If your post-smoothie energy boost is quickly followed by a frustrating crash, you're not alone. Many foods marketed as nutritious can cause rapid spikes in blood glucose, leading to fatigue, mood swings, and cravings, according to dietitians.
The Science Behind the Sugar Crash
Maintaining steady blood sugar levels is crucial for sustained energy and focus throughout the day. Registered dietitian nutritionist Chelsey Amer explains the rollercoaster effect: "If your blood sugar spikes too quickly, you can get a 'sugar high,' followed by a sugar crash, which can lead to fatigue, moodiness and cravings."
Biochemist and author Jessie Inchauspé, founder of Glucose Goddess, describes the process: "When we give our body too much glucose, then blood sugar increases too fast, and then after a spike, there's a crash when you feel tired, hungry, craving more sugar or caffeine."
Long-term management is equally important. RDN Toby Smithson of the American Diabetes Association compares it to Goldilocks: levels must be just right. Prolonged periods out of range not only make you feel unwell but can increase the risk of diabetes complications over time.
Understanding GI and GL: The Key Metrics
Two critical terms dominate the blood sugar conversation: Glycemic Index (GI) and Glycemic Load (GL). RDN Sharon Palmer, author of "The Plant-Powered Plan to Beat Diabetes," clarifies the difference.
"The GI is a scale that ranks carb-containing foods and beverages by how quickly they raise blood sugar levels after eating," Palmer said. High GI foods (70-100) like candy, cookies, and white bread raise levels faster than low GI foods (55 or less) like most vegetables, fruits, and nuts.
However, Glycemic Load provides a more complete picture. "It's a more accurate reflection of how foods impact glucose, as it takes into account the amount of carbs in a food, too," Palmer notes. For example, watermelon has a high GI but a low GL because a typical serving contains relatively few carbohydrates. According to the University of Sydney's Glycemic Index Research Service, low-GL foods have less than 10 grams of glucose per serving.
Unexpected Blood Sugar Spikers in Your Diet
Nutrition experts warn that several common "health" foods can cause significant glucose spikes when consumed alone. Inchauspé suggests re-categorizing these items as "pleasurable" rather than inherently healthy. "It's fine to eat them, but we need to put them back where they belong: in the dessert category," she advises.
Here are the top culprits identified by dietitians:
1. Plant-Based Milks and Sweetened Beverages: "Many plant-based milks, especially oat and rice milk, contain naturally occurring or added sugars that raise blood sugar more than people realize," said registered dietitian Marissa Karp. A splash in coffee may be fine, but a full latte could spike levels. Oat milk has a GI of 49. RDN Dawn Jackson Blatner also flags sports drinks (65 to 100 GI) and some kombuchas.
2. Protein and Snack Bars: "They can be a sneaky source of blood sugar spikes, especially if there are hidden sugars," Amer cautions. Many lack sufficient protein or a balanced macronutrient profile to prevent a spike. Studies show snack bar GIs range from 43 to 56, with PowerBars in the 56 to 58 range.
3. Oatmeal: Its impact varies greatly by type. "While many brands contain some fiber, oatmeal alone tends to raise blood glucose levels more than you would expect," Smithson says. Steel-cut oats have a GI of 42, rolled oats 55, and instant oats can spike to 74. Smithson recommends choosing high-fiber, no-sugar-added varieties and pairing them with a lean protein like eggs or nuts.
4. Sushi: "It can often be an unexpected cause of blood sugar spikes," Amer notes. While basic sushi has a GI of 45 to 55, the type, sauces, and portion size matter greatly. Sushi rice alone has a high GI of 85 due to its short-grain, sticky nature, which makes it digest quickly.
5. Sauces, Dressings, and Smoothies: Hidden sugars are the enemy. Salad dressings can range from 55 to 69 on the GI scale. Smithson advises checking nutrition labels, aiming for 1 gram of carbohydrate or less per 2-tablespoon serving. Smoothies and protein shakes, even with healthy ingredients, concentrate carbohydrates. Recent research notes smoothie GIs ranging from 35 to 40, but commercial versions are often not in major GI databases.
6. Rice Cakes: Often chosen for being light and low-calorie, they have a GI of about 85, classifying them as high GI and capable of causing a quick glucose spike, Karp explains.
Simple Strategies for Stable Energy
Experts offer straightforward methods to avoid these spikes and maintain balanced energy.
First, become a label detective. "Check both the total and added sugar content," Karp advises, noting that granola, yogurt, and smoothies can be surprisingly high. The American Heart Association recommends limiting added sugars to no more than 6% of daily calories—about 6 teaspoons for women and 9 for men.
Also, look for protein, fat, or fiber in a food, as these nutrients slow sugar absorption. RDN Jerlyn Jones suggests avoiding foods where added sugars exceed 20% of the product's total weight, a marker for "high sugar" content.
One of the most effective tools is surprisingly simple: movement. "One of the best things you can do to avoid a blood sugar spike after eating is go for a walk or do some light activity," Amer says. Jones agrees, advising clients to move for 10 to 15 minutes after eating to aid digestion and lower blood sugar.
The Long-Term Toll of Frequent Spikes
Beyond the immediate crash, frequent glucose surges pose serious long-term health risks. "In the short term, occasional spikes make you tired, unfocused and snacky," Blatner said. "But over time, frequent glucose surges may contribute to inflammation, weight gain and potentially insulin resistance."
Karp warns that repeated surges stress your body's systems, disrupting consistent energy supply, mood, focus, and metabolism. Jones adds that research links the pro-inflammatory effects of blood sugar spikes to free radical damage, associated with cancer, heart disease, and aging.
"As a general rule, we want to eat in a way that helps us feel good and energized, not sluggish or sleepy," Karp concludes. "When you keep your blood sugar steady, you feel more energized and focused."
