Understanding the Risk of Malnutrition During Obesity Treatment

Insights
Written By Kathryn Barnette

This information is general in nature and does not replace personalised professional advice. Please contact your healthcare team if you have any questions or concerns.

Obesity treatment can be an important step towards improving your health (1). However, one aspect of treatment that people may not always expect is how it can affect your nutrition.  

Malnutrition can occur in people of any body size, including those living with obesity (2). Changes in appetite, food intake and the way your body functions during treatment can sometimes make it more difficult to meet your body’s nutritional needs (1,2).

Understanding this risk can help you and your healthcare team take steps to support your nutrition throughout treatment.

What is malnutrition?

Malnutrition occurs when the body does not receive enough nutrients (e.g. energy, protein, vitamins or minerals) to support normal body functions (2). Over time, this can affect strength, mobility, energy levels, immune function, wound healing, bone health and long-term weight maintenance (2-4). However, malnutrition is not always visible, and body weight alone does not indicate whether someone is meeting their nutritional needs (1,4).

Malnutrition can occur when:

  • Overall food intake is reduced (2,4).

  • Restrictive dieting practices are followed for long periods of time (5).

  • Diet variety is limited (2).

  • Nutrient needs are increased during illness, increased exercise, recovery from medical/surgical treatment, or periods of nausea and vomiting (2,4).

  • The body is unable to absorb nutrients effectively, e.g. after some types of bariatric (metabolic) surgery (2).

  • Obesity management medications reduce hunger, cause nausea, or lead to skipping meals (6).

How can obesity treatment affect nutrition?

Bariatric surgery

Bariatric surgery supports weight loss by reducing how much food feels comfortable to eat, changing hormones that influence hunger and fullness, and in some procedures, affecting how nutrients are digested and absorbed (7).

After surgery, people may:

  • Feel full sooner (7,8).

  • Eat smaller portion sizes (8).

  • Experience low appetite (8,9).

  • Have changes in taste or the types of food they can tolerate (10).

  • Have less desire or motivation to eat regularly (8).

  • Be unable to absorb as much nutrition from the food they eat as efficiently

Obesity management medications

Obesity management medications, such as GLP-1 medicines including Ozempic® (semaglutide), Wegovy®, and Saxenda®, as well as similar medicines like Mounjaro® (tirzepatide), support weight loss by reducing appetite and slowing how quickly food leaves the stomach (1).

This may lead to:

  • Reduced hunger (6,11).

  • Eating smaller portion sizes (6).

  • Feeling full quicker (6).

  • Nausea or reduced interest in food (6,11).

  • Have less desire or motivation to eat regularly (6,11).

What to eat to avoid malnutrition

Getting enough protein is critical

Eating less food may make it more difficult to meet your protein needs if meals are not carefully planned (7). During weight loss, if protein intake is too low, the body may lose more muscle instead of fat, even if body weight stays the same (12).

Good protein sources include (7):

  • lean meats

  • chicken without skin

  • fish (including tinned varieties)

  • eggs

  • dairy foods such as milk, yoghurt and cheese

  • soy foods such as tofu

  • legumes such as lentils, chickpeas and beans

Protein-rich foods are also important sources of minerals such as iron, zinc and calcium (13).

Ask your healthcare team for guidance on how much protein you should aim to eat each day. How much protein you need will be unique to you and your situation, so it is important to get advice from qualified health professionals.

Include some healthy carbohydrate foods

Including a small amount of good quality carbohydrate at each meal (such as wholegrains like oats, wholegrain bread or brown rice, fruit or legumes) can help keep your energy levels steady and support healthy blood sugar levels (14).

When eaten as part of a balanced meal, carbohydrates can also help you feel fuller for longer. This is especially important if you are doing moderate to higher intensity physical activity, as carbohydrate is the body’s main fuel source during exercise (14,15).

Including enough energy from carbohydrate helps to ensure that the protein you eat is used for its important functions – see above (rather than being used for energy).

Include a small amount of healthy fats

Including a small amount of healthy fats (such as nuts, seeds, avocado, olive oil or oily fish - like salmon, trout and sardines) supports overall health (16).

Fats help the body absorb certain vitamins and play an important role in brain health, hormones and the immune system. Only small amounts are needed, with around 3-4 teaspoons per day usually enough (16,17).

Get enough vitamins and minerals

Reduced appetite during obesity treatment may lead to lower overall food intake.

Over time, this may increase the risk of low levels of certain vitamins and minerals, which may not always cause noticeable symptoms at first (9). After bariatric surgery, it is essential to take appropriate multivitamin and mineral supplements every day (7). For some people taking obesity management medications, a daily multivitamin supplement may also be required (18).

