It's normal to want to lose weight as fast as possible.
But you've probably been told that it's better to lose weight at a slow, steady pace.
That's because most studies show that people who lose weight slowly are more likely to keep it off long-term. Losing weight slowly also comes with far fewer health risks (, , ).
However, several recent studies have found that rapid weight loss might be just as good and safe as slow weight loss (, ).
So is it actually bad for you to lose weight fast? This article digs into the research to uncover the truth.
What Is Considered Fast Weight Loss?
According to many experts, losing 1–2 pounds (0.45–0.9 kg) per week is a healthy and safe rate (, , ).
Losing more than that is considered too fast and could put you at risk of many health problems, including muscle loss, gallstones, nutritional deficiencies and a drop in metabolism (, , , ).
The most common ways that people try to lose weight fast are by exercising a lot, and by following a"crash diet" or a very low-calorie diet of fewer than 800 calories per day.
People often prefer the option of eating a very low-calorie diet, since it is often easier to lose weight through diet than exercise ().
However, if you're just starting a diet or exercise plan, then you may lose much more than 2 pounds (0.9 kg) in your first week.
For this initial period, fast weight loss is perfectly normal. The weight you lose during this time is commonly called "water weight."
When you consume fewer calories than your body burns, your body starts dipping into its stores of energy, known as glycogen. The glycogen in your body is bound to water, so when you're burning glycogen for fuel, the body also releases that water (, ).
This is why you might experience a major drop in weight during your first week. Once your body uses up its glycogen stores, your weight loss should stabilize at 1–2 pounds (0.45–0.9 kg) per week.
Summary: According to experts, losing 1–2 pounds (0.45–0.9 kg) per week is a healthy and safe rate, while losing more than this is considered too fast. However, you may lose more than that during your first week of an exercise or diet plan.
Can You Maintain Fast Weight Loss?
Losing weight is only half the battle. The real challenge is keeping it off for good.
Most people who follow a diet regain half the weight they've lost after only a year. Even worse, nearly everyone who follows a diet regains all the weight they've lost after 3–5 years (, , ).
That's why experts often suggest losing weight at a slow but steady pace. Most studies show that people who lose weight at a slow but steady pace are more likely to keep it off long-term (, , ).
Also, plans that encourage slow weight loss usually help you build healthy eating behaviors like eating more fruits and veggies and drinking fewer sugar-sweetened beverages. Behaviors like these can help you keep weight off long-term (, , , ).
However, several studies have found that rapid weight loss may be just as effective as slow weight loss, even for the long term (, ).
In one study, 103 people followed a rapid weight loss diet for 12 weeks, while 97 people followed a slow but steady weight loss diet for 36 weeks.
Nearly 3 years later, roughly 70% of people in both groups had regained all the weight they had lost. This means that both diets were equally effective in the end ().
Although these studies found that rapid weight loss was just as effective as slow but steady weight loss overall, it's unlikely that a person at home would get similar results.
People in the rapid weight loss groups had support from doctors and dietitians during the weight loss and weight maintenance phases. Research shows that having support from a health professional can improve your chances of long-term weight loss success (, ).
Also, doctors and dietitians try to minimize the health risks that come with eating very few calories. These risks include muscle loss, nutritional deficiencies and gallstones.
People who try these diets alone have a higher risk of these medical conditions.
In short, you are more likely to lose weight and keep it off by losing weight slowly. This approach will help you build healthy eating behaviors to keep the weight off, and is safer to do than fast weight loss, especially if you don’t have the support of a health professional.
Summary: Most research shows that gradual weight loss is easier to maintain over the long-term. It helps you develop healthy eating behaviors and has fewer health risks than fast weight loss.
Risks of Losing Weight Too Fast
While it's tempting to try and lose weight fast, it's usually not recommended.
Diets that promote rapid weight loss are often very low in calories and nutrients. This may put you at risk of many health problems, especially if you follow a rapid weight loss diet for many weeks.
Here are a few risks of losing weight too fast.
You May Lose Muscle
Losing weight is not always the same as losing fat.
While a very low-calorie diet may help you lose weight fast, a lot of the weight you lose may come from muscle and water (, ).
In one study, researchers put 25 people on a very low-calorie diet of 500 calories per day for 5 weeks. They also put 22 people on a low-calorie diet of 1,250 calories per day for 12 weeks.
After the study, the researchers found that both groups had lost similar amounts of weight. However, the people who followed the very low-calorie diet lost over six times as much muscle as those on the low-calorie diet ().
It May Slow Down Your Metabolism
Losing weight too fast may slow down your metabolism.
