If we could design a pill that would concentrate all the necessary nutrients for our survival, we would solve in a jiffy the hunger in the world and all the environmental problems that it entails.
Our diet is based on the correct proportion of nutrients needed for the growth and development of our cells.
According to the WHO, to have a healthy diet it would be necessary to cover with carbohydrates at least 55-60% of the total caloric intake, the fats should not exceed 30% of the total calories ingested and the proteins should account for 15 % remaining in the diet. (1)
Now imagine that a single pill contains all the necessary nutritional intake (including fiber, vitamins and micronutrients), could we live only on pills?
Unfortunately (or rather, fortunately), there are no pills that can concentrate all the energy requirements to such a small size. Think that the average daily caloric intake of an adult person is 2500 kcal for a man and 2000 kcal for women and that would be equivalent to almost 300 grams of olive oil (oil being one of the most caloric foods). Now, imagine a 300 gram pill!. Also keep in mind that you should add the amount of fiber (26-40g daily), protein and other nutrients. It would be almost a miracle to concentrate all that on just a few grams of substance.
Although vitamins and micronutrients are quantified in the daily diet in milligrams, their only consumption through pills would not be recommended either, since some studies suggest that their effectiveness is drastically reduced compared to vitamins extracted from natural foods.
This is because when you eat natural food, you're not consuming only single nutrients, but rather a whole range of minerals, vitamins, co-factors and enzymes that allow for optimal use by the body.
Without these additional compounds, synthetic nutrients are unlikely to be used by the body in the same way as their natural counterparts (2), for example vitamin E which in natural form (RRR-alpha tocopherol) is better absorbed than in synthetic form (all-rac-alpha tocopherol). (3)
Not even the food served in the space station is based on pills, but on dehydrated foods that are reconstituted on board, so the idea of replacing the food with pills (also eliminating the pleasure of the food) could lead to problems of long term health.
Fortunately, feeding on pills is in the field of science fiction!