How Undereating Causes Bloat

The equation seems simple, right? Eat less and look skinnier. If there is nothing in your stomach, you can’t bloat, right? By now you should know that it is not that simple. Our bodies are of course smarter than that!

I too have fallen victim to this “logic” to try and gain control over any potential bloat. Unfortunately, this only left me feeling uncomfortable, physically and frustrated and critical, emotionally. Why couldn’t I control what my body looked like despite having such rigid control over my food intake? I must be the problem then, right? And thus began the cycle of more restriction and self-loathing and less energy to manage life’s stressors. As an athlete, I will let you just imagine how this affected my performance.

Many of my clients come to me with the same problem. They feel upset that their bodies aren’t listening to them or feel like their bodies are broken. In reality, the body is doing exactly what it is programmed to do, we are just mistreating it. 

In this blog post, I will be explaining why underfueling leads to bloat, gas, discomfort, and ultimately worse body image. I will discuss the physiological and emotional effects of underfueling and how we can prevent these negative side effects.

How the Nervous System Reacts to Underfueling

Our body sends and receives signals from our vagus nerve, which drives the shift between the different nervous system states: ventral vagal, hyperarousal, and hypoarousal. Each of these 3 states is described in detail in my module “Nervous System Regulation for Athletes”, but here is a quick run down. When the body is in ventral vagal state, we are able to respond to life’s stressors without feeling overwhelmed and it is easy to connect to ourselves and those around us.¹ Hyperarousal is that “fight or flight” state where we feel overwhelmed and anxious.¹ Finally, hypoarousal is basically “shut down” mode, where the body becomes so overwhelmed that everything slows and we immobilize.¹ The ability to shift between these states is an evolutionary tool that developed millions of years ago to keep us safe from danger and death, however it does not have the ability to differentiate between restrictive food intake caused by environmental reasons, such as famine, or self-inflicted reasons, like disordered eating. 

Initially when restriction begins, the body enters a hyperarousal, or “fight or flight” state. This state tells the body that we should be anxious and promotes the release of stress hormones, such as adrenaline and cortisol. This state mobilizes stored energy in our body while shunting blood away from our GI tract and into the arms and legs so we will be motivated to get up and find food, but in a state of intentional restriction, we do not listen to those signals.¹ As you can imagine, the shift in stress hormones alters our body’s cravings, which when the body is starving, makes it impossible to make sound choices around food or to eat our food mindfully, giving way to gut issues, especially, since our body is focusing on shunting blood to our extremities thanks to the fight or flight state.

When those signals are ignored long enough or undereating is chronic, the nervous system shifts to hypoarousal, or the “shut down” state in order to conserve energy. This state’s purpose is to conserve energy because the body senses that food is not and will not become available in adequate amounts any time soon, as if we were in a famine. The body starts cutting corners where it can. Your heart doesn’t need to beat 60 times per minute, 40 should be sufficient, right? Since we are in a famine, it would not be smart to bring a child into the world, let’s shut reproduction down! Blood doesn’t need to go all the way to your fingertips as long as we are hitting the major extremities! Why are we working so hard to digest food in a timely manner when we could take our time and spend less energy? 

What is dangerous about the hypoarousal state is that it promotes a feeling of “numbness” that makes it challenging to listen to the body’s hunger and fullness cues. This shut down state can ultimately lead to binge and restrict cycles, which lend their way to their own digestive issues, as we will discuss next.

How the Body Reacts to Underfueling

If we think of our digestive system as a highway, we want a steady and efficient flow of traffic throughout the day. We do not want too many cars on the road at once, but we do want some spread out throughout the day. Our digestive system is the same with food, it wants a steady flow throughout the day. Inconsistent eating mimics a construction zone where the speed limit is slower than normal. As I said previously, when the body is chronically underfueled, digestion is going to slow down, causing bloating, gas, constipation, and overall discomfort.

So why does your stomach hurt, even if you have not eaten anything? When digestion is slow, food sits in our stomach and intestines for longer. If food is hanging out in our intestines, our gut bacteria ferment the carbs from our food, producing gases (AKA air), which because motility is slow, leads to the build up of that air, and ultimately causes bloating (and pain). 

