The third day of any diet is absolutely the worst.

Author Martha Beck wrote a book called “The Four-Day Win” in which she explains that if you can make it through day three of any diet you are 90 percent more likely to get to your goal.

Firstly, we often eat out of habit, because we want to, not because we are hungry. Eating is extremely enjoyable.

Hunger is something very different. Hunger is a demand, not an option. Real hunger, although part of the daily life of millions of people on this planet, is uncommon in our Western world except among the very poor and those engaged in dieting. One thing you learn from the worst moments in a diet is what hunger really is.

Getting familiar with hunger teaches you how disconnected your desire to eat is from your body’s need for calories and how important it is, because of that, to control what you eat by some means other than instinct or will power.

This is where meal planning comes in. Planning meals in advance may seem like a pain and a real joy-kill as far as ruining the spontaneity that makes life enjoyable.

Eating is important; it’s one of very few things in life that isn’t optional. If you don’t eat, you die. But it is also true that if you eat too much for too long, you die.

You wouldn’t entertain the idea of investing in a company that didn’t have a budget, where the staff said, “We just spend whatever we feel like most days. It’ll all work out in the long run.” Not only would such a company probably go bankrupt, its department heads would have no way of knowing where the money was going and they’d have no way to measure actual performance against goals to discover where problems lay.

But by trying to “play it by ear” about what you eat or to try to balance your long term calorie intake meal by meal or by making every food decision on the spur of the moment, you’re placing something even more precious than your money, your own health, in the hands of a process that inevitably leads to major health complications.

Two hundred calories might not seem like much but if you are on a diet this is a 10 percent increase over the number of calories you need to eat maintain your weight.

If this happens regularly enough, there’s a small shift in the balance between calories in and calories out.

If your balance slips up by as little as 150 calories a day — the rest of your sister’s smoothie, a piece of leftover bagel — the scale will start to creep up. Granted, it will only be a third of a pound a week, but the trend line will start to rise.

That’s why meal planning is key.

No matter how long or severe your diet, the first 72 hours are the worst. This sad fact forces most people to abandon their diet which, if continued, would soon yield sustainable weight loss without undue discomfort.

Planning a diet from an understanding of how weight loss really works gives you a handle on how long you’ll have to endure its unpleasantness. Knowing how feedback can control your weight equips you with at least intellectual confidence that once you’ve lost weight you’ll never have to go through that again.

This explains the rocky start every dieter must endure. There is a delay, usually between 48 and 72 hours, between the time you cut back on calories and when fat burning begins in earnest. In those hours, you will experience the most severe shortage of nutrition in the entire course of your diet. You’ll feel cold, weak, irritable, tired yet prone to sleep poorly, and a constant, gnawing hunger that urges you toward the refrigerator and implores you to rethink your resolve to lose weight. It will feel worse than the idea of your vacationing friends on the beach in their bikinis.

What must be endured in the first few days of a diet is an investment that will pay off in reduced suffering later on. It’s worth comparing the undeniable aggravation of dieting with the inestimably less enjoyable by-product of excess weight: heart attacks, strokes, and premature death. If you think of a balance sheet with three days of hunger on one side and six weeks of recuperation from a coronary on the other, it’s a lot easier to get through the first days into the long haul where dieting becomes at most a nuisance to be tolerated.

You won’t be as hungry after four days. That’s only 96 hours. Get four days behind you. It’ll be over before your friends step off the plane complaining that their belt is too tight.

Make sure to follow Penny on Twitter @pennylovehoff or visit her website for more health tips.