What Happens To Your Body When You Lose Weight Too Quickly?

What Happens To Your Body When You Lose Weight Too Quickly? Hero Image
Photo: Javier Díez

The desire to lose weight, and lose weight fast, is common. There’s a reason why diets like the cabbage-soup diet exist: People want a quick fix.

As a registered dietitian who counsels private clients (and who has experienced my own weight-loss journey), I understand that desire. But unlike gradual weight loss, rapid weight loss can be dangerous and ultimately counterproductive when it comes to shedding unwanted fat. But how? What’s the difference between losing 15 pounds in two weeks versus two months? A lot.

The first issue is the risk of dehydration.

A lot of the lost weight you experience on crash diets is actually the loss of water weight. You're not only losing the fat that you want to lose, it also means your body isn’t getting the water that it needs. Dehydration doesn’t just cause discomfort from fatigue, headaches, and constipation; if prolonged, it can lead to more-serious issues, like the formation of kidney stones or even impaired kidney function.

Cutting back drastically on your food intake is such a shock to your system that it can also throw off the body’s equilibrium.

Our bodies are constantly adjusting for minor changes to keep things running smoothly, but there are instances where change is too rapid and too drastic for the body to respond and regulate. This is particularly true when it comes to our electrolyte balance.

Electrolytes play a major role in keeping us alive since they provide the electrical current that allows our muscles, particularly the heart, to contract at the right speed. When there is a sudden decrease of food intake, there is also a sudden decrease in the electrolytes that our bodies are used to getting, particularly potassium and magnesium. If the body can’t handle the deficit, this can lead to impaired cardiovascular function and irregularities such as heart arrhythmias.

Just like electrolyte balance needs to be maintained, the amount of sodium in our body also needs to be regulated to keep our blood pressure at just the right spot: Too high and it can increase our risk for heart disease, too low and we can feel faint since oxygen isn’t getting to our cells quickly enough. So while there is talk about low-sodium diets, which are necessary for some, having too little sodium isn’t good either. During a crash diet, sodium levels in the body can plummet, which can make you tired, light-headed, and even cause fainting spells.

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Aside from disrupting heart function and blood pressure, crash diets can lead to deficiencies in other minerals and vitamins.

For example, too little iron can cause anemia. Not enough calcium and vitamin D can lead to osteoporosis. Brittle hair and nails, dull skin, reduced immune function—all of these are the result of undernourishment.

Then there’s the fact that on crash diets, your metabolism slows down.

At such a calorie deficit, the body goes into starvation mode in order to conserve energy and metabolism slows. You’re also losing muscle because the body isn’t getting the protein it needs. Muscle doesn’t just make you strong and toned, it also helps to boost metabolism. When you lose muscle, your metabolism slows down even more, making it harder to lose weight.

While there are definitely situations that call for rapid weight loss, those situations should be closely monitored by a doctor and a dietitian to ensure that it is done safely. Otherwise, severe restriction should never be a means of shedding unwanted pounds. Instead, eating more of the right foods, choosing nutritious combinations, and properly scheduling meals and snacks to increase metabolism should be part of a healthy, balanced, and successful weight-loss plan.

Want to kick-start your weight loss in a healthy way? This one-day blood sugar–balancing meal plan will get you started. Also, here are the best foods for eliminating belly fat, according to doctors.


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