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  • Writer's pictureDr. Brian Lum

Why Do I Feel Sick When I Eat Sugar?

Updated: Oct 18, 2023

As a functional medicine doctor who has specialized in chronic and complex illness for over a decade, I often see sugar sensitivity among my new patients.

You may have been diligently sticking to your elimination diet when you notice a wave of symptoms after eating a piece of fruit or a serving of dessert. Or you may notice that you feel sick when you have sugar of any kind.

Nausea, shakiness, headaches, sinus congestion, dizziness, digestive upset, sluggishness, stomach cramping, gas, and bloating can all be the consequence of your momentary indulgence.

Below we will look into a few potential causes of feeling sick after eating sugar - and how to overcome them naturally.

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Potential Causes of Feeling Sick After Eating Sugar

1. Underlying Infection

Sugar, especially refined sugar (which is found in processed treats like donuts, candy and cupcakes), can feed opportunistic bacteria that is not meant to be in the digestive tract. This invasive bacteria can crowd out the beneficial bacteria that helps you retain digestive balance.

The overgrowth of this invasive bacteria can create an imbalance and make you feel sick, this is called small intestinal bacterial overgrowth - or SIBO.

SIBO often occurs after an illness, high levels of stress, an operation, a bout of food poisoning or a course of antibiotics. [1]

Sugar can also feed yeast in your intestinal tract and lead to a yeast overgrowth in all of your mucus membrane covered tissue in your body.

This is why a yeast infection like candida can lead to symptoms from your digestive tract all the way up to your sinus cavity - causing chronic sinusitis, chronically plugged ears, and the feeling of a stuffy head.

In the example cases of SIBO and Candida, your body is always in a state of attempting to maintain equilibrium. It can 'fight off' the invasion for a while, but when you shower the invasive bacteria or yeast with a food source like white sugar, it replicates so quickly that your body can no longer hold it back.

This especially applies to those who stop eating processed sugar for an extended period of time and then start again - like when you are on an elimination diet and have a 'cheat meal.'

When you temporarily stopped the sugar, the body started to function better and the infection had less fuel. Once sugar was reintroduced, symptoms flare up because the infection is roaring back to life.

It is important to note that the health of your microbiome affects every other aspect of your health and can directly lead to conditions like insulin resistance and obesity. [2] This is why addressing the underlying cause of your sugar sensitivity is so important.

SIBO and Candida can be effectively treated using a combination of dietary and lifestyle changes in conjunction with targeted supplementation. [3]

Symptoms of an Intestinal Infection
  • dizziness

  • head 'fullness'

  • sinus congestion

  • nausea

  • stomach cramping

  • abdominal pain

  • gas

  • rashes or hives

  • bloating

  • diarrhea

  • weight loss

  • difficulty losing weight

  • malnutrition

2. Blood Sugar Regulation

If your body's blood sugar regulation system is unbalanced, an overload of sugar into your body will cause chaos as your body is unable to utilize and store the excess sugar properly.

These problems can range from insulin resistance to type II diabetes.

Blood sugar problems stem from the body's inability to utilize glucose in the blood stream. This inability can arise from a wide variety of underlying problems, like rampant inflammation, a long term infection or high levels of stress and sleep problems.

In the case of insulin resistance, the body's cells cannot access glucose easily, which leads to more and more insulin being released which further adds to the problem.

Insulin resistance can eventually lead to type II diabetes in addition to poorer outcomes in the case of viral infections and an increased risk for developing other chronic conditions.

Insulin resistance affects the ability of our cells to access of life sustaining energy, which is why it can make whatever symptom you are dealing with more severe.

Testing will be required to confirm a diagnosis - the good news is that with the assistance of a functional medicine practitioner, these conditions are usually reversible through natural interventions like dietary and lifestyle changes and natural supplementation. [4]

Symptoms Indicating a Blood Sugar Problem
  • feeling faint

  • nausea

  • shakiness between meals

  • anxiety

  • fatigue

  • irritability

  • excessive thirst

  • difficulty losing weight

  • feeling sick when you are hungry

  • poor quality sleep

  • feeling nauseous in the morning

  • 'crashing' soon after you have sugar

  • needing to eat very often

  • anger and agitation when missing meals

3. Underlying Inflammation

You may have heard inflammation in reference to an infection or a sports injury. You can also see signs of it as a new cut on your hand becomes red and hot - a sign that white blood cells are speeding to the site, aiding in your recovery.

Inflammation is how your body responds to a threat in order to neutralize it, and to get you back in to balance as quickly as possible.

