I was tested for hypoglycemia (fasting and then a blood test) and I was found to be negative. But after eating high sugar foods, such as pastries, donuts, etc. my stomach feels funny and I feel tired.
What’s causing the tiredness? I do not have diabetes either, by the way.
Your symptoms are not uncommon and typical for how the human body responds to high levels of fats and concentrated sweets. Carbohydrates (especially foods high in simple sugars) are sleep inducing and protein (meats or cheese) will help keep you awake. (See Eating Habits topic for more information on this.)
In addition, concentrated sweets will draw water into your gastrointestinal tract as well which may contribute to your “funny stomach”. Foods high in fats can cause indigestion depending on your body’s ability to produce bile by the liver. Simply put, maybe your body cannot use that many calories at one time. Next time try a smaller amount (less than one serving) of the same food to see if you get the same reaction.
How much do you eat at a time? If you only eat one pastry or one donut and feel discomfort, you may want to talk to your doctor about this.
While pastries and donuts are high fat, high sugar foods should be an exception to a healthy diet, I do not believe in eliminating whole groups of foods even in a healthy diet.