Some nutrients require special attention. These include:

  • Iron

  • Zinc

  • Iodine

  • Calcium

  • Vitamin A

  • Vitamin B12

  • Vitamin D

Ask your healthcare team to check your vitamin and mineral levels regularly. Ask for advice on appropriate supplements for you.  This is particularly important after bariatric surgery, when your stomach and gut have changed and does not digest and absorb nutrients normally.

Practical ways to reduce your risk of malnutrition

The goal of obesity treatment is to support weight loss while still meeting your body’s nutrition needs. Building good nutrition habits early can help prevent nutrient deficiencies (20).

You can reduce your risk of malnutrition by:

  • Building your team early

  • Working with your healthcare team. Especially working with a dietitian, before and after starting treatment can help you stay well nourished (13)

  • Choosing nutrient-dense foods

  • When appetite is lower, there is less room for food, which means each bite matters (7).

  • Including a protein food at meals and snacks (e.g. dairy, eggs, legumes, lean meats, chicken, fish including tinned varieties, tofu, nuts or seeds) (7).

  • Including a small amount of healthy carbohydrate at each main meal (e.g. grain bread or crackers, wholegrain cereal such as oats, legumes, corn, sweet potato, basmati rice, fruit or yoghurt). Try not to replace your protein or vegetables/salad with these foods but remember to include them as a small part of your meal (14). If you are doing a lot of exercise, you may need a little more carbohydrate at snacks between meals (14,15).

  • Including small amounts of healthy fats during the day (e.g. olive oil as a salad dressing, or sliced avocado in a salad or on toast) (16).

If symptoms such as nausea affect your intake, your team may recommend:

  • Food fortification (adding extra protein or energy to meals you already tolerate) (21).

  • Combining a few good sources of protein in the same meal i.e. adding cheese to eggs, or yoghurt to casserole/curry, or adding chickpeas/cheese to a tuna/chicken salad.

  • Oral nutrition supplements (high-energy, high-protein drinks) as short-term support (21). Your healthcare team can recommend these if required.

Take recommended vitamin and mineral supplements as recommended:

Particularly after bariatric surgery, you are unlikely to prevent vitamin and mineral deficiencies developing, unless you take appropriate supplements. This is because your digestion and absorption of vitamins and minerals are interrupted by the changes to your stomach and intestine. 

  • For bariatric surgery:

    • Specialised post bariatric surgery vitamin and mineral supplements are usually recommended for life (7). Ask your dietitian or health care provider what these supplements are best for you.

  • With obesity management medications:

    • Supplements are not always needed long term. However, your healthcare team may recommend them if food intake is low or blood tests show low levels of certain nutrients (18).

Monitoring your nutrition with blood tests:

  • For bariatric surgery:

    • Blood tests are commonly recommended before surgery, at 6 and 12 months after surgery, and then yearly long term (7,13) - they may need to be done more regularly for some people.

  • With obesity management medications:

    • Blood tests are usually based on your needs. They may be recommended before starting treatment and repeated if intake is low or symptoms suggest a deficiency (18).

Know when to reach out for help:

Contact your healthcare team if you experience:

  • Ongoing nausea or vomiting

  • Very low intake most days

  • Regularly forgetting to eat or missing meals because of low hunger

  • Ongoing constipation

  • Reduced tolerance to foods overtime

  • Fatigue, dizziness or feeling faint

  • Hair loss

  • New tingling or numbness in your hands or feet

  • Obvious muscle loss

  • Changes in your skin or nails

Get more support in the Prospero Health App

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Prospero Health App is FREE, and made for Australians using GLP-1 medicines (like Wegovy® and Mounjaro®) or recovering from bariatric surgery.

References

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  2. Cederholm T, Jensen GL, Correia MITD, Gonzalez MC, Fukushima R, Higashiguchi T, et al. GLIM criteria for the diagnosis of malnutrition - A consensus report from the global clinical nutrition community. Clinical Nutrition. 2019;38(1):1-9. https://doi.org/10.1016/j.clnu.2018.08.002

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  8. Al-Najim W, Docherty NG, le Roux CW. Food Intake and Eating Behavior After Bariatric Surgery. Physiological Reviews. 2018;98(3):1113-41. https://doi.org/10.1152/physrev.00021.2017

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  13. Ganipisetti VM, Naha S. Bariatric Surgery Malnutrition Complications [Internet]. Treasure Island (FL): StatPearls Publishing; 2026 [updated 2023 May 19]. Available from: https://www.ncbi.nlm.nih.gov/books/NBK592383/

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  21. Baldwin C, Kimber KL, Gibbs M, Weekes CE. Supportive interventions for enhancing dietary intake in malnourished or nutritionally at-risk adults. Cochrane Database Syst Rev. 2016;12(12):Cd009840. https://doi.org/10.1002/14651858.CD009840.pub2

Kathryn Barnette
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