A low-carb diet is any eating pattern that keeps daily carbohydrate intake below about 130 grams, which is the Recommended Dietary Allowance set for adults. The typical American diet includes more than 250 grams of carbs per day, so going low-carb means roughly cutting that in half or more. Below 50 grams per day is generally considered “very low carb,” and that’s where ketogenic diets live.
There’s no single official definition, but most medical professionals use the 130-gram line as the cutoff because it’s the point where you’re eating less than what’s broadly recommended. From there, the spectrum ranges from moderate restriction (around 100 to 130 grams) all the way down to strict ketogenic levels (under 50 grams).
How It Works in Your Body
Carbohydrates are your body’s preferred quick fuel. When you eat them, your blood sugar rises and your pancreas releases insulin to shuttle that glucose into cells. Insulin also tells your fat cells to hold onto their stores and stop releasing fatty acids. In other words, when insulin is high, your body is in storage mode.
When you eat fewer carbs, insulin levels drop. With less insulin circulating, your body shifts toward burning fat for energy instead of storing it. This is the core idea behind why people lose weight on low-carb diets: lower insulin means your fat cells release more fatty acids, and your muscles and organs burn those fatty acids as fuel. If carbs drop low enough (below roughly 50 grams per day), your liver starts converting fatty acids into ketones, which your brain can use in place of glucose. That metabolic state is called ketosis.
Low Carb vs. Keto vs. Other Diets
Not all low-carb diets are the same. A standard ketogenic diet typically gets 70 to 80 percent of its calories from fat, only 5 to 10 percent from carbohydrates, and 10 to 20 percent from protein. The goal is to stay in ketosis, which requires strict carb limits, usually under 20 to 50 grams per day.
Other popular low-carb approaches take a different path. Diets like Paleo, South Beach, and Dukan are high in protein but moderate in fat. They restrict carbs enough to stay below the 130-gram threshold but don’t aim for ketosis. For many people, these feel easier to maintain because the rules are less rigid and the food variety is broader. Choosing between them depends largely on how much restriction you’re willing to sustain and what metabolic goals you’re after.
What You Actually Eat
The backbone of any low-carb diet is protein and fat. Chicken, turkey, beef, pork, lamb, and fish are all naturally carb-free. Eggs contain less than half a gram of carbs each. Fats and oils, including olive oil, butter, and avocado, add virtually zero carbs. These foods form the base of most meals.
On top of that, you fill in with non-starchy vegetables like leafy greens, broccoli, cauliflower, zucchini, and peppers. Nuts, seeds, cheese, and plain yogurt are common additions. What you cut back on or eliminate entirely: bread, pasta, rice, cereal, potatoes, sugary drinks, baked goods, and most fruit (though berries in small amounts are a common exception).
Counting Net Carbs
Many people following a low-carb diet count “net carbs” rather than total carbs. The idea is that fiber passes through your digestive system without raising blood sugar, so it shouldn’t count against your daily limit. The basic formula is: total carbohydrates minus fiber equals net carbs.
Sugar alcohols (commonly found in sugar-free products) fall somewhere in between. They’re partially absorbed, so the standard practice recommended by diabetes educators at UCSF is to subtract half the grams of sugar alcohol from the total carbs. If a protein bar has 29 grams of total carbs and 9 grams of sugar alcohol, you’d count it as about 20 grams of net carbs.
Benefits for Blood Sugar and Metabolic Health
The strongest evidence for low-carb eating involves blood sugar control and related metabolic markers. In a meta-analysis of randomized controlled trials involving over 700 people with type 2 diabetes, those on low-carb diets saw their A1c levels drop by an average of 0.44 percent compared to higher-carb diets. That’s a meaningful reduction, roughly equivalent to what some medications achieve.
Triglycerides, a type of blood fat linked to heart disease risk, also dropped significantly. HDL cholesterol (the protective kind) tends to rise on low-carb diets. LDL cholesterol shows a neutral to small increase in most studies, which is worth monitoring but hasn’t translated into higher cardiovascular event rates in the trials conducted so far.
For people with type 2 diabetes specifically, the results can be dramatic. Prospective trials using very low-carb approaches (under 14 percent of calories from carbohydrates) have shown significant reductions in both insulin and oral diabetes medications, with high adherence rates at 12 months. Some participants saw their A1c drop below the diagnostic threshold for diabetes entirely. A joint consensus statement from the American Diabetes Association, the Endocrine Society, and several international organizations defines diabetes remission as an A1c below 6.5 percent for at least three months without glucose-lowering medications.
The Adjustment Period
The first week or two of a low-carb diet can feel rough. Common symptoms include fatigue, headaches, brain fog, irritability, and muscle cramps. This cluster of side effects is sometimes called “keto flu,” though it can happen at any level of meaningful carb restriction, not just on a ketogenic diet. It occurs because your body is shifting from relying on glucose to burning more fat, and that transition isn’t instant.
Blood ketone levels typically reach a new steady state within about a week, but subjective energy and exercise performance take longer to recover. In controlled studies, the ability to do vigorous exercise took several weeks to a few months to stabilize after starting a ketogenic diet. Uric acid levels also spike during the first week (they can temporarily double) before gradually declining, which is why some people experience joint aches early on. Full metabolic adaptation, where all the body’s parallel responses have settled into their new patterns, is measured in months rather than days.
Staying hydrated and keeping up electrolyte intake (sodium, potassium, magnesium) helps ease these symptoms considerably. Most people report feeling significantly better by weeks two to four.
What Happens Over the Long Term
The metabolic improvements from low-carb diets are most pronounced in the first six months. A recent meta-analysis found that at six months, low-carb diets achieved higher rates of type 2 diabetes remission and greater improvements in weight loss, fasting insulin sensitivity, and triglycerides compared to control diets. By 12 months, those differences had narrowed, though they didn’t disappear.
This pattern likely reflects both biological adaptation and the practical reality that strict diets are harder to maintain over time. The degree of carb restriction that works best for you depends on your health goals, your starting metabolic health, and what you can realistically sustain. Many people find that a moderate low-carb approach (80 to 130 grams per day) is easier to keep up indefinitely than a strict ketogenic diet, while still delivering meaningful improvements in blood sugar, weight, and energy levels.