A ketogenic diet can be effective for prediabetes, and the evidence is stronger than for many other dietary approaches. By drastically cutting carbohydrates, keto directly reduces the blood sugar spikes that define prediabetes, and measurable improvements in HbA1c (a key marker of long-term blood sugar control) can appear in as few as three weeks. But the diet comes with real trade-offs in adherence, nutrient gaps, and cholesterol changes that are worth understanding before you commit.
Why Keto Works for Blood Sugar Control
Prediabetes means your body is struggling to move sugar out of your blood efficiently. Your cells have become partially resistant to insulin, so your pancreas pumps out more and more of it to compensate. Over time, this cycle wears down the system and edges you toward type 2 diabetes.
A keto diet interrupts that cycle at multiple points. When you eat very few carbohydrates (typically 20 to 50 grams per day), less sugar enters your bloodstream after meals, which means your pancreas doesn’t need to produce as much insulin. Lower insulin levels, in turn, give your cells a chance to regain sensitivity to it. At the same time, reduced carbohydrate intake forces your liver to burn through its stored sugar and shift toward breaking down fat for fuel. This clears fat buildup in the liver, which is one of the key drivers of insulin resistance in prediabetes.
The ketone bodies your liver produces during this process also play an active role. They trigger fat cells to release adiponectin, a hormone that improves insulin sensitivity throughout the body. Ketones also reduce inflammation by blocking a specific inflammatory pathway that contributes to insulin resistance. So the benefits aren’t purely about avoiding carbs; the state of ketosis itself appears to help your metabolism recover.
How Quickly Blood Sugar Improves
One of the most encouraging findings is the speed. HbA1c, which reflects your average blood sugar over the past two to three months, begins dropping within the first three weeks on a ketogenic diet. Most of the improvement happens in the first 70 days, though levels continue to decline for months afterward.
In a 12-month trial that included people with prediabetes and type 2 diabetes, those on a very low-carb ketogenic diet saw their HbA1c drop from 6.6% to 6.1%, compared to a smaller drop (6.9% to 6.7%) in a group following a moderate-carb, calorie-restricted, low-fat diet. Longer studies have shown reductions from 7.5% to 5.9% over 15 months. For context, prediabetes is defined as an HbA1c between 5.7% and 6.4%, so these reductions can potentially bring people back into the normal range.
Notably, these blood sugar improvements don’t depend entirely on weight loss. Case reports have documented significant HbA1c reductions in people who lost less than 5% of their body weight, suggesting the metabolic effects of carbohydrate restriction and ketosis contribute independently of shedding pounds.
What Happens to Cholesterol
This is where the picture gets more complicated. In people with prediabetes, ketosis tends to improve some cardiovascular markers while worsening others. Triglycerides, a type of blood fat strongly linked to heart disease risk, drop substantially. One study found people with enhanced ketone production had triglycerides around 110 mg/dL compared to 244 mg/dL in those without. HDL cholesterol (the protective kind) also tends to rise.
LDL cholesterol, however, often increases. In the same study of prediabetic and newly diagnosed diabetic patients, those with higher ketone levels had LDL around 133 mg/dL versus 108 mg/dL in the comparison group. The reassuring detail is that the LDL particles in the ketosis group were larger in size, and larger LDL particles are generally considered less harmful than small, dense ones. Levels of oxidized LDL, the form most directly linked to artery damage, were comparable between groups. Still, if you already have elevated LDL or a history of heart disease, this is something to monitor with blood work.
What You Actually Eat
The version of keto most studied for blood sugar management limits carbohydrates to 20 to 50 grams per day, with about 60% to 75% of calories from fat and moderate protein (roughly 1 to 1.5 grams per kilogram of your ideal body weight daily). In practical terms, 20 to 50 grams of carbs is about one or two servings of non-starchy vegetables and a small handful of berries. Bread, rice, pasta, potatoes, most fruit, and anything with added sugar are essentially off the table.
Fat sources matter. Clinical guidelines for this approach emphasize unsaturated fats: olive oil, nuts, avocados, and fatty fish like salmon. A keto diet built around bacon, butter, and cheese will hit the macronutrient targets but may not deliver the same metabolic benefits and will likely worsen your lipid profile.
Nutrient Gaps to Watch For
Cutting out most fruits, many vegetables, and all whole grains removes significant sources of vitamins and minerals. Low-carb diets are commonly low in thiamin, folate, vitamins A, E, and B6, as well as calcium, magnesium, iron, and potassium. Even carefully planned ketogenic diets tend to fall short on vitamin K, certain essential fatty acids, and most water-soluble vitamins.
Without a daily multivitamin, people on long-term keto are at risk of outright nutritional deficiencies. Fiber intake also drops dramatically, which can cause constipation and may affect gut health over time. If you plan to stay on keto for more than a few months, supplementation isn’t optional.
The Adherence Problem
Keto is one of the harder diets to stick with long-term. In a trial comparing low-carb, low-fat, and Mediterranean diets over two years, adherence to the low-carb diet was 78%, compared to 90% for low-fat and 85% for Mediterranean. That gap matters because the blood sugar benefits of keto only last as long as you maintain the carbohydrate restriction. If you cycle on and off, you may see your numbers bounce back.
This is worth being honest with yourself about. A less restrictive approach, like a Mediterranean diet, produces meaningful HbA1c improvements as well. Some head-to-head trials have shown comparable 1% HbA1c reductions after a year on either a very low-carb or a higher-carb, low-fat diet. The best diet for prediabetes is ultimately the one you can sustain, and for many people, a moderate approach they follow consistently will outperform a strict one they abandon after four months.
Who Should Be Cautious
If you have any degree of kidney disease, keto requires careful consideration. The diet tends to be higher in protein than a standard diet, and impaired kidneys have difficulty handling both the extra protein and the acid load that ketosis produces. When kidney filtration drops below a certain threshold, the risk of metabolic acidosis increases. If you’re taking blood pressure medications that affect kidney function or newer diabetes drugs, the interactions with a ketogenic diet aren’t well studied.
People with a history of kidney stones should also be aware that keto may increase stone risk, particularly for those with a genetic predisposition. And anyone on blood sugar-lowering medications needs to adjust doses when starting keto, because the combined effect can push blood sugar dangerously low. This is one situation where coordinating with a healthcare provider before making the switch is genuinely important, not just a standard disclaimer.
Keto vs. Moderate Low-Carb for Prediabetes
You don’t necessarily need to go full keto to get meaningful results. The sharpest blood sugar improvements happen when you first cut carbohydrates significantly, and going from a typical 250 to 300 grams per day down to even 100 grams captures a large portion of the benefit. Full ketosis (under 50 grams) adds the additional effects of ketone production, including the anti-inflammatory and insulin-sensitizing properties described above, but the marginal gain over moderate carb restriction may not justify the difficulty for everyone.
If you’re motivated and can see yourself eating this way for at least six months to a year, a ketogenic approach offers one of the fastest and most effective dietary paths to reversing prediabetes. If you know strict restriction tends to lead to burnout, a more moderate low-carb or Mediterranean-style diet will still move your numbers in the right direction and may be easier to maintain for the years it takes to keep prediabetes from progressing.