Biggest Loser winner Rachel Frederickson shocked audiences Tuesday when she revealed she had dropped 155 pounds, nearly 60% of her starting weight. Earlier this week a Saudi man also made headlines for losing an astronomical 700 pounds, shedding 50% of his original weight. With the success of shows like The Biggest Loser, Extreme Makeover, and My 600-lb Life, extreme weight loss has become somewhat of a gawking pastime among American audiences. But while audiences can witness these people’s external changes in appearance, what’s happening internally when a body shrinks to half its size?

Obesity is typically measured by body mass index (BMI), with a BMI of 30 and above considered obese, and BMI of 40 and above considered severely obese. For people with a BMI above 40 to reach a healthier weight and actually maintain it, weight-loss surgery is usually the only option. For a 5’10” man, that’s about 280 pounds, and for 5’5″ woman, approximately 240 pounds.

Once the pounds start shedding, people’s perception of their own size remains skewed while they internalize their new bodies. As they adjust, they continue to make a lot of space for themselves, like selecting large spaces to sit. “Internally, people still think they are large. They swing their arms further out from their body like a helicopter, thinking their hips are still as wide as they used to be, even though they aren’t,” says Dr. Roxanne Sukol, a preventive medicine specialist at Cleveland Clinic.

The first 25 to 30 pounds are the easiest to drop, and usually accompanied by immediate improvements in blood pressure, blood sugar, and breathing. It becomes harder to lose the pounds after that initial period, but with each additional pound lost, physicians notice improvements in virtually every organ system.

However, if an individual’s weight has caused significant health problems, like heart issues or diabetes, such problems don’t go away so easily. Even when a person recovers, ailments developed along the way can remain. “We see blood pressure and sugar improve rapidly, but if your obesity caused you to have a dilated heart, that might take longer to heal–if it ever heals,” Dr. Sukol says. Excess skin can also remain after weight is dropped, but it usually adapts to the body after a period of time.

Physical therapy is nearly always needed to continue the healing process. If an individual has not been mobile for years, their muscle and skeletal systems are likely damaged. Our knees and lower extremities aren’t meant to hold the amount of weight severely obese individuals carry, and that weight can interfere with blood flow to the heart, which is one of the reasons obese people experience bloating. The good news, according to Dr. Sukol, is that, with every five pounds lost, an enormous amount of pressure on the knee caps is relieved.

Appetite can also change. When individuals replace foods like white breads and potato chips with intact carbohydrates like beans, vegetables, fruits, and whole grains, they tend to lose weight and feel more satiated from eating more nutritious food.

Lastly, the mental health effects that come from significant weight loss are immeasurable. From a biological level, neurotransmitters in the brain work better when a person is on a healthy diet. But socially, the effect of weight loss is just as great. “Being obese is such a stigma in our society, that I think the stress of being obese and having to cope with how people look at you is something impossible to relate to,” says Dr. Sukol. And that’s a considerable weight off someone’s shoulders.

(MORE: The Biggest Loser Goes Too Far)

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