Taking GLP-1 Medication? Why Body Composition and Gut Health Matter.

When you start GLP-1 treatment, it is natural to focus on the number on the scales. But that number cannot tell you everything about your progress.

For lasting health, it helps to understand what weight you are losing, how well you are nourishing your body, and whether you are maintaining your strength. Body composition assessment, including DEXA scanning where appropriate, can add useful information. Looking after your gut health also deserves attention—particularly when your appetite and eating habits change. At the CWC we also have access to specialist mineral testing and omega 3 quant levels blood testing to make sure you are absorbing the correct ratios of nutrients.

Weight loss and fat loss are different

GLP-1 medicines can support significant weight loss. However, weight lost can include both fat and lean tissue.

In the SURMOUNT-1 body composition substudy, approximately 75% of the weight lost with tirzepatide—a medicine acting on both GIP and GLP-1 receptors—was fat, and 25% was lean mass. These are group averages, not a prediction for every patient. Read the study.

Crucially, lean mass is not the same as muscle. It includes water, organs and other non-fat tissues. A reduction in lean mass on a scan does not automatically mean the same amount of muscle has been lost.

Nevertheless, maintaining muscle strength and physical function is an important part of a supported weight-management programme.

What can a DEXA body composition scan tell you?

DEXA, also called DXA, uses low-dose X-rays. With the appropriate body composition software, it estimates fat mass and lean tissue across the body, providing more detail than weight or BMI alone.

At the CWC we are lucky to have access to diagnostic equipment that is widely considered to be market leading, and that is the Horizon hologic A DEXA scanner.

A baseline assessment and a clinically appropriate follow-up can help your clinician interpret changes during treatment. Results are most useful alongside information about your nutrition, activity and strength. DEXA does not directly measure muscle strength, and it is not essential for everyone taking GLP-1 medication. Joint clinical advisory on nutrition and GLP-1 treatment.

A body composition scan should also be distinguished from the dedicated hip and spine measurements used to assess osteoporosis. Tell the scanning team if you are pregnant or could be pregnant, as DEXA is generally avoided during pregnancy. NHS guidance on DEXA.

Supporting muscle while your appetite is smaller

A reduced appetite can make it harder to meet your nutritional needs. Useful priorities include:

• Including protein-containing foods in meals, with an individual target agreed with your clinician or dietitian.

• Doing regular resistance exercise suited to your ability, such as weights, resistance bands or supported bodyweight exercises.

• Reviewing persistent nausea, poor intake or declining strength with your treatment team.

Protein alone is not enough: nutrition and resistance exercise work together to support muscle health. Joint clinical advisory.

Where does the gut microbiome fit in?

Your gut microbiome is the community of microorganisms living in your digestive tract. These organisms help process food, and research shows that diet can influence their activity and nutrient absorption. NIDDK research on diet and the microbiome.

During GLP-1 treatment, smaller meals and changing food preferences make dietary quality especially relevant. Digestive symptoms also deserve attention: nausea, constipation or diarrhoea should be discussed with your prescriber rather than automatically attributed to an “imbalanced microbiome”.

There is a suggestion that because GLP-1 agonists slow down bowel movement (in some patients cause constipation), this could indirectly affect the local microbiome as your gut relies on adequate movement and pH gradients to absorb nutrients along the GI tract.

For many people, practical support starts with food:

• Eat a variety of vegetables, fruit, wholegrains, beans, nuts and seeds, as tolerated.

• Increase fibre gradually, particularly if you are prone to bloating.

• Drink enough fluid, following any medical advice about fluid restrictions.

The general UK recommendation is 30g of fibre daily for adults, but your intake may need adjusting to your symptoms and medical needs. NHS advice on fibre.

Do you need a microbiome test?

Gut health matters, but routine microbiome testing is not an established requirement for GLP-1 treatment.

Commercial tests can describe microorganisms in a stool sample, but there is currently insufficient evidence to recommend their routine use or to reliably use results to choose a GLP-1 medicine, adjust its dose or select supplements that improve weight-loss outcomes.

If you are considering testing, discuss its limitations and whether the result would meaningfully change your care. Persistent digestive symptoms may require conventional medical assessment instead.

A more complete picture of your progress

At Cardiff Wellness Centre, our GP-led approach brings together lifestyle medicine and personalised assessment. Our services include DEXA body composition scanning and nutritional support. Explore Cardiff Wellness Centre.

If you are taking GLP-1 medication, a consultation can help you consider whether body composition assessment would be useful and how to support your nutrition, strength and digestive wellbeing alongside your prescribing clinician.

Book a consultation to discuss your individual needs and goals.

This article provides general information and does not replace individual medical advice. Seek urgent medical attention for severe, persistent abdominal pain, especially if it spreads to your back or occurs with vomiting, as this can indicate pancreatitis. MHRA safety advice.