When Low Body Weight Has Nothing to Do with Wanting to Lose Weight

Low body weight is almost always framed around one thing: wanting to lose weight, or relate to an eating disorder. The narrative is so dominant that when someone is underweight for entirely different reasons, they fall into a gap. They don't fit a typical ‘underweight’ profile, and they often, don't get the right support.

I have been working with clients who are underweight because of chronic medical conditions for many years including: COPD, gastroparesis and chronic gut conditions, childhood bowel resection, type 1 diabetes, medical trauma and neurofibromatosis type 1. These conditions can create physiological barriers to eating and absorbing food that have nothing to do with choice or psychology.

But here's what I see in clinic: low body weight almost always comes with compromised gut function, and they fuel each other. Food fears can then develop on top of that, which they almost always do, and clients get stuck in a vicious circle with increasing digestive distress but also growing fears about what to eat, which can lead to a limited list of foods that they deem as ‘safe’.

They start eating less, often because they cannot manage large meals anymore but also due to fear, their food window narrows, their anxiety increases, their nervous system becomes more dysregulated, they thyroid and metabolism starts to slow in as part of the survival response, and their gut slows further. Everything gets worse together.

The good news is that this cycle can be reversed. But it requires a completely different approach than what most people are offered.

The Gap Between Diagnosis and Support

When someone presents with low body weight and subsequent digestive symptoms, they're usually offered one of two things: reassurance that "eating more" will fix it, or a restrictive protocol like Low FODMAP. Neither addresses the root cause.

Personally, I don’t favour, or use, the Low FODMAP diet in my clinic practice.

Low FODMAP is marketed as a solution for functional gut disorders. But in my experience, it's often part of the problem, especially in chronic or complex cases. Here's why.

The main aim of the Low FODMAP diet is to relieve uncomfortable gastrointestinal symptoms, such as bloating, gas, stomach pain, diarrhoea or constipation, by removing certain foods (specific carbohydrates) for those who have been with medically diagnosed Irritable Bowel Syndrome (IBS).  It is not designed to be a permanent lifestyle change, but rather to act as a short-term, three-phase process designed to achieve the following:

• Identify trigger foods – which specific carbohydrates or FODMAPs (Fermentable Oligosaccharides, Disaccharides, Monosaccharides, and Polyols) cause your digestive symptoms.

• Reduce Inflammation: Give your gut lining a chance to rest and recover by restricting highly fermentable carbohydrates.

• Personalize Your Diet: Transition to a long-term, nutritionally balanced diet that only restricts the exact foods you are sensitive to, allowing you to eat as widely as possible without experiencing symptoms.

Even for clients who aren’t low body weight, here’s the first challenge – none of this addresses the root cause of IBS. 

• In the majority of people, stress is often the biggest cause of IBS. But the other side of it is your gut microbiome which gets overlooked.

• In my experience, 99% of the problem is that IBS clients have low commensals (good bacteria) and high opportunistic bacteria or dysbiosis (bad bacteria).  This imbalance means that those ‘bad bacteria’ affect digestion, absorption, gut health, gut integrity and more.

• Removing trigger foods may help relieve symptoms but the carbohydrates that are removed are essential food to your gut microbiomes (think fibre, antioxidants and polyphenols) so you could well be making matter worse.

• It is designed to be a an 8-week process, and yet all too often I see clients ‘stuck’ on it for years.

• I have never, in sixteen years of practice, seen two clients react the same way to the exact same foods. Yet Low FODMAP assumes they will. It's a population approach applied to people who need individualised care. There was even a 2016 study which demonstrated how identical twins have different gut microbiomes, so why would every client with IBS respond positively to a Low FODMAP approach?

• Removing foods does not address the root cause of your digestive issues, and in fact could well make things worse long-term.

As for low-body weight clients, IBS is the least of their issues.  They typically have functional gut disorders, delayed gastric emptying, SIBO, slow digestive motility, as well as gut dysbiosis of their microbiome.

However, it is often recommended by their GP, gastroenterologist or they are referred to a dietician to implement the Low FODMAP approach, or they try to do it by themselves as understandably they are desperate for a resolution.

They start removing certain foods, but many of my clients are already anxious about eating and don't replace them or don’t increase then non-Low FODMAP foods (like protein), further compounding weight loss. Then their food window gets smaller, and they start to lose confidence. The fewer foods they eat, the more they fear the ones they've cut out, and the narrower the diet becomes, making weight gain even harder to restore and the harder it becomes to restore gut function.

