Fitbits can also watch your health
Wearable fitness monitors such as Fitbit and smart watches like Apple Watch can tell if you are getting ill even before any symptoms become apparent, a new study showed.
Scientists said sensors could be be valuable in spotting infections, illnesses and even insulin resistance.
The devices which range from £25 to over £650 monitor heart rate, activity, skin temperature and other measurements that can reveal a lot about what is going on inside a person.
They provide a continuous supply of data and computer algorithms could pick up distinctive patterns of deviation from normal baseline readings for each person.
It raises the possibility of identifying inflammatory disease in individuals who may not even know they are getting sick, researchers at the Stanford University School of Medicine said.
For example Professor Michael Snyder who was wearing seven sensors for the study noticed changes in his heart rate and blood oxygen levels on a long flight to Norway for a holiday.
From previous trips, the professor of genetics knew his oxygen levels normally dropped during flights while his heart rate increased at the beginning of a flight, like other participants.
Yet they usually returned to normal over the course of a long flight and after landing. but this time they did not.
Therefore he wasn't completely surprised when he went on to develop a fever and other signs of illness as two weeks before he suspected he had been bitten by a tick and infected with Lyme disease.
Snyder was impressed that the wearable biosensors picked up the infection before he even knew he was sick and said: "Wearables helped make the initial diagnosis."
Subsequent data analysis confirmed his suspicion that the deviations from normal heart rate and oxygen levels on the flight to Norway had indeed been quite abnormal.
Professor of genomics Dr Eric Topol at the Scripps Research Institute commented: "The fact that you can pick up infections by monitoring before they happen is very provocative."
The study involved 60 volunteers who wore between one and seven commercially available activity monitors and other monitors that collected more than 250,000 measurements a day.
The team collected data on weight; heart rate; oxygen in the blood; skin temperature; activity, including sleep, steps, walking, biking and running; calories expended; acceleration; and even exposure to gamma rays and X-rays.
Altogether nearly two billion measurements including continuous data from each participant's wearable biosensor devices and periodic data from laboratory tests of their blood chemistry, gene expression and other measures were collected.
For example, in several participants, higher-than-normal readings for heart rate and skin temperature correlated with increased levels of C reactive protein in blood tests.
C reactive protein is an immune system marker for inflammation and often indicative of infection, autoimmune diseases, developing cardiovascular disease or even cancer.
Prof Snyder's own data revealed four separate bouts of illness and inflammation, including the Lyme disease infection and another that he was unaware of until he saw his sensor data and an increased level of C reactive protein.
The wearable devices could also help distinguish participants with insulin resistance, a precursor for Type 2 diabetes.
The study also revealed that declines in blood-oxygen levels during flights were correlated with fatigue but people recover quickly as oxygen rises they feel less tired.
Prof Topol added: "The desaturation of oxygen in flight was not something I anticipated,
"Whenever you walk up and down the aisle of a plane, everyone is sleeping, and I guess there may be another reason for that besides that they partied too hard the night before.
"That was really interesting, and I thought it was great that the authors did that."
Prof Snyder said: "We have more sensors on our cars than we have on human beings."
But in the future the situation will be reversed and people will have more sensors than cars do.
There is already millions of the devices out there including more than 50 million smart watches and 20 million other fitness monitors.
The professor added most monitors are used to track activity, but they could easily be adjusted to more directly track health measure.
With a precision health approach, every person could know his or her normal baseline for dozens of measures and any abnormality flagged up.
The study was published in PLOS Biology.