I follow a paleo diet with emphasis on getting proper protein and fiber. I also do 20:4 intermittent fasting. During the 20 hours of fasting, the first time I start thinking about food or what I'm going to cook later in the day, I suddenly get nauseated, have excess saliva production, and suddenly start sweating even when I'm sitting still. This lasts about five minutes, then passes like it never happened. I'm completely fine the rest of the day (or more if I'm doing a longer fast) even if I think about food or even go grocery shopping.
Time of day and activity doesn't seem to have an effect. It doesn't seem to be an electrolyte issue because taking a bit of salt or electrolyte drops doesn't help, nor does it exacerbate length or severity of symptoms.
I've had a full bloodwork panel done, and my results come back normal except for my HDL is a little low, LDL a little high. However, I've had bloodwork done every six months, and all my markers have improved over the last three years of doing paleo and time restricted eating. Other than this oddity, I am objectively healthier than I've been in decades. I just really wish I could stop what appears to be thought-related nausea bursts!