The problem with that is that proteins supply some 20 amino acids, about half of which are needed for normal growth and tissue renewal, and if energy - yielding nutrients are in short supply, amino acids may be used as energy sources. This can lead to what's known as protein-calorie malnutrition, a condition apparent in the stunted growth of Third World children, who may subsist almost entirely on starchy foodstuffs derived from cereal and root staples.
Foods high in protein — most meats, fish, eggs, dairy products — are the expensive foods of the comparatively rich. Surprisingly, though, most human diets around the world only vary from about 10 to 15 per cent protein content. The big difference is in the quantity of fats and carbohydrates — as high as 90 per cent carbohydrates in poor diets and as low as 40 per cent in better diets. The remainder, if any, of the diet is made up of fats, perhaps as much as 45 per cent among wealthy eaters.
Fats and carbohydrates, though low in amino acids, are our best energy source. Fats, such as oils and especially butter, have an extremely high energy content (that is, calories) but consumed in excess they can build up as hard deposits known as plaque in the arteries, as in artherosclerosis, one of the major causes of heart attacks. Because most fats are scarce in vitamins, they're called empty calories: They've generally been considered the chief contributor to body fat, an excess of which can be a real problem for people with osteoarthritis, though recent studies have questioned — with some justification — whether in fact fats are the real culprits in excess weight.
The other main source of body energy is the carbohydrate family — sugars and starches. Most of them are loaded with energy but contain relatively little protein. Thus, a high carbohydrate diet will be short on growth potential and leave the body ill-equipped to fight infections. Some starchy foods, such as potatoes, do have protein content (though less than whole-grain cereals) and vitamins, especially C and A. Before the terrible Potato Famine in 19th-century Ireland and Scotland, many crofters survived on little else.
Foods also contain a wide array of minerals useful to normal metabolic processes. You may have two or three pounds of calcium in your body — mostly in the skeletal system — as well as magnesium, which shares some of calcium's functions. Iron, an essential component of hemoglobin in the blood and myoglobin in muscles, is of special importance to menstruating, pregnant or lactating women; it's aided in its work by copper, while cobalt helps form red blood cells. The body is a veritable mineshaft of other exotic minerals, such as zinc, manganese, selenium and molybdenum, whose functions are varied but sometimes interrelated; all are important and require the appropriate food sources.
Excerpts from - http://www.dietitians.ca.