For the sake of science my meters probes are in good contact with my tongue just now left and right side. On AC (200mv scale) it reads 0.0mv. When it's on DC I get between 10 and 50mv depending on how hard I'm biting it against my tongue. Removing the meter leads I can't get anything but small mv spikes on DC where as against my tongue the DC signal was sustained. No AC readings against my body were ever more than a few mv's although they jumped to 30mvs sustained when floating. Higher when moved around.
Also for the sake of science, I used my Fluke 787 to conduct an experiment just now. On the AC scale, with one lead touching my skin and the other lead touching grounded metal, I read approx. 700 millivolts. I switched to the frequency function and read 60 Hz AC. Removing the lead from my skin and leaving the other lead contacting ground dropped the voltage to fluctuating between 50 mV and 150 mV.
I conducted another experiment using the DC scale. I replicated the tongue experiment by placing the two leads on either side of my tongue. I got fluctuating readings between 100 to 120 mV DC (positive). I swapped the leads, placing them in the same position on my tongue as before, but using the opposite lead configuration, and I still got 100 to 120 mV DC (positive) DC voltage has polarity with respect of one voltage source to another. If 120 mV of DC potential exists from one point on my tongue to the other, then there should be a polarity change when I swap leads. There is no polarity change when I do so. I repeated this a dozen times with no difference in outcome.
The DC scale experiment is flawed, because we have no frame of reference from which to measure voltages on our bodies. Where is ground or low potential? How do we determine where the higher potential exists vs the lower one? This is DC voltage we are supposed to be measuring, but it doesn't measure as DC. If I measure two points on a circuit, in order for me to detect a voltage I have to have a higher potential vs a lower potential. If I detect a voltage, swapping the leads will demonstrate a polarity reversal each and every time if the voltage is DC. Yet the DC experiment on my body does not prove where sources of higher potential exist vs lower potential. Furthermore, the test isn't repeatable. After turning my meter off and back on and retesting, the voltages I read changed.
The problem with this discussion is a failure to recognize how a voltmeter functions and what is required for a reliable measurement. My AC test was a test vs a known reference, grounded metal. I detected nearly 3/4 a volt of AC potential between my body and that ground point. I can conclusively say that this is coupled AC from MAINS sources, because the frequency function verifies 60 Hz.
I am not a medical doctor, but I do understand DC electricity and my judgement says that we are NOT reading anything more than random noise or static potentials on our skin with the DC meter. In fact, I just now waved the leads around in the air and as I moved them the mV scale jumped all over the place without physically contacting anything. If someone can demonstrate how the DC meter is referencing its reading against a lower potential, I might be inclined to believe the noise we see is human produced rather than human coupled.