Before we discuss the low-protein diet that can be so beneficial in patients with kidney failure, let's look at why we need to eat the way we do.
We all need calories for energy. Our caloric requirements vary with our size, gender, and activity levels. It is possible to calculate your caloric requirement, based on these factors. But two much simpler approaches are to quit eating when you are no longer hungry, and to watch your weight.
Don't be fooled by the many things that can make you accumulate or lose fluid (and therefore weight) without flesh. One of these is salt, which we'll discuss fully in Chapter 8. People who have substantial accumulation of retained salt and water cannot use their weight as an index of their caloric needs. Ankle swelling is an early hallmark of this problem. Less common is water retention or deficit without accompanying salt or water deficit, which we'll also discuss in Chapter 8; this is usually a short-lived problem. Another is fluid accumulation caused by air travel, which takes a few days to recede. Obviously acute illnesses, especially diarrheal disease, can cause temporary weight loss owing to fluid loss.
Some people can eat all they want to and never gain or lose weight. They are very fortunate. No one knows what accounts for this. As far as I know, there is no measure of what fraction of the population they comprise, but we all know such people.
There are four possible food sources of energy: alcohol, protein, fat, and carbohydrate. Alcohol theoretically contains 7 calories per gram, but, for reasons only partially understood, in fact provides little energy. Fat contains 9 calories per gram; carbohydrate and protein each contain 4 calories per gram.
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