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Research · Diets

Low-carb diet

A Cochrane review of 61 trials found low-carbohydrate diets lowered weight about 1 kg more than balanced-carbohydrate diets, which its authors call little to no difference. In type 2 diabetes, HbA1c and remission improved more at 3 to 6 months but not at 12 to 24. Cohort studies link low carbohydrate intake to higher mortality, and thiamine status was lower.

Add to Bio HackChecked 2026-11-13
Status
A dietary pattern, not a regulated product.
Also called
low-carbohydrate diet, low carb, low-carb high-fat, carb cutting, reduced-carbohydrate diet, carbohydrate-restricted diet
Followed for
weight loss, blood sugar, type 2 diabetes remission, heart health, triglycerides, PCOS

Strong several reviews agree · Moderate one review or consistent trials · Limited one or a few small studies · Untested no study in people, listed with what it rests on

Shown to help4

Reported risks4

What people follow it for, and what was found

Shown to help4

Strong

Weight loss

Shown to produce about 1 kg more weight loss than balanced-carbohydrate or low-fat diets in adults with overweight, a gap a Cochrane review calls little to no difference.

Compared with balanced-carbohydrate diets [1]; low-fat diets [2]; no diet, and low-fat diets in the same network meta-analysis [3]; a healthy low-fat diet [4]; a calorie-restricted low-fat diet and a calorie-restricted Mediterranean diet [5]

The numbers and sources
Effect
In adults without type 2 diabetes, weight fell 1.07 kg more than on balanced-carbohydrate diets over 3 to 8.5 months (95% CI 0.59 to 1.55, 37 RCTs, 3,286 participants) and 0.93 kg more over 1 to 2 years (95% CI 0.04 to 1.81, 14 RCTs, 1,805 participants). In adults with type 2 diabetes the differences were 1.26 kg at 3 to 6 months and 0.33 kg at 1 to 2 years (95% CI -1.46 to 2.13) [1]. Against low-fat diets in weight-loss trials of a year or more, weight fell 1.15 kg more (95% CI 0.52 to 1.79, 18 comparisons, I² 10%) [2]. Against no diet, low-carbohydrate diets lowered weight 8.73 kg at 6 months and 7.25 kg at 12 months, and low-fat diets 7.99 kg and 7.27 kg [3]. In a 12-month trial, weight fell 6.0 kg on a low-carbohydrate diet and 5.3 kg on a low-fat diet, a difference of 0.7 kg (95% CI -0.2 to 1.6) [4]. In a 2-year trial, mean loss was 4.7 kg low-carbohydrate, 4.4 kg Mediterranean and 2.9 kg low-fat [5].
Population
6,925 adults with overweight or obesity in 61 RCTs, with and without type 2 diabetes [1]; 53 trials with 68,128 participants [2]; 7,286 adults in 48 trials [3]; 609 adults aged 18 to 50 without diabetes [4]; 322 adults with moderate obesity, 86% men [5].
Sources
[1], [2], [3], [4], [5]
Caveats
Risk of bias across the Cochrane trials was mostly high, from missing outcome data, and certainty was moderate [1]. The difference between diet types is small next to the loss on either diet [3]. In the 2-year trial, the low-carbohydrate arm had no calorie limit and the other two did [5].
ModerateVery common

HbA1c and remission in type 2 diabetes

Shown to lower HbA1c and raise remission rates at 3 to 6 months in type 2 diabetes, with no clear difference at 12 to 24 months.

Compared with diets with more carbohydrate

The numbers and sources
How common
Very common: more than 1 in 10 people.
Effect
At 6 months, 57% on low-carbohydrate diets reached HbA1c under 6.5% against 31% on control diets (risk difference 0.32, 95% CI 0.17 to 0.47, 8 studies, 264 people, I² 58%). With remission defined as HbA1c under 6.5% without medication, the effect was smaller and not significant. At 12 months, remission data were sparse [6]. Diets under 26% of energy from carbohydrate lowered HbA1c 0.47% more at 3 months (95% CI 0.23 to 0.71) and 0.36% more at 6 months (95% CI 0.09 to 0.62), with no difference at 12 or 24 months. Moderate restriction at 26% to 45% of energy did not differ from higher-carbohydrate diets [7]. Over 1 to 2 years, HbA1c showed little to no difference against balanced-carbohydrate diets [1].
Population
1,357 adults with type 2 diabetes in 23 trials [6]; 2,412 adults with diabetes in 25 RCTs [7]; adults with type 2 diabetes and overweight [1].
Sources
[1], [6], [7]
Caveats
Remission fell markedly in studies that included people using insulin [6]. Certainty was moderate to low [6], and the authors of [7] note issues with evidence quality.
ModerateStudied in women

