It is normal for people to lose between 2 and 4% of their total body weight during intense physical activity because of the dehydration caused by sweating. However, severe dehydration – losing more than 8% of total body weight - can be fatal. Being able to measure hydration levels is therefore important when looking after, for example, malnourished children, the elderly or athletes. Although methods that rely on taking blood samples are highly accurate, they can be impractical in emergencies. Other methods, such as those that measure the electromagnetic impedance of the body, are not sensitive enough.

Shapiro and co-workers weighed 12 young male volunteers –average age 24, average weight 71.5 kg – before and after 30 and 60 minutes of exercise. At the same time, they observed how radio waves of different frequencies were absorbed by the subjects using a small radio-frequency absorption device placed on their wrists (see figure).

They found that the average loss in body weight was about 0.78 kg after 30 minutes of exercise, and 1.59 kg after 60 minutes. These results indicate levels of dehydration that are between 1 to 2.5% of total body weight. More importantly, they observed a definite correlation between weight loss and the absorption of radio waves by the subjects.

The Israeli team now hopes to repeat the experiment with female volunteers and people from different age groups to further establish the reproducibility and validity of the method.