Low-carb vs low-fat diets: what long-term weight loss shows

Blood tests and body composition complicate the verdict

Similar weight results do not mean identical changes in blood tests. In the meta-analysis of 38 trials, low-carbohydrate diets were more favorable for lowering triglycerides and raising HDL cholesterol at 6–12 months. Low-fat diets were more favorable for changes in LDL cholesterol and total cholesterol.[18] HDL and LDL are cholesterol measures distinguished in blood testing. Looking at only the marker that improved most would omit part of the comparison.

The between-group differences were −0.10 mmol/L for triglycerides and 0.05 mmol/L for HDL. For LDL, the difference was 0.07 mmol/L, pointing toward relatively higher values with low-carbohydrate eating; its 95% confidence interval was 0.02 to 0.12 mmol/L.[18] You do not need to interpret these laboratory units unaided. The practical message is that losing weight and improving every blood marker do not necessarily move together.

The 2-year trial showed a similar mixture. The low-carbohydrate group had greater HDL increases and larger early reductions in triglycerides and some other measures. However, its early LDL reductions were smaller, and participants reported more adverse symptoms than those in the low-fat group.[12] These findings belong beside the weight results, rather than in a footnote that disappears when an approach is presented as beneficial overall.

If you already have regular blood tests, tell your doctor about dietary changes and discuss the results together. A favorable change in one measure does not settle the meaning of the others. Symptoms also matter. The trial’s combination of laboratory outcomes and reported symptoms makes clear why a diet decision cannot be reduced to whether the scale moved in the desired direction.[12]

Changes in risk-related blood markers are also different from demonstrating fewer cases of disease. The 148-person trial that favored low-carbohydrate eating acknowledged that it did not evaluate clinical cardiovascular disease outcomes.[20] Evidence of improved markers should therefore be described at that level. It cannot support a claim that the intervention has been shown, within that trial, to prevent future cardiovascular illness.

Body-fat location adds another layer. An analysis using imaging data from 449 DIETFITS participants found a greater reduction in estimated visceral fat with the low-carbohydrate diet. Participants were 60% women, and average age was 39. Visceral fat was estimated from scans at the start, at 6 months, and at 12 months. The between-group difference in reduction was 10.6 cm² at 6 months, with a 95% confidence interval of 5 to 16.2 cm², and 6.3 cm² at 12 months, with an interval of 0.6 to 12 cm².[4]

Men experienced greater low-carbohydrate-associated visceral-fat loss than women in that analysis.[4] A separate secondary analysis of DIETFITS found that men lost more weight with low-carbohydrate than low-fat eating: the difference was −2.98 kg, with a 95% confidence interval of −4.47 to −1.50 kg. Women did not show the same significant between-diet difference. In men, the greater loss included both fat mass and lean mass.[5]

Lean mass refers to the nonfat part of the body. A larger fall in total weight is therefore not automatically the same as a larger fall in fat alone.[5] For a fuller discussion of the distinction, see our feature on body composition beyond the scale. The scale provides a useful outcome, but it does not reveal which components contributed to the change.

These subgroup findings do not establish a rule that men should choose low-carb and women should choose low-fat. The absence of a significant difference among women does not demonstrate low-fat superiority.[5] The useful lesson is to keep weight, fat location, and nonfat mass separate when judging an outcome. Each provides information, and none can simply stand in for all the others.

Food quality and adherence matter on both sides

Deciding a nutrient ratio does not finish the work of planning a menu. A secondary analysis of DIETFITS examined 448 participants with complete dietary records, looking at food quality and adherence together.[2] Adherence means how closely eating followed the assigned approach. A secondary analysis uses an existing trial’s data to investigate another question; it does not mean researchers newly randomized people to high-quality and low-quality diets.

Within both diet groups, participants classified as high in both quality improvement and adherence had a greater 12-month BMI decrease than those classified as low in both. The differences in BMI change were −1.15 kg/m² for low-carbohydrate eating and −1.11 kg/m² for low-fat eating. Their respective 95% confidence intervals were −2.04 to −0.26 and −2.10 to −0.11 kg/m².[2] Higher quality alone or higher adherence alone did not show a significant difference from the low-in-both subgroup.

