Is there any substitute for butter (or oil) on puff pastry? I tried to create puff pastry.
For medical reasons I should minimize the fat on my recipe. Puff pastry is mainly made by 3 important ingredients, flour, water, and fat (butter or oil).
Unfortunately, I learned that butter and oil, consist of 100% fat. I've successfully created puff pastry using oil in a pan before. It works! I create 2 doughs, one dough is formed using flour and water, the other one is using flour and oil. Then, I do the pastry folding.
So as no-fat alternative I tried using a flour-water dough and a flour-applesauce dough, laminated as for regular puff pastry. The result was crisp at the outside, but uncooked on the inside.
I baked twice as long, and the result were very thick hard crackers. I can see the layers with different color, but there is no air in between the layers.
I've also tried only using flour-water dough pastry, folded. Again I can see different colors of the layera, but no air in between.
I haven't tried making the second dough with egg yolks though. It may interesting to see the result, since egg yolks supposedly have around 63% of fat.
Research
After that failed experiment, I read some articles about the science behind the pastry. It says that the pocket of air is formed because of the boiled water that becomes gas, trapped between the fat layers.
I assume, it happens because the oil and water are not soluble. On the other hand, the boiling point of water is 100 C (212 F), yet the boiling point of oil is around 300 C (572 F). I see here that vodka has a boiling point around 78.3 C, which is lower than water. I  haven't tried vodka for the mixture with flour because vodka actually is also soluble in water.
My question is:
Is there any food grade liquid that has boiling point over 100 C and is not soluble in water?
No. An edible organic liquid that does not dissolve in water, almost by definition, is an oil.
That's not the important thing, though. Substances like mineral oil are edible yet non-nutritive; they pass through the body unchanged and would be compatible with any dietary condition. The problem is that, because they are not digestible and not water-soluble, they, ah... lubricate things. Down there. The amount you'd have to use for puff pastry would cause some real digestive issues.
Bottom line: there are no straightforward non-nutritive substitutes for fats and oils which do not cause diarrhea or loose stools.