Eating any diet long-term will create adaptations. We have to remember that we eat to feed ourselves in part from what we digest and absorb from the upper digestive tract and then the rest of what we eat goes on to feed our lower intestine where there is a whole microscopic biome.
If we eat a low variety diet, of any form, the person’s metabolism responds and adapts to that diet and the microbes in the gut biome that grow well on that food will predominate and have their effects on the human’s health.
There can be negative effects, usually in times of diet change as the body and biome readjust.
From these observations some low variety diets reduce dietary adaptability, and the gut or metabolic reactions to those diets can have long-term, cumulative, negative health effects.
The mass experiment that is going on with processed foods, industrial seed oils provides observations and data that a diet that sets up the wrong metabolic and hormonal and gut digestive responses in a majority of people can cause major health problems on a national population scale to the point of bankrupting a country.
Finding a way of eating that result in correct hormone responses with the ability to store and access energy efficiently, and that also feeds the body’s biomes, so that their synergy with the human organism is optimised, is a first step. Then, after healing those parts (often required), testing to see, if a variety of potential food sources can be tolerated follows. Negative effects often claimed of the low carbohydrate ketogenic diet is that the body becomes intolerant to carbohydrate rich foods. Health practitioners that use the low carbohydrate ketogenic diet as a therapy do observe that carbohydrate responses and tolerance improve after some healing. That does not mean that a metabolically damaged person can return to eating the diet that in part caused health issues. It demonstrates that for short periods when the highest nutrient dense and nutritionally bioavailable foods are not available, a person can eat alternate food sources without hurting themselves.