Your metabolism determines how many calories you burn each day. A slower metabolism means you burn fewer calories per day ().
Several studies have found that losing weight fast by eating fewer calories may cause you to burn up to 23% fewer calories per day (, ).
Two reasons why the metabolism drops on a very low-calorie diet are a loss of muscle and a fall in hormones that regulate your metabolism, such as thyroid hormone (, ).
Unfortunately, this drop in metabolism may last long after you finish dieting ().
It May Cause Nutritional Deficiencies
If you're not eating enough calories regularly, you may be at risk of a nutritional deficiency.
This is because it's hard to consume enough important nutrients like iron, folate and vitamin B12 on a low-calorie diet.
Below are a few consequences of nutritional deficiencies.
- Hair loss: When you eat too few calories, your body might not get enough nutrients to support hair growth, which may cause hair loss (, ).
- Extreme fatigue: You may not be getting enough iron, vitamin B12 and folate on a very low-calorie diet, which may put you at risk of extreme fatigue and anemia (, ).
- Poor immune function: Not getting enough calories and nutrients may weaken your immune system and increase your risk of infections (, ).
- Weak and brittle bones: May be caused by a lack of vitamin D, calcium and phosphorus in the diet (, ).
Fortunately, you can avoid a nutritional deficiency by eating a diet rich in whole, unprocessed foods. These foods contain fewer calories per gram and are also quite filling, which may help you lose weight ().
It May Cause Gallstones
Gallstones are hardened pieces of material that form inside the gallbladder. They can be a painful side effect of losing weight too fast (, , ).
Normally, your gallbladder releases digestive juices to break down fatty food so it can be digested. If you're not eating much food then your gallbladder won't have to release the digestive juices ().
Gallstones can form when substances inside the digestive juices sit for a while and have time to join together.
The gallstones can become stuck inside the opening of the gallbladder and cause a gallstone attack. This may cause severe pain and indigestion ().
Other Side Effects
Losing weight fast on a "crash diet" or very low-calorie diet is linked to several other side effects, including (, ):
- Feeling cold
- Muscle cramps
- Constipation or diarrhea
Summary: Losing weight too fast comes with many health risks. These include losing muscle, decreased metabolism, nutritional deficiencies, gallstones and other side effects.
Tips to Help You Lose Weight at a Healthy
Although slow weight loss might not sound appealing, there are many things you can do to help speed up the process safely.
Here are a few tips to help you lose weight at a healthy rate.
- Eat more protein: A high-protein diet can help boost your metabolism, keep you fuller for longer and preserve your muscle mass (, , ).
- Cut back on sugar and starches: Research tends to show that people who follow a low-carb diet lose more weight. Cutting back on sugar and starches helps you reduce your carb intake (, ).
- Eat slowly: Chewing your food thoroughly can help you feel fuller for longer and eat less food (, ).
- Drink green tea or oolong tea: Research has shown that drinking green tea may boost your metabolism by 4–5%, and may increase fat burning up to 17% (, , ).
- Get plenty of rest: A lack of sleep may boost your levels of ghrelin, the hunger hormone, and lower your levels of leptin, the fullness hormone. This means that poor sleep could leave you hungry, making it harder to lose weight ().
- Try resistance training: Resistance training or lifting weights can help fight muscle loss and the drop in metabolism that may happen with weight loss ().
- Try a high-intensity workout: High-intensity interval training (HIIT) involves short, intense bursts of exercise. Unlike regular aerobic exercise, also known as cardio, HIIT continues to burn calories long after you work out (, ).
- Eat soluble fiber: Research shows that soluble fiber may help you burn fat, especially belly fat (, ).
Summary: There are many ways to safely lose weight faster. For example, you can try eating more protein, eating slowly, cutting back on sugar and starches, and doing resistance training or high-intensity interval workouts.
The Bottom Line
If you want to lose weight and keep it off, aim to lose it at a slow but steady rate of 1–2 pounds (0.45–0.9 kg) per week.
Research shows that slow, steady weight loss is easier to maintain long-term because it’s better for developing healthy eating behaviors, and is much safer than very fast weight loss.
Losing weight too fast may increase your risk of side effects including muscle loss, lower metabolism, nutrient deficiencies, gallstones and many other risks. This is especially true if you try to lose weight quickly without support from a health professional.
Although slow weight loss might not sound as appealing as fast weight loss, there are plenty of ways to help speed up weight loss safely. For example, you can increase your protein intake, cut back on sugar and starches, and drink more green tea.
Slowly changing your eating and exercise behaviors will help you lose weight and keep it off in the long run.