If you ever tried to lose weight, you may heard about intermittent fasting or low-carb diet(s). When you following this type of diet, you can't drink or eat anything sweet or anything contains carbs (or calories), so your only option is to drink black coffee, especially in the morning. But here is a reason why drinking coffee first thing in the morning, may sabotage your efforts.
I do intermittent fasting many months now and I love it, as it fits into my lifestyle. I tried also a low-carb diet before, the type of diet when you can only consume low-carb foods (Wiki), but I don't recommend this diet as safe or effective.
Anyway, what I realized is that coffee stops ketones production, even if you drink it black. I believe that the reason behind this is the impact caffeine have on insulin.
To see if this was true, I made an experiment with Ketostix test strips and there was a difference in ketones' concentration, when I did drink coffee and when I didn't. So, when you drink coffee, insulin levels rise and it stops ketones production. My opinion is to drink coffee when you want to break the fast, with 1-2 tablespoons of heavy cream in it or at least, a few hours after you wake up.