The HCG diet has been popular for many years.

It's an extreme diet, claimed to cause fast weight loss of up to 1–2 pounds (0.5–1 kg) per day.

What's more, you're not supposed to feel hungry in the process.

However, the FDA has called this diet dangerous, illegal and fraudulent (, ).

This article examines the science behind the HCG diet.

HCG Diet 101

HCG, or human chorionic gonadotropin, is a hormone present at high levels in early pregnancy.

In fact, this hormone is used as a marker in home pregnancy tests ().

HCG has also been used to treat fertility issues in both men and women ().

However, elevated blood levels of HCG may also be a symptom of several types of cancer, including placental, ovarian and testicular cancer ().

A British doctor named Albert Simeons first proposed HCG as a weight loss tool in 1954.

His diet consisted of two main components:

  • An ultra-low-calorie diet of around 500 calories per day.
  • The HCG hormone administered via injections.

Today, HCG products are sold in various forms, including oral drops, pellets and sprays. They are also available through countless websites and some retail stores.

Summary HCG is a hormone produced in early pregnancy. The HCG diet uses a combination of HCG and extremely low calorie intake to achieve dramatic weight loss.

HCG is a protein-based hormone produced during pregnancy that tells a woman's body that it's pregnant.

HCG helps maintain the production of important hormones like progesterone and estrogen, which are essential for the development of the embryo and fetus ().

After the first three months of pregnancy, blood levels of HCG decrease.

Summary HCG is a hormone produced in large amounts in the first three months of pregnancy. It stimulates the production of essential pregnancy hormones.

Proponents of the HCG diet claim that it boosts metabolism and helps you lose large amounts of fat — all without feeling hungry.

Various theories attempt to explain HCG’s weight loss mechanisms.

However, multiple studies have concluded that weight loss achieved by the HCG diet is due to ultra-low-calorie intake alone and has nothing to do with the HCG hormone (, , , ).

These studies compared the effects of HCG and placebo injections given to individuals on a calorie-restricted diet.

Weight loss was identical or nearly identical between the two groups.

Furthermore, these studies determined that the HCG hormone did not significantly reduce hunger.

Summary Several studies indicate that weight loss on the HCG diet is due only to drastic calorie restriction. It has nothing to do with HCG — which is also ineffective in reducing hunger.

One common side effect of weight loss is decreased muscle mass ().

This is especially common in diets that severely restrict calorie intake, such as the HCG diet.

Your body may also think it's starving and reduce the number of calories it burns in order to conserve energy ().

However, proponents of the HCG diet claim that it only causes fat loss, not muscle loss.

They also claim that HCG elevates other hormones, boosts metabolism and leads to a growth-promoting, or anabolic, state.

However, there is no scientific evidence to support these claims (, ).

If you're on a low-calorie diet, there are much better ways to prevent muscle loss and metabolic slowdown than taking HCG.

Weightlifting is the most effective strategy. Likewise, eating plenty of high-protein foods and taking an occasional break from your diet can boost metabolism (, , ).

Summary Some people claim that the HCG diet helps prevent muscle loss and metabolic slowdown while severely restricting calories. However, no evidence supports these claims.

The HCG diet is a very low-fat, very low-calorie diet.

It is generally divided into three phases:

  1. Loading phase: Start taking HCG and eat plenty of high-fat, high-calorie foods for two days.
  2. Weight loss phase: Continue taking HCG and eat only 500 calories per day for 3–6 weeks.
  3. Maintenance phase: Stop taking HCG. Gradually increase food intake but avoid sugar and starch for three weeks.

While people seeking minimal weight loss may spend three weeks on the middle phase, those seeking significant weight loss may be advised to follow the diet for six weeks — and even repeat all phases of the cycle several times.

During the weight loss phase, you're only allowed to eat two meals per day — usually lunch and dinner.

HCG meal plans generally suggest that each meal should contain one portion of lean protein, a vegetable, a piece of bread and a fruit.

You may also get a list of approved foods to select from in specific amounts.

Butter, oils and sugar should be avoided, but you're encouraged to drink a lot of water. Mineral water, coffee and tea are allowed as well.

Summary The HCG diet is usually divided into three phases. During the weight loss phase, you take HCG while eating only 500 calories per day.

Most of the HCG products on the market today are homeopathic, meaning that they don’t contain any HCG.

Real HCG, in the form of injections, is administered as a fertility drug and available only through a doctor's prescription.

Only injections can raise blood levels of HCG, not homeopathic products sold online.

Summary Most of the HCG products available online are homeopathic and do not contain any real HCG.

HCG has not been approved as a weight loss drug by the FDA.

On the contrary, government agencies have questioned the safety of HCG products, as the ingredients are unregulated and unknown.

There are also a number of side effects associated with the HCG diet, such as:

These may be largely due to its starvation-level calorie intake, which is almost guaranteed to make people feel miserable.

In one case, a 64-year-old woman was on the HCG diet when blood clots developed in her leg and lungs. It was determined that the clots were likely caused by the diet ().

Summary The safety of HCG products has been questioned by official agencies like the FDA, and numerous side effects have been reported.

The HCG diet limits calorie intake to around 500 calories per day for weeks at a time, making it an extreme weight loss diet.

Any diet that is this low in calories will make you lose weight.

However, numerous studies have found that the HCG hormone has no effect on weight loss and doesn't reduce your appetite.

If you're serious about losing weight and keeping it off, there are plenty of effective methods that are much more sensible than the HCG diet.