This blog was stimulated by the recent surge to have government endorse/require lower salt in the diet. There is, of course, no direct evidence that a low salt diet prolongs life. In the recommendations, lowering salt in the diet is taken as a surrogate for lowering blood pressure. It is assumed that lowering the amount of salt in the diet by 3 grams/day will lower systolic blood pressure between 3.5 and 5.5 mm; no estimate is given for the effect on diastolic blood pressure (is there any?). It is claimed that this will reduce death by heart attacks and other problems.
High blood pressure is a demonstrated risk (i.e. there a correlation, but an 0.80 correlation is a 0.8x0.8 or 64% estimate) for strokes, hypertensive heart disease, and abdominal aortic aneurysms (the latter in male smokers). However, every study of lowering blood pressure (usually by diuretics) only shows a reduction in the risk of strokes; there is NO demonstrated reduction in mortality or hypertensive heart disease. I should also mention that reduction of blood pressure by beta-blockers alone has shown no reduction in any risk of stroke or heart attack. In addition, there has been no prospective study to see if lowering the amount of salt in the diet has any injurious effect, and a lot of medicine is counter-intuitive. I recall a study that lowering the salt in the diet INcreases the risk for heart failure, but who really knows?
A related dietary question deals with lowering the amount of sugar in the diet (?by taxing sodas). I pause here to remind my lay readers that just because diabetes is evidenced by elevated blood sugar, this does NOT mean that increasing the amount of sugar in your diet causes diabetes, assuming you gain no weight. And we will also ignore the white lab rat study that shows that if we keep lab rats 30% below their IBW (and who determines the Ideal Body Weight for a lab rat?) they live longer. The flip side is the recent studies that demonstrate that overweight adult humans (BMI 26-29) have an increased chance of surviving a heart attack.
Now let us assume that we wish to lose weight. What is the most efficient and surest way to do this? (Remember that when you lose weight as an adult, your fat cells shrink, and get smaller and have a smaller net surface area for metabolism.) Of course, the obvious method is to eat less. No one was overweight in a concentration camp. The lap-band surgery shrinks the functioning volume of your stomach, so you can't ingest many calories, and again you lose weight, and usually cure your diabetes as well. Anorectics have absolutely no problem in losing weight. In another vein,liposuction sucks the fat cells away from your body,and these cells can never increase in size again.
But the brain demands homeostasis, and strives to keep your body parameters as constant as possible. Most people's weight varies very little from year to year. All nutritionists agree that 3500 calories = 1 pound. If you divide 3500 by 350 days, you find that you have to micromanage your food intake to plus or minus 10 calories a day to keep your weight constant for the year. Not even the most dedicated fanatic can do this. Obviously, the brain has a mechanism to raise or lower your metabolism to keep your weight constant, sort of a feedback mechanism, which has nothing to do with leptin. This metabolic/mass inertia is what makes it equally difficult to gain or lose a lot of weight. In fact studies have shown that when you gain weight, your metabolism increases, and when you lose weight, your metabolism decreases, in a true negative feedback manner, which is why most dieters hit intermittent weight plateaus.
In order to lose weight, you must feel good about yourself, and be relatively happy and free from anger. Food was the first pleasure, given to you by your mother. It is very difficult to deny yourself this pleasure if you are not "in a good place". If you are mentally at peace, then you can walk around hungry without feeling (emotionally) deprived. You can be satisfied by the flavor of half a chocolate chip cookie rather than a whole box of Mallomars. So losing weight is not a matter of pure will-power, but rather a matter of not being too out of rhythm with the world and ego-dystonic. This is not to say that all overweight people are neurotic. Many are very happy with themselves, and see no need to change their body habitus. Your self-image is everything to you, and this is what we all rely on and judge ourselves by. This is why I don't think special diets are the answer. We all eat the way we like, and no one does Weight Watchers forever. The answer is always portion control. Why torture yourself by eating foods you don't like , instead of simply eating less of the foods you do like?
Let me close by saying eating "healthy" food is less important than eating less and losing weight. We know very little about nutrition. When I went to school as a child, the food pyramid was the exact inversion of what it is now. We don't even know (i.e. have good evidence) if the biggest meal of the day should be breakfast, lunch, or dinner. Should we have one large meal a day or six small ones. For every study, there is a counter-study. Do lefties live longer than righties? Do short people live longer than tall people? Why does overall health correlate so strongly with economic position in life? Maybe we all would be healthier if everyone were a millionaire? Any time any person or authority tries to tell you that something is "good" for you, or "good for your health" always ask the same question we asked reflexively in physics and on hospital rounds: "Do you have a published article to support that opinion?".