Why you should reduce sugar and refined carbs—and the smart way to do it
Added sugars and white flour drive high blood sugar and all its negative consequences. Here’s what they do to your body, and the best ways to reduce your intake.
From white bread and pasta to cookies and crackers, refined carbs—mostly wheat flour and sugar—dominate the American diet. And that’s at least partly by design. Food manufacturers strip away fiber, vitamins, and minerals during processing, then add sugars and additives to the refined flours to enhance the flavor and extend the shelf life of packaged foods. In turn, we tend to eat more: Sugar appears to activate the same brain reward pathways as some drugs and has been linked to compulsive eating behaviors.
These processed carbs are a major contributor to the metabolic health crisis. Countless studies have linked refined carbs to obesity, metabolic dysregulation, chronic inflammation, oxidative stress, and chronic disease.
Most nutrition and health experts agree we’re eating too many refined carbs, which generally have little to no health benefit. But carbohydrates in general are part of a healthy diet—they power our brain, help our muscles recover, keep digestion humming, and promote higher-quality sleep.
How many refined carbs your body can tolerate without inducing disease, and how many carbs you need, are areas of debate. However, at the levels consumed in the standard American diet—about 42 percent of the average American's daily calories—refined carbs are wreaking havoc on metabolic health and exacerbating existing health problems.
This article explores the scientific rationale behind reducing refined carbs and added sugars, focusing on their impact on blood sugar, inflammation, and overall health.
The facts about sugars
The average American consumes about 17 teaspoons (71.4 grams) of added sugars daily, translating to roughly 277 calories, or almost 14 percent of a 2,000-calorie diet. The main source? Ultra-processed foods. Nearly 90 percent of added sugars consumed in the United States come from these highly palatable, packaged foods, which contain five times more added sugars than minimally processed foods do. Moreover, many of these processed foods masquerade as health foods, like bars, store-bought smoothies, energy or hydration drinks, flavored yogurts, sauces, and dressings, so we may not realize how much we eat.
Most people agree we should cut back, though there’s disagreement on how much sugar is okay. The Dietary Guidelines for Americans recommend that less than 10 percent of daily calories come from added sugars, meaning someone on a 2,000-calorie diet should eat, at the very most, 50 grams of added sugars per day. The American Heart Association recommends that women consume no more than 25 grams and men consume no more than 36 grams daily. But other doctors, including many Levels advisors, believe we should try not to consume any added sugars, since they can harm our health while providing no benefit.
What are refined carbs and sugars, and how do they affect the body?
Refined carbohydrates include most forms of sugar and flour. Along with protein and fat, carbohydrates are one of the three macronutrients the human body needs to function. During digestion, most carbohydrates are broken down into glucose, which is absorbed into the bloodstream and serves as the primary fuel source for our cells.
Different types of carbs enter the bloodstream at different rates, leading to varying metabolic effects. “Complex” carbohydrates (starches and fiber) consist of multiple linked sugar molecules that must be broken down before they can be absorbed. This leads to a slower rise in blood glucose levels and fewer adverse metabolic effects. “Simple” carbohydrates (sugars and refined flours) contain one or two sugar molecules, leading to rapid entry into the bloodstream, sharp rises in blood sugar and insulin, and the potentially harmful short and long-term outcomes that follow from high glucose and insulin resistance.
Table sugar is made of sucrose, a compound that's 50% glucose and 50% fructose—so in 100 grams of sugar, about half is glucose in a form your body absorbs quickly. White flour, on the other hand, is mostly starch, which is made of long chains of glucose molecules, and because of the milling, it is absorbed quickly. While it doesn’t contain free glucose, it’s broken down into glucose during digestion. In short, sugar delivers more glucose in a fast-absorbing form, while flour has slightly less carbohydrate per unit of weight but still quickly becomes glucose in the body. Both can spike blood sugar, though sugar tends to act more rapidly.
Flour
Flour has undergone processing—such as grinding or milling—that reduces particle size and removes the bran (a source of fiber, B vitamins, and antioxidants) and germ (which provides healthy fats, minerals, and more B vitamins). Refined carbohydrates are found in foods like white bread, pasta, white rice, pastries, breakfast cereals, and crackers.
Without fiber to slow digestion and absorption, consuming refined carbs leads to high blood sugar spikes, which can have long-term consequences for metabolic health. A study in the American Journal of Clinical Nutrition found a positive correlation between higher intakes of refined carbohydrates and the development of Type 2 diabetes, and a negative correlation between higher intakes of fiber and Type 2 diabetes. Intake of refined carbohydrates is also associated with obesity and cardiovascular disease.
Added sugars
Added sugars are incorporated into food during manufacturing or preparation. At least 61 different sugar names can be listed on food labels, including sucrose, barley malt, rice syrup, dextrose, maltose, and high-fructose corn syrup.
Higher consumption of these sweeteners has been linked to insulin resistance. When your body breaks down any type of carbohydrate, the glucose enters the bloodstream, and the pancreas secretes the hormone insulin. Insulin signals to the cells to take up that glucose for energy or storage, which allows blood glucose levels to return to normal levels. When too much glucose is entering the bloodstream regularly, the pancreas has to produce more and more insulin. Over time, the body's cells become less responsive to the hormone, a condition called insulin resistance, which can develop into Type 2 diabetes.
Epidemiologic studies have shown that people who consume higher amounts of added sugars tend to gain more weight and have a higher risk of obesity, Type 2 diabetes, poor cholesterol, hypertension, and cardiovascular disease (CVD).