The word “high” needs context here. The researchers divided participants using the median changes within the study. They combined changes in a diet-quality score with changes in carbohydrate or fat intake, creating four subgroups inside each assigned diet.[2] The analysis also cannot prove that improving both factors will necessarily cause precisely the reported BMI difference.

Its practical contribution is to show why labels are incomplete.[2] You can say that you follow low-carb or low-fat without saying what your meals contain or how consistently the pattern fits daily life. Discussing the actual foods and the situations in which you can maintain the approach gives a clearer account than claiming membership in a dietary camp.

Fiber provides a concrete example. In another DIETFITS secondary analysis, average intake at 12 months was 23.04 g in the low-fat group and 18.61 g in the low-carbohydrate group. Low-fat participants consumed more overall, but nonstarchy vegetables were the largest fiber source in both groups. Whole grains and fruit were other important sources for low-fat participants; nuts, seeds, and other plant foods contributed in the low-carbohydrate group.[15]

Fiber from nonstarchy vegetables averaged 4.13 g in the low-fat group and 5.13 g in the low-carbohydrate group. In the low-fat group, whole grains provided 3.90 g and fruit provided 3.40 g.[15] These are amounts of fiber supplied by those food groups, not weights of vegetables, grains, or fruit to put on a plate. Keeping that distinction clear avoids converting a descriptive study result into an unsupported serving prescription.

The food choices show that low-carb need not mean pushing vegetables aside, and low-fat need not mean ignoring the kind of carbohydrate eaten.[15] Look at where plant foods remain in the menu. If you reduce one component of dinner, consider the rest of the meal rather than treating the removed component as the entire intervention. That makes the research more useful when shopping than a simple list of forbidden foods.

For a household with a rushed breakfast, lunch outside, and shared evening dishes, examine those situations before adopting a label. Is it easier to adjust the rice portion separately, or to change a dish that uses a lot of oil? Which option still leaves an acceptable meal for you? DIETFITS placed finding a maintainable level of restriction at the center of its support.[1]

Include late returns home and meals out with family in your thinking. A plan that works only with one stocked refrigerator may become harder to use when shopping or eating arrangements change. The studies do not prescribe a single solution to those situations. They do provide a reason to distinguish the strictest restriction you can briefly achieve from the eating pattern you can continue.[1]

Common misconceptions

“Low-carb always produces more weight loss than low-fat.” Some pooled evidence favors low-carbohydrate diets on average, but large long-term trials also report no significant weight difference.[1][18] An average advantage does not identify which approach will work better for you. Nor should a headline erase differences in participants, support, or follow-up. The evidence supports a qualified comparison rather than a universal winner.

“There is no long-term difference, so food choices do not matter.” DIETFITS compared two approaches that emphasized food quality and included support for changing behavior.[1] Participants were not simply left to eat anything they liked under different labels. The lack of a significant between-group weight difference describes those supported interventions. It does not demonstrate that every possible version of either diet would produce the same result.

“An insulin test can tell me which diet to choose.” In the primary DIETFITS analysis, baseline insulin secretion was not significantly associated with the dietary effect on weight loss, and the genotype patterns studied did not help identify the better approach for a person.[1] Insulin secretion was assessed using blood insulin after a glucose challenge. This finding does not provide a validated shortcut from a test result to a personal diet prescription.

An exploratory reanalysis of the same data adds complexity. It found strong associations between carbohydrate-related measures and weight loss, and suggested that reducing glycemic load, a measure related to the blood-glucose impact of eating, might help explain weight change. Differences in response associated with baseline insulin secretion and glycemic-load reduction appeared at 3 and 6 months, but were not significant at 12 months. The authors explicitly advised caution because the analysis was exploratory.[7]

The primary analysis and reanalysis tested different questions using different methods.[1][7] Exploratory findings can inform thinking about mechanisms without becoming a settled method for selecting an individual’s diet. The food-quality analysis also reused DIETFITS participant data.[2] Several papers may illuminate the same trial from different angles. They do not automatically represent several independent trials, so counting papers as votes can give a misleading impression of how much separate evidence exists.

