Low-Carb Eating and Blood Sugar: What the Evidence Shows

Evidence: Moderate

“Cut back on carbs” is some of the most common advice people with high blood sugar hear, but a low-carb diet is rarely explained well. How low is low? Does it actually work? And is it safe? Here’s what the research shows, and how to start in a way you can stick with.

What a low-carb diet actually means

There’s no single low-carb diet. Instead, researchers usually group them like this, based on a 2,000-calorie day:

Approach	Carbs per day	What it looks like
Moderate-carb	About 130–225 g (26–45% of calories)	Smaller portions of bread, rice and pasta; no sugary drinks
Low-carb	Under 130 g (under 26% of calories)	Mostly vegetables, protein, healthy fats; starches in small amounts
Very low-carb (keto)	Under about 50 g (under 10% of calories)	Almost no grains, starchy vegetables, fruit or sugar

For comparison, a typical American diet contains roughly 45–55% of calories from carbohydrate. In practice, most of the benefit of a low-carb diet for blood sugar comes from cutting the fastest-digesting carbs: sugary drinks, sweets, white bread and other refined grains.

Why carbs affect blood sugar most

First, carbohydrates are broken down into glucose, which goes straight into your bloodstream. By contrast, protein and fat have a much smaller effect on blood sugar. However, in type 2 diabetes and prediabetes, the body doesn’t respond well to insulin, so glucose from a carb-heavy meal stays in the blood longer. As a result, eating fewer carbs, and choosing ones that come with fiber, means smaller blood sugar spikes and less demand for insulin.

What the 2026 diabetes guidelines say

The American Diabetes Association’s Standards of Care in Diabetes—2026 recommend prescribing an evidence-based eating pattern, such as Mediterranean or low-carbohydrate, to people with prediabetes to prevent type 2 diabetes. For adults at high risk who have overweight or obesity, the guidelines also recommend a diabetes prevention program aiming for 5–7% weight loss and at least 150 minutes of moderate activity a week.

That’s a meaningful shift, because older advice often centered on regular, carbohydrate-based meals. Today, a low-carb diet sits alongside Mediterranean eating as one of the best-supported options, and the guidelines stress choosing a pattern you can sustain.

What results to expect from a low-carb diet

The best summary of the research comes from a 2021 review in the BMJ that pooled 23 randomized trials of low- and very-low-carb diets in adults with type 2 diabetes. After six months:

• More people reached non-diabetic blood sugar levels. 57% of people on low-carb diets had an HbA1c below 6.5%, compared with 31% on comparison diets (mostly low-fat). When people who still needed medication were excluded, the difference was much smaller.
• Weight came down more, by about 3.5 kg (7.6 lb) more than on comparison diets.
• Triglycerides and insulin sensitivity improved, and many people were able to reduce their medication.

However, there’s an important catch: most of the advantage faded by 12 months, largely because people found it hard to keep eating that way. At 12 months, the researchers also saw a possible worsening in LDL (“bad”) cholesterol and quality of life, though these changes weren’t statistically certain. Overall, they rated the evidence as moderate to low certainty. Interestingly, very-low-carb diets didn’t beat moderately low-carb ones for weight loss once adherence was taken into account. In other words, the version you can stick with matters more than how strict it is.

Who should be careful with a low-carb diet

Talk to your doctor before cutting carbs if you take diabetes medication. Lowering carbohydrates quickly can cause low blood sugar if you use insulin or certain other glucose-lowering drugs, and very-low-carb eating should be avoided with SGLT2 inhibitors (such as empagliflozin or dapagliflozin) because of the risk of ketoacidosis. Your doses may need adjusting, sometimes within days.

Very-low-carb plans also aren’t recommended during pregnancy or breastfeeding, for children, for people with kidney disease, or for anyone with a history of disordered eating.

Also, if you have high LDL cholesterol or heart disease, ask your doctor about checking your cholesterol a few months after you start. Choosing fish, poultry, eggs, olive oil, nuts and avocado over large amounts of butter and processed meat helps.

How to start a low-carb diet

• Drop sugary drinks first. Soda, juice and sweetened coffee drinks are the single biggest source of added sugar. See Does Sugar Cause Diabetes?
• Fill half your plate with non-starchy vegetables, such as leafy greens, broccoli, peppers, zucchini or cauliflower.
• Make protein the anchor of each meal, such as eggs, fish, chicken, Greek yogurt, tofu or beans.
• Shrink, don’t ban, the starches. A fist-sized portion of whole grains or potatoes is a reasonable starting point for moderate low-carb.
• Read labels for added sugar. Our 60-second label guide shows you where to look.
• Track how you respond. If you have diabetes, a glucose meter or continuous glucose monitor (CGM) can show which meals spike your blood sugar.

The bottom line

A low-carb diet is a well-supported way to improve blood sugar, and it’s now one of the American Diabetes Association’s top-recommended patterns for preventing type 2 diabetes. Above all, the biggest wins come from cutting sugary drinks and refined starches, not from extreme rules. Pick a level you can keep up, and loop in your doctor if you take medication. Comparing your options? See Keto vs. Low-Carb vs. Mediterranean vs. Carnivore.

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This article is for general education and isn’t medical advice. See our medical disclaimer.

Sources

1. American Diabetes Association. Prevention or Delay of Diabetes and Associated Comorbidities: Standards of Care in Diabetes—2026. Diabetes Care. 2026;49(Suppl 1):S50–S60. PMC.
2. American Diabetes Association. Standards of Care in Diabetes—2026 (press release). diabetes.org.
3. Goldenberg JZ, et al. Efficacy and safety of low and very low carbohydrate diets for type 2 diabetes remission: systematic review and meta-analysis. BMJ. 2021;372:m4743. PMC.
4. Gardner CD, et al. Effect of a ketogenic diet versus Mediterranean diet on glycated hemoglobin in individuals with prediabetes and type 2 diabetes: the Keto-Med randomized crossover trial. Am J Clin Nutr. 2022;116(3):640–652. PubMed.