This is one of the most frequent questions that we get asked. Most people are thinking about their experiences with bioelectrical impedance, since those body fat test devices are very sensitive to hydration levels. We've always told our clients that our DXA scans aren't as sensitive to water. But how sensitive are they? We decided to find out!

To do so, we scanned BodySpec's intrepid co-founder Jason. After his first scan, Jason gamely chugged a whole gallon of water. Afterwards, we scanned him again (before he ran off to use the mens room).

(Note: drinking too much water too fast is not recommended and can lead to water intoxication. Jason took over an hour to drink his water.)

Here's what we found:

Before Water After Water Difference Total Mass (lbs) 179.6 186.0 6.4 Body Fat % 14.6% 13.6% -1.0% Fat Tissue (lbs) 26.2 25.2 -1.0 Lean Tissue (lbs) 145.5 152.9 7.4 Bone Mineral Content (lbs) 7.9 7.9 0.0

As can be expected, the majority of the water's effect is on lean tissue. As you probably know, the DXA scan categorizes all non-bone, and non-fat matter as lean tissue, including blood and other fluids. By increasing the amount of water in your body, you're effectively increasing your lean tissue amount – and decreasing your body fat percentage. The fat tissue numbers are also changed slightly as water is dispersed throughout the body over the course of an hour, changing the average tissue density.

However – notice that it took an entire gallon of water to drop Jason's body fat percentage by 1 percent. How much is one gallon of water?

That's right – it takes 16 cups of water to affect your body fat percentage by 1%. That means that when you drink just 1 or 2 cups before your scan, the difference in your results should be negligible.

Nevertheless, it's always best to try to test under similar circumstances if you can – which means refraining from chugging a gallon of water right before your scan!