Effect of high impedance Opamp buffer on current lost
There will be no significant current flowing into the input of the OpAmp. Depending on your measurement resolution and your specific OpAmp this will not even be detectable.  

The Input bias current (check the datasheet of your OpAmp) will be in the range of picoamp (FET input stage) to nanoamp (bipolar input stage).

Example of impact:
Assuming an input bias current of 10nA, this would be a fraction of the full scale input of \$ \frac{10~nA}{20~mA} = 0.5 \cdot 10^{-6} \$. This would require a resolution of \$ N = log_2 \frac{1}{0.5 \cdot 10^{-6}} = 21\$ Bit to change the result by only one single LSB.