Weight and insulin resistance in PCOS

Shown in one meta-analysis of 8 trials to lower BMI and insulin resistance more than a regular diet in women with PCOS.

Compared with a regular diet with about 45% of energy from carbohydrate [13]; before the diet, in the same women [14]

The numbers and sources
Effect
Against a regular diet, low-carbohydrate diets lowered BMI (SMD -1.04, 95% CI -1.38 to -0.70), HOMA-IR (SMD -0.66, 95% CI -1.01 to -0.30), total cholesterol (SMD -0.68) and LDL cholesterol (SMD -0.66, 95% CI -1.30 to -0.02) [13]. A second review of 25 diet studies in PCOS found insulin resistance fell from before to after diets with under 50% of energy from carbohydrate (standardized paired difference -0.86, 95% CI -1.23 to -0.50) [14].
Population
327 women with PCOS in 8 RCTs [13]; women with PCOS in 25 prospective diet studies [14].
Sources
[13], [14]
Caveats
Trials were small. The second review measured change within each diet group rather than against another diet [14].
ModerateStudied in women

Testosterone and SHBG in PCOS

In one meta-analysis, testosterone fell and SHBG rose more than on a regular diet only in trials of women with PCOS lasting longer than 4 weeks.

Compared with a regular diet with about 45% of energy from carbohydrate [13]

The numbers and sources
Effect
In trials of low-carbohydrate diets lasting more than 4 weeks, testosterone fell (SMD -1.79, 95% CI -3.22 to -0.36), SHBG rose (MD 5.98, 95% CI 3.51 to 8.46) and FSH rose (MD 0.39) more than on a regular diet. Diets low in both fat and carbohydrate raised SHBG and FSH more than higher-fat versions [13].
Population
327 women with PCOS in 8 RCTs [13].
Sources
[13]
Caveats
These are subgroup results by trial length, from small trials. The abstract does not report an overall pooled effect on testosterone or SHBG [13].

Mixed results1

Moderate

Heart risk factors

Mixed results: triglycerides and cholesterol ratio improved in some trials, while a Cochrane review found little to no difference in LDL or blood pressure over 1 to 2 years.

Compared with diets with more carbohydrate

The numbers and sources
Effect
Over 1 to 2 years, diastolic blood pressure and LDL cholesterol showed little to no difference from balanced-carbohydrate diets, with and without type 2 diabetes [1]. Over 2 years, the ratio of total to HDL cholesterol fell 20% on a low-carbohydrate diet and 12% on a low-fat diet [5]. In type 2 diabetes, triglycerides improved substantially at 6 months and less at 12 months. LDL cholesterol worsened at 12 months by an amount judged clinically important but not statistically significant [6].
Population
Adults with overweight or obesity [1][5]; adults with type 2 diabetes [6].
Sources
[1], [5], [6]
Caveats
Mean baseline LDL and diastolic pressure were in the normal range across the Cochrane trials [1]. No trial here measured heart attacks or strokes.

How trials defined it

The Cochrane review defined low-carbohydrate diets as more than 50 g to 150 g a day or under 45% of energy (42 trials), very low as 50 g or less or under 10% of energy (14 trials), and compared them with balanced diets at 45% to 65% of energy from carbohydrate. The most common design paired low carbohydrate with 20% to 35% of energy from fat and more than 20% from protein [1]. The diabetes remission review used under 130 g a day, or under 26% of a 2,000 kcal diet, and under 10% for very low [6]. In a 12-month trial taught through 22 group sessions, people on the low-carbohydrate diet ate 30% of energy from carbohydrate, 45% from fat and 23% from protein [4]. A diabetes review split restriction into under 26% and 26% to 45% of energy [7].