Natural sugars
Despite clever marketing, “natural” sugars such as brown sugar, agave nectar, beet sugar, brown rice syrup, cane sugar, and honey are not healthier for you. Some may contain trace levels of minerals, but the amounts in a typical serving are insignificant to benefit your health. Your body breaks down these sugars in the same manner as white table sugar, triggering the same rise in blood glucose and insulin, and the same metabolic effects when eaten in large quantities.
Health impacts of sugar and refined carbs
Consuming refined carbs and sugars in excess can have short- and long-term health consequences.
Energy and mental clarity
In the short term, carbs can be detrimental to our energy and mental clarity. For example, a meta-analysis of 31 studies showed that eating carbohydrates lowered alertness within 60 minutes of consumption and increased fatigue within 30 minutes, with exact results varying depending on the type and dose of carbohydrates administered. The mechanism is unclear, but high-carbohydrate foods appear to trigger increased synthesis and uptake of key neurotransmitters (like serotonin and tryptophan) in the brain. Meals that cause a sharp blood sugar spike can also lead to reactive hypoglycemia—a blood sugar crash—which means less glucose is available to the brain, potentially leading to fatigue and brain fog.
Insulin resistance and diabetes
Refined carbs and sugars are considered high-glycemic index foods because eating them causes a steep spike in blood glucose. This stresses the pancreas to produce more insulin and contributes to insulin resistance. One meta-analysis of 16 cohort studies found that each additional daily serving of sugar-sweetened beverages increases risk for Type 2 diabetes by 27 percent.
Cardiovascular risk
Studies show that an increased intake of refined carbohydrates contributes to worse cholesterol levels, including increased triglycerides and small LDL particles (which are more likely to form plaque buildup) and reduced HDL cholesterol (the type that helps remove LDL from the bloodstream). People with Type 2 diabetes are between two and four times more likely to have cardiovascular problems such as heart disease and stroke. In a 16-year study published in JAMA Internal Medicine, consuming 17 to 21 percent of daily calories from added sugars led to a 38 percent higher risk of dying from cardiovascular disease compared with consuming 8 percent of calories from added sugars.
Weight gain and obesity
Studies suggest sugar, especially added sugars, can disrupt appetite regulation, leading to weight gain. One small study on 16 women found that a higher-sugar breakfast led to increased hunger and food intake at lunchtime. Meanwhile, a prospective cohort study published in 2023 found that every 100-gram daily increase in starch or added sugar daily was associated with an extra 3.3 pounds (starch) or 2 pounds (sugar) gained over four years. But every 10-gram increase in daily fiber was associated with 1.7 pounds less weight gain. Some researchers believe that high insulin—the result of consuming high carbohydrates—is the primary driver behind obesity, since insulin blocks fat burning and promotes fat storage.
Inflammation and oxidative stress
Excessive intake of refined carbohydrates and sugars can exacerbate chronic, low-grade inflammation and oxidative stress—both drivers of disease—through multiple mechanisms:
- Both blood sugar spikes and chronically elevated blood glucose can ramp up the production of reactive oxygen species (ROS) in cells. While we need ROS as part of a healthy immune response, too much ROS and too few antioxidants in our cells leads to oxidative stress, inflammation, and metabolic dysfunction.
- Sugar consumption promotes the formation of advanced glycation end products (AGEs), which also trigger oxidative stress. Over time, the accumulation of these compounds is linked to the development of age-related diseases, such as neurodegenerative diseases, atherosclerosis, and chronic inflammatory diseases.
- Refined carbs and sugars may directly increase inflammatory markers like nuclear factor-kB activation and binding.
- High-sugar intake may alter the gut microbiome, increasing pro-inflammatory proteobacteria while decreasing bacteroidetes. Bacteroidetes help reinforce the gut barrier and neutralize toxins in the gut, so their reduction may increase gut permeability and contribute to systemic inflammation.
Chronic disease
Inflammation, metabolic dysfunction, and oxidative stress are the underlying factors in many illnesses. Research shows that sugars and other refined carbohydrates are principal factors in the growing prevalence of other diseases. For example, drinking high amounts of fructose—the sugar commonly added to soft drinks and fruit juice—has been linked to non-alcoholic fatty liver disease. The liver metabolizes fructose, but it can only handle so much. If you consume more than this, it can’t process it all and converts the excess to fat. And a study on older adults showed participants in the highest sugar intake group had a 110 percent higher risk of developing dementia compared to those in the lowest sugar intake group.
How to choose healthier carbohydrates
Carbohydrate quality is essential. Cutting out all carbohydrates would mean missing fiber, vitamins, minerals, and other beneficial nutrients. Instead, focus on consuming whole or minimally processed carbohydrates high in fiber, and reducing your intake of refined carbs and added sugars. In the prospective cohort study mentioned earlier, fiber and carbohydrate intake from fruit, whole grains, and non-starchy vegetables were inversely associated with weight gain.
To help, Levels experts advise thinking about three categories of carbs based on how much and how often to eat them:
- Carbs to eat often = non-starchy veggies (spinach, kale, broccoli, cauliflower, asparagus, peppers, mushrooms, tomatoes) and low-sugar fruits (berries, kiwis, grapefruit, watermelon)
- Carbs to eat in moderation = starchy veggies (potatoes, beans, corn, sweet potatoes, lentils), higher-sugar fruits (mangos, cherries, grapes, apples, bananas), and whole grains (wheat, rice, oats, barley, quinoa)
- Carbs to avoid = sodas, juices, sweeteners, flour-based products, and other highly-processed foods
Prioritizing whole, unprocessed foods and minimizing your intake of refined carbohydrates will support your short- and long-term metabolic health and reduce your risk of chronic disease.
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