Protein is notably declining across staples. Protein is not substitutable like carbohydrates. Meaning that your body will trigger you to eat until you get enough protein. Conversely, if you have an abundance of protein and the various micronutrients or vitamins your body can happily convert protein to carbohydrates via ketogenesis. One can subsist on a nearly complete protein diet so long as nutrient deficiencies causing diseases like scurvy are addressed. The same is not true of fat or carbs.