Risks

Reported risks4

LimitedVery common

Thiamine

Linked to lower thiamine status: 43% of people on low-carbohydrate diets fell short of reference intake from food, against 14% of controls.

The numbers and sources
How common
Very common: more than 1 in 10 people.
Effect
From food alone, 21 of 49 people on low-carbohydrate diets (43%) did not meet the reference nutrient intake for thiamine, against 14% of controls. With supplements counted, intakes did not differ, but red blood cell thiamine was lower (407 against 633 ng/g hemoglobin). Three people on low-carbohydrate diets were thiamine deficient, against one control. Magnesium, zinc, copper and selenium did not differ [8].
Population
98 adults in the UK, 49 following a low-carbohydrate diet for a median of 9 months at 16% of energy from carbohydrate, including ketogenic, paleolithic and Atkins versions, and 49 controls [8].
Sources
[8]
Caveats
A cross-sectional observational study. The control group was younger and had more men [8].
LimitedHow often is not known

Fiber

Linked to fiber intake below guideline goals in US adults on low-carbohydrate diets, a shortfall the general population also showed.

The numbers and sources
How common
How often is not known: the sources give no rate.
Effect
Neither men nor women on low-carbohydrate diets met fiber goals. Intake was also short for magnesium, potassium and vitamins A, E and D, vitamin C in men and folate in women. The general population showed a comparable picture [9].
Population
307 US adults following a low-carbohydrate diet, from national survey data [9].
Sources
[9]
Caveats
Cross-sectional survey data, not a trial. The shortfalls were not unique to low-carbohydrate diets [9].
LimitedHow often is not known

Low blood sugar and low blood pressure on medication

An expert consensus states that starting a low-carbohydrate diet on insulin, sulfonylureas or blood pressure drugs carries a risk of low blood sugar and low blood pressure.

The numbers and sources
How common
How often is not known: the sources give no rate.
Effect
An international consensus on adjusting medication states that starting a low-carbohydrate diet on medication requires close monitoring and medication changes to reduce the risk of hypoglycemia and hypotension. It names insulin, sulfonylureas, SGLT2 inhibitors, blood pressure medications and diuretics as the most immediate adjustments [10].
Population
Adults with type 2 diabetes on medication [10].
Sources
[10]
Caveats
A consensus document, not a trial measuring hypoglycemia rates.
ModerateHow often is not known

Long-term mortality

Linked to 20% to 31% higher all-cause mortality in cohort studies, higher when carbohydrate was replaced with animal foods and lower when replaced with plant foods.

The numbers and sources
How common
How often is not known: the sources give no rate.
Effect
Across 4 cohorts (272,216 people), those with high low-carbohydrate scores had higher all-cause mortality (RR 1.31, 95% CI 1.07 to 1.59). Cardiovascular death (RR 1.10, 95% CI 0.98 to 1.24) and cardiovascular disease (RR 0.98, 95% CI 0.78 to 1.24) were not significantly raised [11]. Across cohorts with 432,179 participants, under 40% of energy from carbohydrate was linked to higher mortality than moderate intake (HR 1.20, 95% CI 1.09 to 1.32), as was over 70% (HR 1.23). Replacing carbohydrate with animal fat or protein was linked to higher mortality (HR 1.18), and with plant sources to lower mortality (HR 0.82) [12].
Population
Prospective cohorts of adults, observational [11][12]; 15,428 US adults aged 45 to 64 followed for a median of 25 years [12].
Sources
[11], [12]
Caveats
Observational studies of habitual intake from questionnaires, not trials of a prescribed diet, so they cannot separate the diet from other differences between people [11][12].

Supplements to consider

The nutrients research linked to this diet running short, each with its own page.

Thiamine

Limited Linked to lower thiamine status: 43% of people on low-carbohydrate diets fell short of reference intake from food, against 14% of controls.

Add both to Bio Hack

Fiber supplements

Limited Linked to fiber intake below guideline goals in US adults on low-carbohydrate diets, a shortfall the general population also showed.

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Claims tested and not found

  1. That genotype or insulin secretion predicts who loses more weight on a low-carbohydrate diet. In a 609-person, 12-month trial, neither three genotype patterns nor insulin secretion interacted with diet type [4].
  2. That a low-carbohydrate diet keeps a clear weight lead over 1 to 2 years. The difference from balanced-carbohydrate diets was 0.93 kg without diabetes and 0.33 kg, not significant, with type 2 diabetes [1].