What you can do today

• If you choose lower-carbohydrate eating, adjust staple portions to a level you can maintain rather than automatically removing them. At 12 months in DIETFITS, carbohydrate still supplied an average of 30% of energy.[1] Use that observed amount as context, not a personal prescription. Keep nonstarchy vegetables and suitable plant foods such as nuts and seeds in the menu.[15]
• If you choose lower-fat eating, consider replacing dishes that use a lot of oil while retaining whole grains, fruit, and vegetables.[15] The 148-person trial used less than 30% of energy from fat as its study target.[20] This concerns the whole diet, rather than oil alone, and is not a target established as appropriate for everyone. Seek a maintainable pattern with dietary support.
• Leave room to adjust your eating after the initial change. DIETFITS began with 2 months of fat or carbohydrate restriction, then increased intake toward a comfortable maintenance level.[3] Treat that timing as a description of a supported study, rather than a reason to undertake severe restriction on your own. Plan how the approach could accommodate family meals and eating out.
• In dietary counseling, explain where adjusting staples or cooking fats has proved difficult. DIETFITS included 22 small-group sessions over 12 months, so continuing support was part of the intervention.[1] Choose a consultation schedule that matches the support available to you rather than assuming that its study schedule is a universal requirement.
• If you are treated for diabetes and use medication or insulin, consult your doctor before starting carbohydrate restriction. If you receive treatment for another ongoing condition, check with your doctor before making a major dietary change, and at the visit explain what you want to change and discuss relevant test results, including triglycerides, HDL, and LDL.

Sustaining food choices and movement at On the Shore Tachikawa

Choosing between low-carb and low-fat means considering how eating fits shared meals and changing schedules. Alongside those food choices, On the Shore Tachikawa offers a place to maintain a movement habit with visits that fit your routine.

On the Shore Tachikawa is open every day 8:00–23:30, a 1-minute walk from the North Exit of JR Tachikawa Station. Its address is 3F Etoile Bldg, 2-14-10 Akebonocho, Tachikawa, Tokyo. Compare those hours with family meals and your journey home when considering a regular visit.

Pregnant guests cannot join lava-stone hot yoga; room-temperature maternity yoga is available. Guests told by a doctor not to exercise cannot join. Many of our yoga instructors speak English, and the studio’s owner often helps guests from the US bases in English herself.

The yoga menu includes more than 25 kinds of lessons, with lava-stone hot yoga and room-temperature yoga among the options. Pilates, personal training, HIIT, boxercise, and women-only kickboxercise are also available.

For ¥1,980 including tax, the yoga trial includes a 60-minute lesson, mat rental, 1 face towel, and 2 bath towels. Check the studio information and prices, then use trial booking to arrange a visit that fits your schedule.

