: Bad Brain fog 15 min after eating but sugar test is negative The problem started when I was about 18. anything I eat makes the following symptoms happens 15 min after: Brain Fog Sleepiness and Tirdness Very thirsty

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It's possible that sugar is not causing the problems, but perhaps a reaction to gluten. If your reaction comes from sugar in the form of wheat products or even products containing trace amounts of gluten, this could be the issue. I went off of wheat over a year ago and my brain fog disappeared and my weight naturally started to drop. I recommend looking into the book Grain Brain by Dr. Perlmutter. Gluten sensitivity doesn't always show itself with gut issues. It can present many of the symptoms that you describe without gut discomfort. I recommend maybe getting a gluten sensitivity test done to rule that out. Note: this is not the same as a test for celiac issues. Gluten is also known to cause anxiety and depression. Many with Celiac (an extreme version of gluten sensitivity) are known to struggle with anxiety and depression. Again, I highly recommend looking into Grain Brain.

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Since the reaction occurs so quickly you might try the following:

Troubleshooting Step #1

collect various food items that you usually eat (a fruit, a vegetable, a carbohydrate, meat, etc.)
eat just as much of one item that you would normally have in a meal (Ie: a quarter of an apple, or a piece of lettuce)
wait 15 minutes

This may help you decide if it is a particular type of food that causes the reaction.

Troubleshooting Step #2

if you have been taking any medication since you were 18 and can safely skip it for a couple of days and see if the reaction happens

Troubleshooting Step #3

if you have any other health issues that seem odd (hair falling out, unexplained rash, etc.), research that, even if it isn't a felt problem as it could be related

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The problem could do be what the food is doing to you, but rather what it is doing for you gut flora.

Bacteria in your small intestine are able to communicate with your central nervous system via receptors lining the apical cell of your GI tract. Many of these receptors are ion gated receptors also found in the CNS and have a direct link to your brain (such as seratonin 3, which is responsible for vomiting reflex). However, recent research has found that activation of these receptors may also affect emotional and congintal areas of the brain (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4367209/).

It may be promising to look into your gut flora as the source of your discomfort after eating.

You may also be interested in intermittent fasting.

Please do not use this as medical advice, but rather more information that you can research further at your own discretion. I do not know your medical history and do not want to pretend that I can make any recommendations.

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First, your symptoms sound severe enough that every doctor should take you seriously. I cannot provide any final solution, but the following may help you discuss this with a doctor.

Two conditions that came to my mind are:

Postprandial (reactive) hypoglycemia (Mayo Clinic)

When you eat a large meal, especially if high in carbohydrates, a great amount of glucose will be absorbed in a short time. This may trigger the release of excessive amounts of the hormone insulin, which will result in a drop of glucose not only to normal but to below normal levels (insulin allows glucose to move from the blood into the body cells). This is called "postprandial hypoglycemia" - symptoms are pretty much the same as you have described them. Brain fog and sleepiness can be from low glucose levels, and irritation and thirst (dry mouth?) from high adrenaline levels (as a reaction to low glucose).

Migraine is not a typical symptom of postprandial hypoglycemia, but here is a description of a migraine-like headache after meals (PubMed Central)

Idiopathic postprandial syndrome can cause very similar symptoms as postprandial hypoglycemia, but without detectable low glucose levels (see the Mayo Clinic article above).