Sources

  1. Naude CE, Brand A, Schoonees A, et al. Low-carbohydrate versus balanced-carbohydrate diets for reducing weight and cardiovascular risk. Cochrane Database of Systematic Reviews. 2022. https://doi.org/10.1002/14651858.cd013334.pub2
  2. Tobias DK, Chen M, Manson JE, et al. Effect of low-fat diet interventions versus other diet interventions on long-term weight change in adults: a systematic review and meta-analysis. The Lancet Diabetes & Endocrinology. 2015. https://doi.org/10.1016/s2213-8587(15)00367-8
  3. Johnston BC, Kanters S, Bandayrel K, et al. Comparison of weight loss among named diet programs in overweight and obese adults: a meta-analysis. JAMA. 2014. https://doi.org/10.1001/jama.2014.10397
  4. Gardner CD, Trepanowski JF, Del Gobbo LC, et al. Effect of Low-Fat vs Low-Carbohydrate Diet on 12-Month Weight Loss in Overweight Adults and the Association With Genotype Pattern or Insulin Secretion: The DIETFITS Randomized Clinical Trial. JAMA. 2018. https://doi.org/10.1001/jama.2018.0245
  5. Shai I, Schwarzfuchs D, Henkin Y, et al. Weight loss with a low-carbohydrate, Mediterranean, or low-fat diet. New England Journal of Medicine. 2008. https://doi.org/10.1056/nejmoa0708681
  6. Goldenberg JZ, Day A, Brinkworth GD, et al. Efficacy and safety of low and very low carbohydrate diets for type 2 diabetes remission: systematic review and meta-analysis of published and unpublished randomized trial data. BMJ. 2021. https://doi.org/10.1136/bmj.m4743
  7. Sainsbury E, Kizirian NV, Partridge SR, et al. Effect of dietary carbohydrate restriction on glycemic control in adults with diabetes: A systematic review and meta-analysis. Diabetes Research and Clinical Practice. 2018. https://doi.org/10.1016/j.diabres.2018.02.026
  8. Churuangsuk C, Catchpole A, Talwar D, et al. Low thiamine status in adults following low-carbohydrate / ketogenic diets: a cross-sectional comparative study of micronutrient intake and status. European Journal of Nutrition. 2024. https://doi.org/10.1007/s00394-024-03459-y
  9. Storz MA, Ronco AL. Nutrient intake in low-carbohydrate diets in comparison to the 2020-2025 Dietary Guidelines for Americans: a cross-sectional study. British Journal of Nutrition. 2023. https://doi.org/10.1017/s0007114522001908
  10. Cucuzzella M, Riley K, Isaacs D. Adapting Medication for Type 2 Diabetes to a Low Carbohydrate Diet. Frontiers in Nutrition. 2021. https://doi.org/10.3389/fnut.2021.688540
  11. Noto H, Goto A, Tsujimoto T, et al. Low-carbohydrate diets and all-cause mortality: a systematic review and meta-analysis of observational studies. PLoS ONE. 2013. https://doi.org/10.1371/journal.pone.0055030
  12. Seidelmann SB, Claggett B, Cheng S, et al. Dietary carbohydrate intake and mortality: a prospective cohort study and meta-analysis. The Lancet Public Health. 2018. https://doi.org/10.1016/s2468-2667(18)30135-x
  13. Zhang X, Zheng Y, Guo Y, Lai Z. The Effect of Low Carbohydrate Diet on Polycystic Ovary Syndrome: A Meta-Analysis of Randomized Controlled Trials. International Journal of Endocrinology. 2019. https://doi.org/10.1155/2019/4386401
  14. Porchia LM, Hernandez-Garcia SC, Gonzalez-Mejia ME, López-Bayghen E. Diets with lower carbohydrate concentrations improve insulin sensitivity in women with polycystic ovary syndrome: A meta-analysis. European Journal of Obstetrics & Gynecology and Reproductive Biology. 2020. https://doi.org/10.1016/j.ejogrb.2020.03.010

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