A glucose test, such as the standard fasting glucose test or the Oral Glucose Tolerance Test (OGTT), measures how effectively your body processes sugar. The results provide insight into blood sugar regulation and are used to screen for or diagnose conditions like diabetes or gestational diabetes. Proper preparation is necessary for an accurate result, as dietary choices in the days and hours leading up to the appointment significantly influence the body’s metabolic response. Incorrect preparation may lead to a false diagnosis or the need to repeat the procedure.
Dietary Preparation in the Days Leading Up to the Test
The diet consumed in the two to three days immediately preceding the test is crucial for ensuring an accurate result. The primary goal during this period is to maintain normal metabolic function by consistently consuming carbohydrates. It is recommended to consume at least 150 grams of carbohydrates each day to prevent artificially high or low readings resulting from carbohydrate restriction.
Consuming sufficient carbohydrates ensures that insulin-producing beta cells are appropriately stimulated and adapted to glucose processing. If you follow a restrictive diet, such as a ketogenic or very low-carb diet, you must temporarily reintroduce carbohydrates for these three days. Failure to do so can result in a false indication of impaired glucose tolerance, as your body may not be primed to handle the glucose load administered during the test.
This required carbohydrate intake can be met by including common foods like breads, pasta, rice, cereals, potatoes, and fruits. Maintaining your typical activity level during this time is also advised, as excessive or unusual exercise can affect glucose metabolism.
Strict Rules for the Overnight Fast
The overnight fast, typically lasting 8 to 14 hours, is the most regulated part of preparation and establishes a true baseline blood sugar level. For most glucose tests, you must fast completely during this window, meaning zero caloric intake. The hours spent asleep count toward this required fasting time, making an early morning appointment often the most convenient option.
Consuming anything other than plain water invalidates the test results because it raises blood glucose or alters the body’s hormonal response. You must abstain from all food, including gum, mints, and hard candy, even if they are sugar-free, as they can prompt an insulin release. Beverages like coffee, tea, juice, and soda are forbidden, including black coffee, as caffeine may affect glucose levels and cause dehydration, making blood drawing more difficult.
You must also avoid smoking or vaping during the fasting period and the test itself, as nicotine can elevate blood sugar levels. If you take medications, discuss them with your healthcare provider beforehand, as some drugs can interfere with results. Typically, only plain water and approved non-caloric medications are permitted during the fast, and drinking water is encouraged to ensure hydration.
What to Eat Immediately After the Test
After the extended fast and the glucose challenge, it is common to feel hungry, lightheaded, or nauseous, especially if blood sugar has spiked and dropped rapidly. Your first meal should be balanced and nutrient-dense to stabilize blood sugar levels and replenish energy stores. Avoid immediately consuming large amounts of simple sugars, like candy or a sugary drink, as this can exacerbate a sugar crash and lead to further discomfort.
A recovery meal should contain a combination of protein, complex carbohydrates, and fiber. Protein and fiber are helpful because they slow the absorption of glucose into the bloodstream, preventing a sharp spike and subsequent crash. Good examples include eggs with whole-wheat toast, a salad with lean protein, or yogurt with berries and nuts.
Prior planning is helpful, so having a meal packed and ready to eat immediately after the final blood draw is a good strategy. Focus on rehydrating, as prolonged fasting can lead to mild dehydration. Choose water over caffeinated drinks until you feel fully recovered.