Low-carb diets for weight loss: are they effective?

Low-carb diets are a popular approach for rapid, short-term weight loss. Many people notice results in as little as 1 to 2 weeks, which is why low-carb plans are so appealing.

However, research shows that initial weight loss on a low-carb diet is usually a combination of fat and water loss, rather than body fat loss alone. There is also some debate about whether low-carb diets are better for long-term weight loss than other diet types.

In this guide, we explain what a low-carb diet actually means, what happens in your body when you cut back on carbs, what to eat, possible risks, and whether low-carb diets work beyond the short term.

There is no single universal definition of a low-carb diet, but clinical guidance commonly defines low carb as under 130g of carbohydrates per day.

You may also see very low-carb, or ketogenic (keto) diets grouped within this category, which are diets with below 50g of carbohydrates per day. This guide refers to a low-carb diet of the 130g range.

Importantly, low carb does not mean no carb. Clinical guidance emphasises the importance of fibre-containing carbohydrate foods such as vegetables, whole fruits, pulses, and whole grains, as they still play an important role in digestive health and overall nutrition.

As a result, carbohydrates still feature in many low-carb diet approaches, which we will explore later in this guide.

When you significantly reduce carbohydrate intake, several changes happen in your body.

First, your body uses up your stored carbohydrates, known as glycogen. Glycogen is the stored form of carbohydrate in the muscles and liver, and it is the body’s preferred short-term energy source. As glycogen is used up, water weight is lost.

Each gram of glycogen is stored with several grams of water, so when glycogen levels fall, water is released. Research shows this is a major reason for weight loss in the early stages of a low-carb diet.

With fewer carbohydrates available, the body begins to rely more heavily on fat and protein for energy, which can support fat loss when a calorie deficit is present.

When carbohydrate intake falls below 20 to 50g per day, research shows that glycogen depletion and ketone production can occur, leading to nutritional ketosis. Ketosis is a process whereby fat and protein become the body’s primary energy source. However, ketosis is not always required for weight loss.

For weight loss, low-carb approaches typically fall between 50 and 130 grams of carbohydrates per day, depending on how restrictive the diet is.

Sustainable, low-carb diets should include a combination of all macronutrients: protein, fat, and carbohydrates, in different quantities.

Foods to eat on a low carb diet

Protein sources are especially important for supporting fullness and preserving lean mass. Examples of high protein foods include:

• eggs
• fish
• poultry
• lean meat
• tofu
• Greek yoghurt

Learn more about the best high-protein foods for weight loss.

Non-starchy vegetables rich in fibre provide essential micronutrients and support digestion. Aim to include:

• leafy greens
• broccoli
• cauliflower
• courgette
• peppers

Healthy, unsaturated fats are encouraged in moderation while following a low-carb diet. These include:

• extra virgin olive oil
• rapeseed oil
• nuts
• seeds
• avocado

Controlled portions of carbohydrates, preferably from high-fibre sources, such as:

• whole grains
• pulses
• fruits
• low-glycaemic-index foods

Foods to limit or avoid

Watch out for the following food types:

• refined carbohydrates – white bread, pastries, and chips
• sugary foods and drinks – fruit juices and sugary alcohol
• ultra-processed foods marketed as “low carb” but high in calories

Sample low-carb diet plan

Here are a few meal ideas* that you can include in your low-carb diet. Remember to aim to keep each meal at around 20 to 40g carbs and snacks under 10 to 15g carbs.

*Portion sizes and values here are approximate and may vary by brand and preparation.

For more low-carb meal ideas with calorie and carb breakdown, view meal plans and recipes on Diabetes UK.

People see different results depending on their personal circumstances. Some people find that low-carb diets are effective, even as early as 2 weeks after starting one.

However, findings from clinical studies suggest that early weight loss is partly due to water loss, as reducing carbohydrate intake leads to depletion of glycogen stores, which are stored with water in the body. This explains why weight can drop quickly at the start, before stabilising.

Evidence also indicates that fat loss does occur with continued adherence to this kind of diet, but the rate of loss typically slows after the initial weeks and becomes similar to other calorie-controlled diets over time.

Results in the initial stages of a low-carb diet depend on overall calorie intake, food quality and how sustainable the approach is long term.

