Nearly every food you eat converts to some form of sugar in your body. Carbohydrates break down almost entirely into glucose, your body’s primary fuel. But starches, fruits, dairy, and even protein all end up raising blood sugar to varying degrees. The differences come down to how fast the conversion happens and how much glucose each food ultimately delivers.
How Your Body Turns Food Into Sugar
Digestion begins converting starch to sugar before food even reaches your stomach. An enzyme in your saliva starts breaking large starch molecules into smaller fragments. Your pancreas releases more of the same enzyme to continue the job in your small intestine. These fragments are then split into individual glucose molecules, which pass through the intestinal wall and enter your bloodstream.
Simple sugars like table sugar (sucrose) skip some of those steps. Sucrose splits into one glucose molecule and one fructose molecule. Lactose, the sugar in milk, splits into glucose and galactose. The simpler the starting molecule, the faster it reaches your blood as usable sugar.
Starchy Foods: The Biggest Source
Starch is just a long chain of glucose molecules linked together. When you eat starchy foods, your digestive enzymes unzip those chains and release pure glucose. This makes starches the most efficient sugar-producing foods in your diet, even though they don’t taste sweet.
Potatoes are among the fastest converters. They contain about 20.4 grams of starch per 100 grams of raw potato, and cooked potatoes consistently score high on the glycemic index, often landing between 82 and 93 out of 100. That means the glucose hits your bloodstream quickly. White rice, white bread, and most refined grain products behave similarly because processing strips away fiber and other compounds that would otherwise slow digestion.
Not all starches convert at the same speed, though. Legumes like red kidney beans consistently produce the lowest blood sugar response among starchy foods. Red sorghum ranks lowest among grains. Both contain higher levels of resistant starch and plant compounds that interfere with the enzymes trying to break the starch apart. Your body still extracts glucose from these foods, but the process is slower and more gradual.
How Cooking Changes the Conversion
Raw potatoes contain a large amount of resistant starch that your enzymes can’t easily access. Cooking changes everything. When starch granules absorb water and heat, their internal structure breaks apart in a process called gelatinization. This makes the starch highly digestible, and the estimated glycemic index of conventionally cooked potatoes can climb above 90.
Cooling cooked starches reverses some of this effect. When cooked rice or potatoes cool in the refrigerator, some of the starch molecules re-link into tighter structures that resist digestion. Reheating doesn’t fully undo this change. So cold potato salad or day-old rice will convert to sugar more slowly than the same food served fresh and hot. The practical takeaway: cooking method matters. The more water and heat involved, the faster the starch in your food will become blood sugar.
Fruits and Fructose
Fruit contains a mix of glucose, fructose, and sucrose. Glucose enters your bloodstream directly. Fructose takes a detour through the liver, where about 41% of it gets converted into glucose within a few hours. Another quarter or so becomes lactate, which other tissues can use for energy. Less than 1% of fructose converts directly to fat, despite common fears about fruit sugar and weight gain.
Whole fruit delivers its sugars alongside fiber, water, and plant compounds that slow absorption. A whole apple raises blood sugar differently than apple juice, even though both contain similar types of sugar. Dried fruit and fruit juices concentrate the sugars while removing the fiber, making them behave more like refined sugar in your body.
Dairy Sugars
Milk, yogurt, and other dairy products contain lactose, a sugar made of one glucose molecule bonded to one galactose molecule. An enzyme in your small intestine splits lactose into these two simple sugars, both of which are then absorbed and used for energy. If you’re lactose intolerant, you produce less of this enzyme, so the lactose passes undigested into your colon, where bacteria ferment it and cause the familiar symptoms of gas and bloating.
Vegetables: A Wide Spectrum
All vegetables contain some carbohydrate, but the amounts vary enormously. Root vegetables sit at the higher end. Carrots and beets each contain roughly 9.5 to 9.6 grams of carbohydrate per 100 grams, while potatoes more than double that with their 20.4 grams of starch alone. Corn and peas fall somewhere in between.
Leafy greens and non-starchy vegetables like spinach, broccoli, and zucchini contain so little digestible carbohydrate that their contribution to blood sugar is negligible. The fiber they contain actually slows the absorption of sugar from other foods eaten at the same meal.
Hidden Sugars in Processed Foods
Many processed foods contain starch-derived sweeteners that convert to blood sugar as fast as, or faster than, table sugar. Corn syrup, maltodextrin, and dextrin are all made by breaking cornstarch into progressively smaller fragments. The degree of breakdown is measured by something called dextrose equivalence: a higher number means the starch has been chopped into smaller pieces closer to pure glucose.
Standard corn syrup has a dextrose equivalence of 42, meaning its solids behave as if they were 42% pure glucose in terms of chemical activity. High-conversion syrups reach 65 or higher. Maltodextrin, commonly added to sauces, dressings, protein powders, and snack foods, falls on the lower end of that scale but still digests rapidly. These ingredients don’t always taste sweet, which is why they’re easy to overlook on a label. Checking ingredient lists for terms like maltodextrin, corn syrup solids, dextrose, and rice syrup can reveal sugar sources that the front of the package doesn’t advertise.
Protein and Fat: Slower, Smaller Contributions
Protein can convert to glucose, but the process is far less efficient than carbohydrate digestion. Your body can produce roughly 60 grams of glucose from 100 grams of protein through a liver process called gluconeogenesis. This pathway ramps up primarily when carbohydrate intake is very low, such as during fasting or on a ketogenic diet. Under normal eating conditions, protein’s contribution to blood sugar is modest and slow.
Fat does not convert to glucose in any meaningful amount. It provides energy through a completely separate pathway. However, fat slows stomach emptying, which means adding fat to a carbohydrate-rich meal can blunt the speed of the resulting blood sugar spike, even though the total glucose produced stays the same.
What Slows the Conversion
Several factors determine how quickly any food becomes blood sugar. Fiber is the most important. It physically slows enzyme access to starch and sugar molecules. Resistant starch acts similarly, passing through digestion without being broken down. Foods higher in plant compounds, particularly those found in beans, lentils, and whole grains like sorghum, also slow the process.
Eating protein or fat alongside carbohydrates delays glucose absorption. The physical form of food matters too: whole grains convert more slowly than flour made from the same grain, because the intact kernel takes longer to break apart. Even something as simple as choosing al dente pasta over soft-cooked pasta reduces the speed of sugar conversion, because firmer starch granules resist enzyme activity longer.