References

1. 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 — C. Gardner et al., 2018, JAMA. DOI: 10.1001/jama.2018.0245
2. Association of dietary adherence and dietary quality with weight loss success among those following low-carbohydrate and low-fat diets: a secondary analysis of the DIETFITS randomized clinical trial — Michelle E. Hauser et al., 2023, The American Journal of Clinical Nutrition. DOI: 10.1016/j.ajcnut.2023.10.028
3. Weight, insulin resistance, blood lipids, and diet quality changes associated with ketogenic and ultra low-fat dietary patterns: a secondary analysis of the DIETFITS randomized clinical trial — L. Aronica et al., 2023, Frontiers in Nutrition. DOI: 10.3389/fnut.2023.1220020
4. Effect of low-carbohydrate vs low-fat diet intervention on visceral fat estimated from dual energy X-ray absorptiometry in a 12-month randomized controlled trial. — Shawna Follis et al., 2025, International journal of obesity. DOI: 10.1038/s41366-025-01989-x
5. Examining differences between overweight women and men in 12-month weight loss study comparing healthy low-carbohydrate vs. low-fat diets — L. Aronica et al., 2020, International journal of obesity (2005). DOI: 10.1038/s41366-020-00708-y
6. Evidence for the carbohydrate-insulin model in a reanalysis of the Diet Intervention Examining The Factors Interacting with Treatment Success (DIETFITS) trial. — A. Soto-Mota et al., 2023, The American journal of clinical nutrition. DOI: 10.1016/j.ajcnut.2022.12.014
7. Comparison of dietary macronutrient patterns of 14 popular named dietary programmes for weight and cardiovascular risk factor reduction in adults: systematic review and network meta-analysis of randomised trials — L. Ge et al., 2020, The BMJ. DOI: 10.1136/bmj.m696
8. Effects of low-carbohydrate vs low-fat diets on weight loss and cardiovascular risk factors: a meta-analysis of randomized controlled trials. — A. Nordmann et al., 2006, Archives of internal medicine. DOI: 10.1001/archinte.166.3.285
9. Weight and Metabolic Outcomes After 2 Years on a Low-Carbohydrate Versus Low-Fat Diet — G. Foster et al., 2010, Annals of internal medicine. DOI: 10.7326/0003-4819-153-3-201008030-00005
10. Changes in Quantity and Sources of Dietary Fiber from Adopting Healthy Low-Fat vs. Healthy Low-Carb Weight Loss Diets: Secondary Analysis of DIETFITS Weight Loss Diet Study — L. Offringa et al., 2021, Nutrients. DOI: 10.3390/nu13103625
11. The Effect of Low-Fat and Low-Carbohydrate Diets on Weight Loss and Lipid Levels: A Systematic Review and Meta-Analysis — S. Chawla et al., 2020, Nutrients. DOI: 10.3390/nu12123774
12. Effects of Low-Carbohydrate and Low-Fat Diets — L. Bazzano et al., 2014, Annals of internal medicine. DOI: 10.7326/m14-0180

Cover photo: A bunless hamburger served on a plate in a food photograph. (Photo: Triplec85 / CC BY-SA 4.0 / Wikimedia Commons)

Information as of October 2026. For your own health, consult a doctor.

More from Diet Science (English)

• Metabolic adaptation and why your weight loss can slow down
• Body composition and weight loss beyond the bathroom scale
• Food logging and weight loss: what matters beyond perfect records
• Protein and weight loss beyond fullness and the bathroom scale
• Dietary fiber and weight loss: what gut research really shows
• Time-restricted eating and weight loss: what trials reveal
• Breakfast and weight loss: what eating or skipping really changes
• Sleep deprivation and appetite: why nighttime snacks add up
• Stress and emotional eating beyond the cortisol explanation
• Aerobic vs resistance training for fat loss and lean mass
• HIIT and body fat loss: what shorter workouts can really offer
• Strength training, metabolism and the science of weight regain
• Yoga and weight loss: what body composition research shows
• Hot yoga calories and sweat: what a lighter scale really means
• Pilates and body composition: what posture and fat studies show
• NEAT and weight loss: what everyday movement can really do
• Daily steps and weight loss: what 8,000 steps really tells us
• Keeping weight off: what long-term maintainers do and how they recover
• Daily self-weighing and weight loss: how to use the numbers
• Water before meals and weight loss: what 500 ml trials show
• Sugary drinks and sweeteners: what replacement means for weight
• Alcohol and weight loss through calories, appetite and fat use
• Ultra-processed foods and overeating through the trial evidence
• Eating rate and weight loss: what slower chewing can change
• Menopause weight gain and why your waist can change first
• Postpartum weight loss through diet, exercise and support
• Sarcopenia and protein: protecting muscle as you grow older
• Visceral fat in men and what exercise changes beyond weight
• Supervised exercise and weight loss: building lasting habits

All Diet Science articles (English)