Additionally, when restriction persists for long enough, the muscles of the gut start to atrophy. Our gut is made of hundreds of muscles that need to be exercised in order to function properly. You would not expect to go to the gym for the first time without feeling sore, would you? This same logic applies to our guts. If we don’t use the muscles, they weaken! To make matters worse, when the gut muscles are weak, they struggle to push stool and gas out effectively, worsening discomfort.

It is not uncommon for people who restrict food intake to be diagnosed with Irritable Bowel Syndrome (IBS) or Small Intestinal Bacterial Overgrowth (SIBO) due to the same reasons listed above: altered gut motility, increased anxiety and nervous system dysregulation, and altered gut flora. One study found that of patients surveyed who were diagnosed with anorexia or bulimia, 90% reported abdominal distention as their most frequent symptom.² Another study found that adults who were diagnosed with an eating disorder during adolescence were three times more likely to be diagnosed with IBS as an adult, proving that restriction has long lasting effects.³ When it comes to SIBO, a hallmark and oftentimes most bothersome symptoms is abdominal bloat. Both IBS and slowed gut motility are listed as causes of SIBO, and the best treatment for SIBO is to address the underlying cause, which in the case of disordered eating is to improve gut motility.⁴ So how do we improve motility and therefore bloat?

How to Prevent Bloating

By now, you get it: undereating has no benefits. Undereating can look like eating portions that are too small regularly or going too long between eating episodes, or a combination of both. Regardless, it shifts our nervous system state, slows gut motility, and alters the microbiome, resulting in painful bloat. To prevent this, we have to do the opposite of what caused the bloat: we have to eat enough throughout the day regularly.⁴ This may look like having 3 meals and 2-3 snacks per day or 5-6 mini meals or a combination of both. Additionally, we want to include a portion of carbs, protein, fat, and fiber at each meal, and for snacks a combo of at least two of these macronutrients. Regardless of what you choose, when the body is adequately nourished,, you will feel it in the following ways:

• More regulated nervous system, allowing us to manage daily stressors without feeling overwhelmed

• Improved energy levels and cognitive capacity to handle stressors 

• Improved gut motility

• Increased microbiome diversity

• More consistent bowel habits (less gas, constipation, and/or diarrhea) 

• More dietary variety 

• Less bloating and discomfort

Many times when someone does experience painful bloating, it inhibits them from eating enough, causing further restriction. Sometimes, it is important to tackle the cause of bloating that is a deterrent to eating, however regardless of whether restriction caused the bloating or vice versa, being adequately fueled is a non-negotiable. If you need accountability or help finding what works for you, book with a SkimWellness RD today to see how you can live and feel your best!  

References

1. Porges S. W. (2025). Disorders of gut-brain interaction through the lens of polyvagal theory. Neurogastroenterology and motility, 37(3), e14926. https://doi.org/10.1111/nmo.14926

2. Salvioli, B., Pellicciari, A., Iero, L., Di Pietro, E., Moscano, F., Gualandi, S., Stanghellini, V., De Giorgio, R., Ruggeri, E., & Franzoni, E. (2013). Audit of digestive complaints and psychopathological traits in patients with eating disorders: a prospective study. Digestive and liver disease : official journal of the Italian Society of Gastroenterology and the Italian Association for the Study of the Liver, 45(8), 639–644. https://doi.org/10.1016/j.dld.2013.02.022

3. Yang, K., Kwon, S., Burton-Murray, H., Kuo, B., Chan, A. T., Field, A. E., & Staller, K. (2024). Maladaptive weight control and eating behaviours in female adolescents/young adults are associated with increased risk of irritable bowel syndrome in adulthood: Results from the Growing Up Today Study (GUTS). Alimentary pharmacology & therapeutics, 60(7), 934–939. https://doi.org/10.1111/apt.18197

4. Dukowicz, A. C., Lacy, B. E., & Levine, G. M. (2007). Small intestinal bacterial overgrowth: a comprehensive review. Gastroenterology & hepatology, 3(2), 112–122.