What Happens to Sugar Levels in the Blood While Fasting?

Low Blood Sugar

Fasting typically doesn't cause low blood sugar levels. However, going to long without food can affect fasting glucose levels for people with diabetes.

Blood sugar, also known as glucose, is the body's main source of energy, according to the Mayo Clinic. The body gets most of its glucose by metabolizing the carbohydrates in food. But going too long without food can cause dangerously low blood sugar in people with medical conditions such as diabetes.

Normal Blood Sugar Levels

The body relies on a complex system to keep blood sugar levels within a healthy range. In order to understand fasting blood sugar levels, it can be helpful to know a bit more about how this system works.

Insulin and glucagon are the two main hormones responsible for maintaining normal glucose levels, according to information published by Kaiser Permanente. Insulin helps move glucose out of the blood and into the cells, which use it for fuel. Through this process, insulin also lowers blood sugar. Insulin does most of its work following a meal.

But food isn't the body's only source of glucose. The liver also stores and produces glucose to keep blood sugar levels from getting too low between meals or overnight, as described by the University of California, San Francisco. If blood sugars do get too low (called hypoglycemia), the body releases glucagon, which signals the liver to release its stored glucose. This brings blood sugar levels back to normal.

A blood sugar level is simply the measurement of how much glucose is in the blood at a given moment. Healthy blood sugars fall into a fairly narrow range. According to Diabetes.co.uk, recommended blood sugar levels for people with diabetes are:

  • Before meals: Between 80 and 130 mg/dL
  • Two hours after eating: Less than 180 mg/dL

Note: A blood sugar level that is lower than 70 mg/dL is defined as hypoglycemia.

Fasting Blood Sugar Levels

A fasting blood sugar level is the amount of glucose in a person's blood after they haven't had any food or drink (other than water) in at least eight hours, according to the American Diabetes Association. But it's not uncommon to go longer than eight hours without food. Maybe you eat breakfast at a later time than normal. You may be fasting for spiritual reasons, or as part of an intermittent fasting diet. How will this impact your blood sugar levels?

The short answer is, it usually won't. "There is no 'normal' situation — that is, [one] without medication or illness — in which a person would develop hypoglycemia," says Samar Hafida, MD, an endocrinologist at Harvard's Joslin Diabetes Center. "The body just has too many defense mechanisms. Even people who don't eat for many days or weeks won't develop hypoglycemia."

That may sound surprising. After all, most people have experienced at least some of the classic signs of hypoglycemia (including hunger and irritability), according to the Mayo Clinic. But if there are no additional medical factors, simply having the symptoms of hypoglycemia doesn't actually mean your blood sugar is low.

The brain depends on glucose to function, as described by a study published in the July 2015 World Journal of Diabetes. Therefore, our bodies have evolved to protect against potential carbohydrate shortages. The body uses a group of counter-regulatory hormones (including glucagon) to make sure it always has enough glucose, even when it isn't getting energy from food.

Read more: Should I Exercise While Fasting?

Is Fasting OK for People with Diabetes?

People with diabetes have systems that either don't make enough insulin (type 1) or can't use the insulin they do have effectively (type 2). Insufficient insulin action causes high blood sugar, also called hyperglycemia, according to the National Institutes of Health. To manage this, people with diabetes often need to take medication such as insulin injections to lower their blood sugar. But these medications can also cause low blood sugar if a person takes too much or doesn't eat enough carbs. "People with diabetes can fast, but they should talk with their clinician beforehand, [as] they may need to adjust their medications [accordingly]," says Hafida.

However, Hafida points out that fasting may be inadvisable for certain patients — particularly those with hypoglycemic unawareness. "[If a person] has frequent hypoglycemia, they can lose their ability to feel the symptoms of low blood sugar," she says. "Fasting is probably not appropriate [for them] unless they have a continuous glucose monitor." Hafida also points out that fasting can have other negative health effects, including dehydration and low blood pressure.

If you have diabetes and are planning on fasting, be sure to discuss it with your doctor. In general, it's a good idea to carry a small high-carb snack with you at all times. That way, if you go longer between meals than you'd planned, you'll still have a source of carbs on hand.

Read more: High-Carb, Low-Fat Foods

Other Causes of Fasting Hypoglycemia

In rare cases, nondiabetic fasting hypoglycemia can be caused by a handful of other medical conditions, according to Hormone.org. These include serious illnesses (particularly those affecting the liver, heart or kidneys), counterregulatory hormone imbalances or a rare pancreatic tumor called an insulinoma. Certain medications (such as the sulfa class of antibiotics) as well as excessive alcohol consumption can also cause fasting hypoglycemia.

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