Have you ever wondered why it's so easy to lose weight one week into a new diet, and after that it gets progressively harder? It isn't just your imagination. A video by Business Insider shows how your body and brain can turn against you when you're trying to eat healthy foods and shed a few pounds.

After the first week of a new diet, during which time you may shed mostly water weight, your metabolism adjusts to changes in your diet and you won't burn as many calories. Also, as you start to burn more fat, you may notice that you feel hungrier. That's because a hormone called leptin, whose function is to let your brain know you're full after finishing a meal, is in shorter supply when you lose weight. As the video points out, one study revealed that obese people who had lost 10 percent of their body weight had lower levels of leptin, which activated certain areas of the brain linked with appetite.

Not only does this make you want to eat more, but your body's attempt to bring your leptin levels back to normal may also make you crave fatty foods that are high in calories. Those who push through the temptation will start to feel the difference, though. Besides lessening the strain on blood vessels and improving overall brain function, every pound of weight lost lifts four pounds of pressure from the knee joints.

Where does all that fat go when you burn it off, though? According to one study, for every 10 pounds of fat you shed, 8.4 pounds are exhaled as CO2. The remaining 1.6 pounds are converted to water and discharged from the body as urine, feces, sweat, and other bodily fluids. So rather than "burning" fat, you're actually excreting it.

[h/t Business Insider]