I have Celiac disease and I have also always had oddly thick blood. Doctors don't ask about that, but I do have a long history of migraines, Raynaud's, and leg pain that I couldn't tell if it was vascular or nerve, but exercise and taking aspirin helped. I have only taken the aspirin on a short-term basis for fear of harming my stomach. I was anemic when I was getting tested for Celiac disease.
Recently I became more concerned about the recurring leg pain and the sluggish feeling with exercise and my duller brain function, despite a gluten-free diet, it occurred to me that perhaps the thick blood was actually a very important condition to treat. I therefore started Ginkgo biloba after doing some reading, and it does seem to be helping. Especially interesting is that my sallow skin color is pinker. It did this temporarily when I went gluten-free, too.
http://www.ncbi.nlm.nih.gov/pubmed/18366270
I'm not sure the why's about sugar and fat thickening the blood, but I did know someone who had this problem later in ife. Her husband was a natural doctor, so he was very particular about what she ate when other people tried to be helpful and bring food to her. Besides the foods that were the problem for her specifically, which I am not sure all of which the foods were, one huge thing they both did was to use reverse osmosis filtered water for all cooking and drinking. In fact, before they used any dish to eat or drink from that someone else brought over or if someone else had been helping with the housework and had washed dishes, they would rinse everything once more with the filtered water before using. This was mainly for the lady with the thickening blood issue. They were both getting older and she needed the help, since he was busy with his practice. I do know from this, though, that a very clean diet is crucial to keeping the blood from getting so thick.
I have had chronic frequent migraines since about age 18, I found out that I have agent v which means thick blood and has to do something with clotting, It is genetic because all my moms brothers and sisters take blood thinners medication, Also both my mom and my sister have had serious blood clots, I have been really lucky so far not to have had a blood clot,