The result is clients feel stuck, despite following ‘medical advice’.  They are often desperate to feel well again and restore weight, as it is affecting their quality of life, but they start to wonder if their health will ever be ‘normal’ again.

What's Actually Happening

When someone is underweight due to a chronic medical condition, several things are usually happening at once.

Metabolism adapts downward. When someone is underweight and undereating over a long period, their thyroid function naturally slows as part of the body’s innate survival mechanism.  It is trying to conserve as much energy as possible because it isn’t getting enough form foods. Basal metabolic rate drops and the body goes is in conservation mode, but the flip side is that it makes restoring weight more challenging as eventually you will requires eating more calories and more frequently than an average person, because the brain and body need to be convinced again that it's safe to return to normal metabolism.  This can also take many weeks or months depending on the individual.

dysfunction is at the root. Often there is slow gastric emptying where food isn't moving through the stomach efficiently, or reduced gut motility whereby the intestines aren't contracting properly to push food along. Many of my clients have SIBO (small intestinal bacterial overgrowth), which causes bloating every time they eat and makes it physically hard to increase food volume. They eat something, feel full quickly, feel bloated, feel uncomfortable, maybe with cramps or pain. The brain then learns food = pain, so food fear increases and eating decreases.

Constipation is almost always present. This seems counterintuitive when someone is underweight, but it's common. Poor motility means poor evacuation. Stagnant gut contents mean bacterial overgrowth and worsening gut dysbiosis. Bloating worsens, and the person eats even less, or they become reliant on over-the-counter medications such as Movicol or Dulcolax, which (in my experience) do very little even at high doses

The nervous system is dysregulated. When someone has been unwell for a long time, whether from chronic illness, medical trauma, or prolonged stress, their nervous system lives in fight-or-flight. This state literally suppresses digestion: whenever the brain senses ‘stress’ (whether real or perceived) blood flow moves away from the gut toward the muscles. Stomach acid production decreases, motility slows, and nutrient absorption drops. There may also be a natural loss of appetite as well, and so the more dysregulated the nervous system becomes, the more the gut suffers, and the more the person's anxiety increases. It's another vicious spiral.

Food fears develop. This is crucial to understand: food fears here are not psychological in origin. They are learned responses to real pain and discomfort. The person eats, their digestive system bloats or causes pain, and their nervous system registers that “this food is a threat”. Over time, more and more foods become threats, the safe food list shrinks and weight loss accelerates.

Medical investigations come back normal. Many of my clients have had numerous medical investigations over the years – colonoscopies, endoscopies, celiac disease has been ruled out, slow gastric emptying studies, MRI scans and biopsies – all of which typically come back as ‘normal’, or slow gastric emptying is confirmed, and medication is prescribed, but often this is a symptom of a bigger picture.

Nutrient malabsorption The longer someone is low body weight, the more likely they are to be missing key nutrients that are needed not just for digestive health, but for their whole body.  Vitamin B12, folate and iron deficiencies are very common, but also so is a lack of fibre ad polyphenols that are needed for optimal gut health and motility.  The wider picture is also that this also impact the nervous system and brain chemistry, as neurotransmitter production and function reduce making it even harder to manage moods, anxiety, sleep and pain.

All of these things reinforce each other. And mainstream medicine often treats them as separate problems, or misses them entirely.

The Approach: Root Cause First, Then Rebuild

Restoring weight when someone has a chronic medical condition requires patience, and personalisation.  I am always honest with clients: it can be a long journey with lots of highs and lows, but by working closely together and tailoring the work specifically to that individual is essential and also significantly increases the chances of successful weight restoration.

Some of the steps that I look at include:

Step 1: Address the gut foundation.

Before we think about increasing food, we address what's preventing food from being tolerated. This might look like:

• Addressing gut motility with appropriate supplementation (digestive enzymes, magnesium glycinate for muscle relaxation, L-glutamine for gut barrier integrity)

• Supporting inflammation reduction (omega-3s, anti-inflammatory herbs tailored to the individual)

• Managing SIBO if present, as part of the overall gut microbiome picture.  Many clients have had a SIBO diagnosis and taken the appropriate antibiotics, only to find the SIBO returns or things have got worse because the antibiotics further increased gut dysbiosis.

