The rubber bag view of the body and considering only the calorie content of food is obviously oversimplified. There is a difference between eating a varied diet and chowing down on a cup of lard and sugar once a day. Programmers know this instinctively: they balance their daily menu among the four major food groups: caffeine, sugar, grease, and salt.
In reality, food satisfies two distinct needs of the body. The first is for energy. A substantial amount of energy is needed just to maintain a constant body temperature and keep the heart, lungs, and the rest of the body's mechanisms running. The energy consumed by a human body is comparable to a 100 watt light bulb. Food also supplies the raw materials the body uses to manufacture all the chemicals it needs, including those needed to build new cells.
From the standpoint of energy, almost any food will do; you can assume that all foods with the same calorie content are interchangeable. Eating the right mix of food only becomes important when you consider food as raw material. For the most part the body breaks food down into small molecules composed of carbon, hydrogen, oxygen, and nitrogen and then manufactures what it needs from these building blocks. However, certain structures in the body require other constituents. For example, iron is needed to form the hæmoglobin that carries oxygen in red blood cells, and calcium forms the matrix that strengthens bones. In addition, there are a number of complicated organic molecules our bodies require but cannot, for one reason or another, manufacture. These raw materials, minerals and vitamins, must be furnished or else the body begins to develop deficiency diseases such as scurvy and rickets.
If you eat a reasonable selection of food, varied within each meal as well as from meal to meal, it's extremely unlikely you'll come up short one of these crucial substances. (Vegetarians have to be careful, as some nutrients abundant in meat are present only in a limited number of plant foods: these considerations are discussed in detail in numerous books describing vegetarian diets, and I won't go into them here.)
The reason we focus entirely on calories when talking about weight control is that the energy-producing aspect of food is what determines whether you gain or lose weight. Unless your diet is wildly out of whack, which particular foods you eat has very little effect on your weight, compared to the calorie total. To lose weight, you have to eat less. When you eat less, you'll not only be putting less energy in the rubber bag, but also supplying less of the raw materials the body needs. It is, therefore, important to maintain a balanced diet as you lose weight.
Be reasonable. I think the main reason so many diet books are packed with information about food, special recipes, and the like is that it's a useful way to pad out the essential message of a diet book, ``eat less food,'' into something thick enough to be visible on the shelf. As long as you vary what you eat and choose your foods from all around the supermarket, the probability you'll develop a deficiency disease whilst dieting is extremely remote. If you supplement your food with a multivitamin every day (any one that provides 100% or more of the RDA of the big name nutrients is fine), you have even less cause for concern.
If you adopt the ``Clam juice and brown rice quick-loss diet'' from the supermarket tabloid, good luck. At least eat some peach fuzz along with it.
By John Walker