And around we go

Surely, what we have here is a feedback system, and like any feedback system, it just keeps on going. Information: daily weight, the trend calculated from it, and the actual calorie balance determined from the trend, leads to action: controlling calories eaten by adjusting your meal plan. Feedback is negative since, when you discover too few calories going in, you compensate by adding calories to the meal plan and vice versa. Control is proportional because adjustments to the meal plan are incremental, based on the degree to which reality, reflected in the trend line, diverges from the goal. (A bang bang diet would, by contrast, let you eat anything you liked as long as you were within 10 pounds of your goal. When the scale cried "tilt," you'd have to stop eating lunch until you got back to the goal. Some people actually do this, but it's stressful to the body and hardly a way to enjoy life.)

The most important aspect of our proportional negative feedback system is the one we've come to expect, it works. It is robust, reliable, stable, and self-adjusting. Since we have a closed feedback loop, even changes that would otherwise require careful compensation are accounted for automatically.

Suppose you take up jogging and run a couple of miles every other day. The number of calories you burn increases and, with meal planning holding calories eaten constant, shows up before long as a falling trend line. How many extra calories are you burning by jogging?

Just look at the trend chart! The calorie deficit calculated from the trend tells you precisely how much more to eat now that you're more active.

If, on the other hand, you have lunch every day at Chez Maintenant, the trendy gourmet drive-thru joint and, unbeknownst to you, Chef Bubba starts making his secret sauce for the Escargot Burger with two tablespoons of mayonnaise instead of one, the extra hundred calories that crept past your meal plan will soon engender a slow, steady rise in the trend line. Even if you don't know what changed, the course of action is clear: cut back 100 calories somewhere. If the rest of your meals are pretty predictable, you may even be able to finger the culprit and switch to the tasty yet less filling Slug Nuggets.

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