If by stiffness you also have some arthritis type pain, this three part explanation on how a keto diet could help relieve it is interesting. https://crownmd.net/the-ketogenic-d...VmEUbHaF8GU5o6k. This article only refers to a ketogenic diet in general, but I also think specific foods might add to the pain and stiffness.

The focus is on reducing blood glucose and insulin resistance, something that can happen well before weight loss (it did for me). In addition to a very low carb diet overall, test certain foods, but two that bother me are alcohol and Peanuts (very high in lectins) Wheat gluten is probably the worse for lectins, Soy too, but those are not part of my diet. The nightshade plants have lectins too but a few tomatoes and peppers in a salad don’t bother me, other foods on Bob's list also cause no problems for me. After starting a ketogenic diet, which eliminates some of the big offenders, then try an elimination diet for you.