As always in this modern high-tech world, email works very well. I have set up a dedicated email account Fat Loss Biology at:
DrDennisBellyFat [at] gmail [dot] com
(I list it in that form to prevent bots, spammers, and hackers from misusing it.)
An old-fashioned phone call also works. I keep my cell phone ringer set to “mute” most of the time for the sake of avoiding interruptions. Nevertheless, if you leave a message, I will get back to you promptly.
Of course, if you want to go super-retro, you can send me a note via (gasp!) snail mail to:
Dr. Dennis Clark
4701 E. Wintu Way
Phoenix, AZ 85044
That should do it.