I’m a non-smoker and I have ridges on my nails. I’m celiac (negative blood test but diagnosed through diet). My intestinal issues (bloating, episodic pain, constipation) resolved on the SCD diet. The pads of the tips of my fingers, toes and my BUTT had flattened from being an undiagnosed celiac for so many years. After improving my gut flora on the SCD, I started absorbing nutrients better and the pads of my fingers, toes AND my butt have returned to normal. I was also obese before the diet and had crazy carb cravings. The diet helps normalize your weight because it heals your intestines so you absorb nutrients better and it helps you to have a normal appetite. Those who are underweight gain, those over tend to lose. (Unless you eat too many nuts!) Alas, the diet didn’t seem to help my horizontal fingernail ridges at all, so for me I think it is being caused by something more than just malabsorption.
BTW, if you have really bad brain fog and and IBS symptoms try the SCD diet for a month. I was shocked how my brain fog dramatically lifted after four days on the diet. Keep it low histamine though, I learned that the hard way.
My best, SweetFeather