Other benefits of a low carb diet

Beyond weight loss, there is evidence to suggest that a low-carb diet may offer additional benefits for some people, including:

✓ improved blood sugar control, particularly in people with type 2 diabetes, due to reduced glucose intake

✓ lower levels of unhealthy fats in the blood and an increase in “good” cholesterol – particularly over the short term

✓ reduced insulin demand, which may support diabetes management and lower medication needs under clinical supervision

✓ potential metabolic benefits, including improved glycaemic control and cardiometabolic markers in some people

However, these benefits vary between people, and long-term success depends on diet quality, adherence, and overall calorie balance rather than carbohydrate reduction alone.

Despite the benefits, research shows that reducing carbohydrates can cause temporary side effects, particularly in the early stages of the diet.

These may include:

• fatigue, headaches, and “brain fog” – often linked to fluid and electrolyte loss during early carbohydrate reduction (known as “low-carb flu” or “keto flu”)
• dizziness or muscle cramps – which may occur due to changes in sodium, potassium, or magnesium balance
• constipation – especially if fibre intake drops when whole grains, fruit, or pulses are reduced
• bad breath is sometimes reported with very low-carbohydrate or ketogenic diets
• low blood sugar could be a risk for people using insulin or certain diabetes medications

Studies show that these effects are usually short-term and can often be reduced by staying hydrated and maintaining fibre intake from vegetables.

Because of the possible risk of low blood sugar, certain groups of people should not significantly reduce carbohydrates without professional support. Conditions that could make following a low-carb diet problematic include:

• type 1 diabetes
• eating disorders
• kidney disease
• pregnancy
• taking insulin or SGLT2 inhibitor medications

Ketogenic diets (also known as the ‘keto’ diet) are a more restrictive form of low-carb eating, where carb intake is usually below 50g per day. Some research suggests that for some people, keto diets may suppress appetite and reduce calorie intake, which can lead to faster short-term weight loss.

Studies also indicate potential benefits for specific groups, such as people with type 2 diabetes, insulin resistance, or certain neurological conditions, when followed under medical supervision.

However, longer-term research shows that keto diets do not produce significantly better weight loss than less restrictive low-carb or balanced diets when calories are similar. Evidence also highlights higher rates of side effects and lower long-term adherence with ketogenic diets.

For most people, keto is an option rather than a requirement, and sustainable weight loss is more strongly linked to overall calorie intake, diet quality, and long-term adherence than to extreme carbohydrate restriction.

Research consistently shows that weight loss is primarily driven by calorie deficit, not carbohydrate level alone.

Large randomised trials show that low-carb and low-fat diets produce similar weight loss at 12 months when calorie intake and diet quality are comparable.

Research also shows that diets that people can maintain long-term lead to better outcomes than highly restrictive approaches that are difficult to sustain. A low-carb diet may be the right option for you if you can stick to it.

Diet-only approaches can be effective, especially in the short term. However, clinically supported weight loss, including injections, may be appropriate for people with:

• higher BMI
• weight-related health conditions
• repeated unsuccessful diet attempts

Low-carb diets are best positioned as complementary, rather than competing with medical weight loss treatments. If you are already on weight loss treatment, a balanced, low-carb diet can support sustainable, healthy progress when followed correctly.

Low-Carbohydrate Diet, StatPearls [accessed 14 January 2026]

The Ketogenic Diet: Clinical Applications, Evidence-based Indications, and Implementation, StatPearls [accessed 14 January 2026]

The Impact of a Ketogenic Diet on Weight Loss, Metabolism, Body Composition and Quality of Life, Cell Reports Medicine [accessed 14 January 2026]

Optimal Diet Strategies for Weight Loss and Weight Loss Maintenance, Journal of Obesity & Metabolic Syndrome [accessed 14 January 2026]

Effect of Low-Fat vs Low-Carbohydrate Diet on 12-Month Weight Loss (DIETFITS Trial), JAMA [accessed 14 January 2026]

Low-carb diet and meal plan, Diabetes UK [accessed 14 January 2026]

Reducing carbohydrates, University Hospitals Sussex NHS Foundation Trust [accessed 14 January 2026]

Low-carb diet side effects, Diabetes UK [accessed 14 January 2026]

Consumer Reports of “Keto Flu” Associated With the Ketogenic Diet, Frontiers in Nutrition [accessed 14 January 2026]