Blood Sugar 164 mg/dl — Too High? Fasting & After Eating

Less water, same sugar: why 164 can be partly a concentration effect

A meter does not count glucose in your whole body. It measures how much glucose is dissolved in a small amount of blood, in milligrams per deciliter. When you lose fluid through sweat, vomiting, diarrhea or simply not drinking enough, the liquid part of the blood shrinks while the glucose stays. The same amount of sugar is now in less water, so the number per deciliter goes up.

Dehydration on its own rarely explains a big jump, but on a hot day or after a stomach bug it can add some points to a reading that would otherwise sit lower. It also makes the body release stress hormones, which nudge glucose up further. People who already run high feel this most, because their bodies have less room to correct the shift quickly, and because thirst itself can be a sign of high glucose that is easy to mistake for ordinary heat.

A loop that can feed itself

High glucose and dehydration can push each other. Once blood glucose rises above roughly 180, the kidneys start passing glucose into the urine, and water follows it. That means more urine, more fluid loss, and an even higher concentration in the blood. At 164 most people are not losing glucose through the kidneys yet, but they are not far from it.

This loop is one reason very high readings during illness can become dangerous. Steady drinking is a basic part of the sick-day advice that care teams give people with diabetes.

Hot days put supplies at risk too

Heat affects more than the body:

• Test strips left in a hot car or in direct sun can give wrong results. Keep them in their closed container and within the temperature range on the label.
• Insulin loses strength when it gets too warm. Insulin that has been overheated may not bring glucose down as expected, which can look like a stubborn 164.
• Insulin absorption can speed up in heat, because warm skin has more blood flow. That can cause an unexpected drop later.
• Some medicines, such as water pills and SGLT2 inhibitors, increase fluid loss and make dehydration more likely.

What to drink, and the signs worth knowing

Plain water is the simplest choice. Unsweetened tea, sparkling water and sugar-free drinks also count. Regular sports drinks, sodas and juice add a quick load of sugar on top of the fluid, so they push readings up unless you are treating a low or doing long exercise.

Signs of dehydration include dark yellow urine, a dry mouth, headache, dizziness when standing and feeling unusually tired. Older adults often feel less thirsty, so they may need to drink on a schedule. If you cannot keep fluids down, or readings keep climbing while you are ill, contact your care team. The what affects blood sugar page covers other everyday factors.