• Supporting the nervous system to come out of fight-or-flight so that digestion can actually happen.

The nervous system piece is not optional. I encourage clients to work with modalities like breathwork, vagal nerve stimulation (including humming or singing), or devices like Neurostym, whatever the individual client is open to and willing to do. A dysregulated nervous system will sabotage everything else we're trying to do, but over time the brain can re-learn that it is safe and therefore foods are safe too.

This phase takes time. There's no rushing it, but it's essential. We're not trying to force the body to accept more food yet because first we are creating the conditions where the body can accept food safely.

However, I may well start increasing food volume of your safe foods in this time.  The quicker we can begin weight restoration the better

Step 2: Introduce food with absolute precision.

Once the gut foundation is more stable, we begin reintroduction starting with one food at a time, in tiny amounts, at a pace the individual client can tolerate.

This is what I call microdosing. It works like this: introduce a single new food (e.g. eggs) starting with 1 teaspoon. If there's no reaction, increase to 2–3 teaspoons a few days later. Then to 2–3 tablespoons a few days after that. Only once that food is genuinely tolerated do we introduce the next one.

All too often, I hear how clients go to introduce a ‘new’ food, with good intentions, but at ‘normal’ amounts. If that food has not been eaten for many months (even years in some cases) then the gut and its microbiome may not recognise it in the usual way and it is then more likely to cause a reaction, which can create more fear again.  I often get clietns to think about Veganuary as an example.  People jump on Veganuary in January of a new year with good intention to be healthy.  They suddenly cut a lot of normal foods and increase beans, pulses and vegetables … and quickly end up bloated and constipated within a few days.  As much as these foods are good for us and our microbiome, it is too much too soon.  The gut needs time to adjust

My approach does something critical: it proves to the nervous system that food is safe. Each successful introduction is a piece of evidence that the body can handle it. Confidence then starts to build, food fears start to decrease and weight starts to increase. The safe food list expands, not from restriction, but from lived experience of tolerance.

This takes time. Lots of it. But it works because it's meeting the client where they actually are at and not where a protocol says they should be.

Step 3: Increase calories and frequency.

As the gut stabilises and food tolerance improves, we increase both the volume and frequency of eating. This usually means eating more often, five to six smaller meals rather than three, because eating little and often is gentler on a compromised system than large meals.

The calorie requirement is often higher than for someone of average weight, because the metabolism has adapted downward, and we need to send consistent signals to the brain and thyroid that it's safe to return to normal function. This is not about willpower. This is about biochemistry.  But I also work with clients to look for the easy wins here – what are high calorie foods that don’t require big volumes.  Some of my go-to’s are extra virgin olive oil, macadamia nuts, avocado, coconut oil, nut butter and yoghurt drinks.

Step 4: Support the whole person.

Throughout this process, we're also managing pain (through supplements and close communication with any pain specialists involved), monitoring medication interactions, supporting nutrient repletion (many of these clients are depleted in zinc, magnesium, B vitamins, iron, and others), and checking thyroid and metabolic markers as weight begins to restore.

I also meet with my clients weekly, and they can message me with questions in-between sessions, because this work requires accountability, cheerleading, reassurance and support.

Weight restoration from this place is not quick, despite clients keenness for good health. A minimum of six to twelve months is realistic for meaningful restoration, but with consistency, weekly weight gain is possible and crucially, it's sustainable because we're addressing the root causes, not just forcing the body to do something it's not ready for.

Who This Is For

This approach is for people who:

• Are underweight due to a chronic medical condition, not an eating disorder

• May be on many medications or a PEG feed

• Have developed food fears, or ARFID, because of prolonged digestive discomfort

• Have been told to "just eat more" or given a restrictive diet that made things worse

• Feel like their body is broken and no one understands

• Are willing to do deep, patient work to restore their health

It's not a quick fix because it does requires time, personalisation, and a willingness to work closely with a practitioner who understands that your body isn't the problem, but that it's the communication between your nervous system, your gut, and your metabolism that needs to be repaired.

But if you recognise yourself in this description, know this: your low weight is not your fault, not are the food fears that come with it. There is soemthing we can do about it, you are not unfixable, but it's been through something difficult, is in 'fight-and-flight' mode, and needs support that meets it where it actually is.

If you would like to find out more about how Nicola can help you restore weight, increase food variety and also improve health symptoms then book a Clarity Call to discuss further.