Unfortunately, in our stressful modern world full of chemical and pesticide laden food, cooked in unhealthy fats and loaded with processed sugar - our diets and lifestyles frequently cause rampant internal inflammation.

This inflammation can damage intestinal lining, weaken artery walls, cause stomach ulcers, lower life expectancy and increase the risk of chronic disease, [5]

Functional medicine tests (mentioned below) can uncover different kinds of inflammation as well as where it is in the body. It is well documented that underlying inflammation can cause metabolic disorders like insulin resistance, making a multifaceted treatment imperative to regaining health. [6}

There are also dietary changes and nutritional supplements that can be used to dramatically reduce inflammation, improve sleep, increase energy and remove symptoms previously thought to be permanent - like migraines, fatigue and poor digestion.

4. An Underlying Condition

If you have an underlying condition, like rheumatoid arthritis, diabetes, long Covid or vascular disease - it may directly contribute to a condition like insulin resistance. [7]

Additionally, having a metabolic condition like insulin resistance may make the development of and complications from chronic conditions more likely. [8]

It is common for individuals with an underlying condition to feel pain in their joints or have brain fog and digestive distress for days after they eat sugar. The inflammation lingers, and causes a chain reaction of effects throughout the body.

Just as your lifestyle can trigger pain, inflammation and disease - it can also hold the key to quelling these processes in the body.

A strong social network, good quality sleep, and physical activities you enjoy are just as important as diet when it comes to your recovery from any symptom or condition.

A diet rich in whole foods and low in processed foods - with lots of vegetables and fruits with and healthy fats like olive oil - can create a great foundation to keep inflammation low.

Some people benefit from a Ketogenic-like diet temporarily while we treat their underlying conditions.[9] This will be highly dependent on a patient's health goals and underlying conditions.

Just as medicine does not work the same on everyone - each person does not thrive on the same diet. The temptation may be to go to a strict Keto or Paleo diet but remember - these diets only work for some, and can exacerbate the underlying health conditions of others.

Just because it sounds good on paper or works well for someone else does not mean a diet or supplement will work with your unique genetic and biological needs.

Functional medicine testing can determine if you have any underlying conditions, if certain factors led to those conditions, and what specific interventions we can do to lessen symptoms and improve overall health. It can also be a guide in crafting an ideal diet for your health condition.

5. Food Allergies and Sensitivities

Although a fructose allergy is certainly possible, it is more common for people to mistake a sugar intolerance or allergy for another kind of food intolerance (or one of the causes listed above).

A patient may notice that whenever they have toaster pastries or a certain bag of candies - their symptoms restart. This may be due to sugar but it also may be due to other ingredients in the food as well.

The problem is, processed foods are loaded with ingredients you could be reactive to.

Gluten, preservatives, dyes, binders, dairy, nuts and soy are all incredibly common allergens that are crammed into foods like candies, take-out foods and processed meals.

A functional medicine doctor can help you prune your diet of the likely culprits and restore your gastrointestinal health.

Once the trigger is identified, it is often necessary to avoid the dietary offender for a time while your body is healing - this will often enable you to tolerate the food in moderation in the future.

A step you can start today if you find yourself highly reactive to sugar is to keep a food journal, you may notice that you feel fine with fruit but awful with corn syrup or feel better overall when you avoid grains - these differences are very important in crafting a healthy, symptom-free diet.

Limitations of a 'Gut Healing Diet'

Despite the popularity of 'gut healing' diets like paleo, keto, high protein, or calorie restriction - usually your body needs a healthy diet in addition to some targeted nutritional medicines to kick an infection for good.

An indication that your diet alone is not treating the infection adequately is when you have been adhering to your diet and one serving of sugar is enough to inflame your body and restart symptoms - this shows that an infection is still dormant in your gut.

A healthy intestinal tract should be able to handle an occasional sugary dessert.

After all, high amounts of sugar were available to our Paleolithic ancestors - wandering upon the occasional fruit tree or bees nest would supply fantastic amounts of fuel that they would make use of.

Instead of staying on a restrictive diet permanently, treating your underlying infection or condition can allow you to have an occasional indulgence without the consequences.

Many patients also find improved overall health when an infection is finally cleared - increased energy, better sleep, improved mood as well as a reduced risk of developing chronic disease in the future.

For my patients, if they feel sick when eating sugar I will often order a series of tests in addition to lifestyle and dietary recommendations to pinpoint the cause of their symptoms.

Functional Medicine Testing

A starting point for many patients who react adversely to sugar is to check their Hemoglobin A1C and to do a comprehensive stool analysis.

