What Are Weight-Loss Injections and How Do They Work? A Dietitian's View

“Weight-loss injection” is a popular name for GLP-1 group medicines; semaglutide (Ozempic, Wegovy), tirzepatide (Mounjaro) and liraglutide (Saxenda) are the main active ingredients in this group. These medicines reduce appetite by increasing the feeling of fullness and slowing stomach emptying; they are used only after a doctor's assessment, with a doctor's prescription and follow-up, together with balanced eating and physical activity.

In recent years these medicines have been talked about a great deal, both in the medical world and on social media. A question I hear often is, “If I take the injection, will I still need to watch what I eat?” In this article I explain, from a dietitian's point of view, how the medicines work, who they might be considered for and what part nutrition plays in the process. This is not a recommendation to take a medicine; the treatment decision belongs entirely to your doctor.

What Is a Weight-Loss Injection?

A weight-loss injection is not actually the name of a particular medicine but the general name for a group of medicines that mimic the action of hormones regulating appetite and blood sugar. Most of these medicines were first developed for the treatment of type 2 diabetes, and some were later approved for use in the treatment of obesity.

The best-known active ingredients in Turkey and worldwide are:

The brand names are given here for information only. Which medicine, at what dose and for how long is decided only by your doctor after assessing your health. Obtaining these medicines without a prescription, from the internet or from unreliable sources can pose serious health risks.

How Do Weight-Loss Injections Work?

These medicines mimic the action of the hormone GLP-1 (and, with tirzepatide, also GIP), which is released from the gut after a meal. As a result the brain receives a fullness signal earlier, the stomach empties more slowly and the desire to eat falls.

• In the brain: They reduce the feeling of hunger and the desire to eat through the appetite and reward centres.
• In the stomach: They slow stomach emptying and prolong fullness, so small portions are satisfying.
• In the pancreas: They support the release of insulin when blood sugar rises.

Weight loss comes not from the medicine burning fat directly but from a fall in the energy taken in. This distinction matters: eating less can also mean taking in less protein, fewer vitamins and fewer minerals. As a dietitian, that is exactly my focus: making sure the food you do eat is of high nutritional quality.

These medicines come as pen injectors that are given under the skin once a week or once a day; there are also forms taken as tablets. Treatment generally starts at a low dose and is adjusted gradually under a doctor's supervision. The aim of this stepwise approach is to let the body get used to the medicine and to limit digestive side effects.

Who Might Weight-Loss Injections Be Considered For?

According to the US National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), weight management medicines are generally considered for adults with a body mass index of 30 or above, or 27 or above with a weight-related health problem such as high blood pressure or type 2 diabetes. A high body mass index alone, however, is not a sufficient reason to use a medicine.

Your doctor assesses your health history, the other medicines you take, family illnesses and possible risks together. A personal or family history of some thyroid cancers, or a history of pancreatitis, can restrict the use of these medicines. These medicines are not used by women who are planning a pregnancy, pregnant or breastfeeding; the NHS advises people who want to conceive while using semaglutide to plan with their doctors when to stop the medicine.

If you would like to find out your own body mass index, you can look at my article on body mass index calculation; but do not use the result alone as a basis for a treatment decision.

What Are the Common Side Effects?

The most common side effects are nausea, vomiting, diarrhoea, constipation, abdominal pain, bloating and tiredness. These complaints are usually more noticeable at the start of treatment and during dose increases, and can ease over time.

In situations such as severe abdominal pain that does not go away (especially pain that spreads to the back), constant vomiting, being unable to keep fluids down, jaundice or signs of an allergic reaction, you need to see a doctor without delay. With rapid weight loss, gallbladder problems and muscle loss are also risks to be borne in mind.

Eating patterns play an important role in easing digestive side effects: small, frequent meals, staying away from fatty and fried foods, eating slowly and spreading fluid intake through the day. You can read the nutrition guide for people using weight-loss injections for detailed advice on this.

Do You Still Need to Watch Your Diet While Using a Weight-Loss Injection?

Yes. In the words of the health authorities, these medicines are approved for use together with balanced eating and physical activity. The medicine reduces appetite but does not choose what you eat; with meals that are eaten in small amounts, inadequate protein and micronutrient intake can pave the way for muscle loss, tiredness and hair shedding.

• Protein first: Having a protein source such as eggs, yoghurt, cheese, chicken, fish or pulses at every meal helps preserve muscle mass.
• Fluids: The feeling of thirst can fall along with appetite; you need to drink water regularly through the day.
• Fibre: Vegetables, fruit, wholegrains and pulses help prevent constipation.
• Movement: With your doctor's approval, walking and resistance exercise matter for both muscle and metabolism.

For advice specific to the active ingredient, you can also look at the article on eating while taking Mounjaro.

What Are Common Misconceptions About Weight-Loss Injections?

A great deal of wrong information about these medicines goes around on social media. Common ones include:

• “The injection burns fat”: The medicine does not burn fat directly; it lowers the energy you take in by reducing appetite. How much of the weight lost is fat and how much is muscle depends on diet and exercise.
• “The less I eat, the better”: Eating very little increases muscle loss, tiredness and nutrient deficiencies. The aim is not to eat little but to eat little that is nourishing.
• “I'll use it for a short time to lose a few kilos”: These medicines are not for short-term cosmetic use; they are used as medical treatment in people your doctor considers suitable.
• “My friend's dose will suit me too”: The dose and dose increases are entirely individual; taking someone else's treatment as an example can be dangerous.
• “Weight doesn't come back when the injection ends”: Research shows that when habits are not changed, a significant part of the weight can be regained.

What Happens When the Medicine Is Stopped?

Clinical studies show that when the medicine is stopped, appetite returns and many people regain some of the weight. For this reason, building sustainable eating habits while using the medicine is the key to long-term success.

I covered this topic in detail in the article on whether weight comes back when you stop weight-loss injections. The article on rapid weight loss with Ozempic can also give you an idea of the possible consequences of fast weight loss.

How Does a Dietitian Help in This Process?

A dietitian does not replace medical treatment; the dietitian complements the treatment your doctor is managing on the nutrition side. Our aim is to make sure that, even with a reduced appetite, you get the protein, vitamins and minerals your body needs, manage side effects through diet and prepare for life after the medicine.

In this process a meal plan, portion adjustment, protein target, fluid tracking and an eating pattern that fits with exercise are built together. If you are under a doctor's care and would like nutrition support during this process, we can work together through online diet counselling from anywhere in the world. I would like to remind you that results vary from person to person and that no method has the same effect for everyone.

Sources

• NIDDK – Prescription Medications to Treat Overweight & Obesity
• NHS – Semaglutide
• NHS – Tirzepatide
• WHO – Obesity and overweight

This article is for general information only; please consult your doctor and dietitian about your own situation.