Because it’s raining hard, you drive to the gym to work out —even though you’re sick and tired of doing that full-body, 12-exercise, circuit training workout you finally committed to memory. You’re also sick and tired, you realize, of that diet you’ve been on for the last three months.

Then, as you wait at a red light, you get an offbeat idea you find oddly appealing. A bit inspiring even. Suddenly, you’re super-psyched to sweat.

You’ll skip the lifting and ride the spin bike — and harder than you ever have before. So hard that five minutes into it you’ll feel as if you sprinted up 10 flights of stairs.

And even if you have to use Jack LaLanne’s name in vain, you’ll force yourself to feel that fatigued and winded for a full 45 minutes. You’ll cool down, crawl into the showers, limp into the car, and then eat whatever you like — that’s right, whatever you like — at McDonald’s.

This spur-of-the-moment sadomasochism works, takes you to an athletic level of pain you haven’t hit since high school. It also takes your carb level so low that you swear you can feel the sugar in that soda you guzzle down at Micky D’s racing through your veins.

As you force yourself to sip and not swig, you feel as euphoric as some strung-out junkie who has mainlined heroin. Oh, and the Quarter Pounder that comes next tastes just as excellent and illegal, too.

But some after-the-indulgence internet investigation interrupts your euphoria.

According to one source, caloric estimates on computerized pieces of training equipment are infamous for giving erroneously high readings. In all likelihood, you burned far fewer than the 1096 calories the readout claimed you did.

In fact, an old study from Medicine & Science in Sports & Exercise you find reveals that when fit subjects pedaled all-out for 47 minutes on stationary bikes, the average caloric expenditure was just under 520 calories.

That’s just about the same number of calories in a Quarter Pounder. And you had a large soda and a large order of fries slathered in ketchup, too.

You laugh to keep from crying. Somehow, someway, you psyched yourself up to work out with an intensity that you haven’t hit since your high school days, and yet you still managed to gain weight in the process.

What did that announcer say while he watched the Hindenburg burn? “Oh, the humanity!”

You now know why so many experts say that a reduction in food along with an increase in exercise is a more effective way to lose weight and keep it off than just exercise alone.

But there’s a reason why you should still feel good, though maybe not quite euphoric, if a splurge follows intense exercise. It’s called the after-burn.

Work out really hard, and your body burns calories at up to seven times beyond your basal metabolic rate. When you stop, your body needs time to metabolically slow down, so it still burns cals at a far higher rate than your BMR.

How much higher? When asked about the research on this, David Neuman, PhD, a professor of exercise science at Appalachian State University, and lead author of the aforementioned study, told Health magazine that if you do two or three similar rides a week, you can drop two pounds of body fat in a month just from the after-burn.

Because the degree of after-burn is based on so many variables — body weight, body fat percentage, degree of fitness, type of exercise, and the intensity of it — it’s hard to know the exact amount you personally will burn in the hours after exercise, but in the last few year researchers have discovered that one hormone in particular influences after-burn.

Irisin.

Irisin is produced during moderate-to-intense endurance activity when your cardiorespiratory system is engaged and your muscles are exerted. This production of irisin activates genes and a protein in your body that combine to change the composition of body fat.

What you think of as the typical love-handle, saddle-bag fat (aka white fat cells), turn into brown fat cells. Brown fat cells require far more energy than white fat cells; therefore, your new fat actually burns old fat.

Furthermore, the production of irisin seems to inhibit the production of new fatty tissue.

Li-Jun Yang, professor of hematopathology at the University of Florida College of Medicine’s department of pathology, immunology, and laboratory medicine and lead researcher in a September 2016 article published in American Journal of Physiology, Endocrinology and Metabolism, feels their findings about irisin’s role in regulating fat cells explains in part how working out allows some people who don’t really monitor food intake to stay slender.

Prior to Li-Jun Yang’s work, a 2014, a study published in the Tohoku Journal of Experimental Medicine found that high-intensity exercise, such as the sort used in the hypothetical scenario that began this column, increases irisin levels more than low-intensity exercise.