:
- The first barrier to seeing muscle definition is body fat. With enough body fat (above 10% in men), you will never see muscle definition no matter how much water you drink.
- Hydration is very important to fat loss and building muscle.
- Intentional dehydration for contests is always a temporary thing and rehydration either happens after weigh ins (for power lifting, weightlifting, and strong man events) or after the contest (for bodybuilding events).
In the interest of fairness, there are two opposing articles surrounding distilled water intake listed below. One is written by a distiller manufacturer and so biased in that way, and the other is about as equally suspect in the way they present the information.
The bottom line is that if you don't rely on your water for minerals, and you take care of your electrolyte needs outside the water consumption then it doesn't really matter. Regarding the acidity claim in the second article, with few exceptions distilled water has a PH of 6.8 to 7--which is essentially neutral.