Hemoglobin A1c

Hemoglobin A1C measures your average blood sugar over the course of 3 months.  It is the most accurate assessment of blood sugar regulation.  Functional medicine levels should be approximately 5.2.  5.7 or higher is classified as prediabetic and 6.5 or higher is type II diabetes. 

Comprehensive Stool Analysis 

A Comprehensive Stool Analysis evaluates specific levels of your protective bacteria, identifies possible hidden gut infections, and assesses your ability to digest food.

Click the link below to see additional testing we offer, the list is not exhaustive as there are ever increasing resources to investigate a wide variety of symptoms and conditions.

Functional Medicine Can Help Sugar Sensitivity

A functional medicine consultation works a little differently from the conventional medicine appointments you are used to having. Rather than a short appointment followed by a routine test, we start with a lengthy hour-long new patient appointment.

The details of symptom onset and progression, family history, and a full medical history are all vital to understand your health condition and in crafting a treatment plan going forward.

Follow up appointments are 30 minutes long during which treatment plans will be adjusted according to your recovery trajectory.

I specialize in treating complex and chronic symptoms naturally and have found that often, patients can resolve many long term symptoms and conditions completely, including ones that they had been told were permanent, or an inevitable effect of aging.

A consultation can help you determine the cause of your symptoms, particularly if your condition is complex, and you have tried many diets and approaches with limited success.

Online Functional Medicine Consultations

I offer online functional medicine consultations and I am currently accepting patients worldwide.

If you would like to speak to me to ask questions about functional medicine, chronic illness, or becoming a patient please schedule a free 15 minute consultation.


Disclaimer: If you think that you may have a medical emergency, call your doctor or 911 immediately. No action or inaction should be taken based solely on the contents of this information. Nor should you ever delay seeking medical advice or treatment due to the information contained on this Website. The information on this Website is not intended to diagnose, treat, cure, or prevent any disease or medical condition. The information discussed is not intended to replace the advice of your healthcare provider. Reliance on the information provided by this Website, Dr. Brian Lum, or Functional Healthcare Institute is solely at your own risk.


[1] Kossewska, Joanna, Karolina Bierlit, and Vladimir Trajkovski. 2022. “Personality, Anxiety, and Stress in Patients with Small Intestine Bacterial Overgrowth Syndrome. The Polish Preliminary Study.” International Journal of Environmental Research and Public Health 20 (1): 93.

[2] Saad MJ, Santos A, Prada PO. Linking Gut Microbiota and Inflammation to Obesity and Insulin Resistance. Physiology (Bethesda). 2016 Jul;31(4):283-93. doi: 10.1152/physiol.00041.2015. PMID: 27252163.

[3] Ostrowski, Bartosz, and Michał Kukla. 2020. “SIBO – What the General Practitioner Should Know.” Paediatrics and Family Medicine 16 (1): 53–56.

[4] "Lifestyle Interventions for Type 2 Diabetes," The Institute for Functional Medicine, Accessed May 21, 2023,

[5]Hunter, Philip. 2012. “The Inflammation Theory of Disease - The Growing Realization That Chronic Inflammation Is Crucial in Many Diseases Opens New Avenues for Treatment.” EMBO Reports 13 (11): 968–70.

[6] "Connections Between Inflammation and Insulin Resistance," The Institute for Functional Medicine, Accessed May 21, 2023,

[7]"Lifestyle Interventions for Type II Diabetes," The Institute for Functional Medicine, Accessed May 21, 2023,

Gonzalez-Gay MA, Gonzalez-Juanatey C, Vazquez-Rodriguez TR, Miranda-Filloy JA, Llorca J. Insulin resistance in rheumatoid arthritis: the impact of the anti-TNF-alpha therapy. Ann N Y Acad Sci. 2010 Apr;1193:153-9. doi: 10.1111/j.1749-6632.2009.05287.x. PMID: 20398022.

[8] Al-Hakeim, Hussein Kadhem, Haneen Tahseen Al-Rubaye, Abdulsahib S Jubran, Abbas F Almulla, Shatha Moustafa, and Michael Maes. 2022. “Increased Insulin Resistance Due to Long COVID Is Associated with Depressive Symptoms and Partly Predicted by the Inflammatory Response During Acute Infection.” MedRxiv.

[9] Yuan X, Wang J, Yang S, Gao M, Cao L, Li X, Hong D, Tian S, Sun C. Effect of the ketogenic diet on glycemic control, insulin resistance, and lipid metabolism in patients with T2DM: a systematic review and meta-analysis. Nutr Diabetes. 2020 Nov 30;10(1):38. doi: 10.1038/s41387-020-00142-z. PMID: 33257645; PMCID: PMC7705738.


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