23 Studies on Low Carb and Low Fat Diets — Time to Retire the Fad

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When it comes to weight loss, nutritionists often debate the issue “carbohydrates versus fat.”

Most mainstream health organizations argue that a diet that is rich in fat can lead to health problems, especially heart disease.

They tend to recommend a low fat diet, which restricts dietary fat to less than 30% of total calories.

However, a growing number of studies have been challenging the low fat approach.

Many now argue that a low carb diet, which is higher in fat and protein, may be more effective for treating and preventing obesity and other conditions.

This article analyzes the data from 23 studies comparing low carb and low fat diets.

All of the studies are randomized controlled trials, and they all appear in respected, peer-reviewed journals.

The studies

Many of the studies comparing low carb and low fat diets focus on people with:

  • obesity
  • type 2 diabetes
  • metabolic syndrome

The researchers usually measure factors such as:

  • weight loss
  • cholesterol levels
  • triglycerides
  • blood sugar levels

1. Foster, G. D. et al. A randomized trial of a low-carbohydrate diet for obesity.New England Journal of Medicine, 2003.

Details: Sixty-three adults with obesity followed either a low fat or a low carb diet for 12 months. The low fat group was calorie restricted.

Weight loss: After 6 months, the low carb group had lost 7% of their total body weight, compared with the low fat group, which lost 3%. The difference was statistically significant at 3 and 6 months but not at 12 months.

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Conclusion: There was more weight loss in the low carb group, and the difference was significant at 3 and 6 months, but not 12. The low carb group had greater improvements in blood triglycerides and HDL (good cholesterol), but other biomarkers were similar between groups.

2. Samaha, F. F. et al. A low-carbohydrate as compared with a low-fat diet in severe obesity.New England Journal of Medicine, 2003.

Details: In this study, 132 individuals with severe obesity (an average BMI of 43) followed either a low fat or a low carb diet for 6 months. Many had metabolic syndrome or type 2 diabetes. Those on the low fat diet had a restricted calorie intake.

Weight loss: The low carb group lost an average of 12.8 pounds (5.8 kg), while the low fat group lost only 4.2 pounds (1.9 kg). The difference was statistically significant.

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Conclusion: Those who followed the low carb diet lost about three times more weight than those on the low fat diet.

There was also a statistically significant difference in several biomarkers:

  • Triglycerides fell by 38 mg/dL in the low carb group, compared with 7 mg/dL in the low fat group.
  • Insulin sensitivity improved on the low carb diet, but it worsened slightly on the low fat diet.
  • Fasting blood glucose levels fell by 26 mg/dL in the low carb group, but only by 5 mg/dL in the low fat group.
  • Insulin levels dropped by 27% in the low carb group, but it rose slightly in the low fat group.

Overall, the low carb diet produced more benefits for weight and key biomarkers in this study.

3. Sondike, S. B. et al. Effects of a low-carbohydrate diet on weight loss and cardiovascular risk factor in overweight adolescents.The Journal of Pediatrics, 2003.

Details: Thirty adolescents with overweight followed either a low carb diet or a low fat diet for 12 weeks. Neither group restricted their calorie intake.

Weight loss: Those on the low carb diet lost 21.8 pounds (9.9 kg), while those on the low fat diet lost only 9 pounds (4.1 kg). The difference was statistically significant.

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Conclusion: The low carb group lost 2.3 times as much weight and had significant decreases in triglyceride and non-high-density lipoprotein (non-HDL) cholesterol levels. Total and low-density lipoprotein (LDL) — or “bad” cholesterol — fell in the low fat group only.

4. Brehm, B. J. et al. A randomized trial comparing a very low carbohydrate diet and a calorie-restricted low fat diet on body weight and cardiovascular risk factors in healthy women.The Journal of Clinical Endocrinology & Metabolism, 2003.

Details: Fifty-three females who had obesity but were in good health followed either a low fat or a low carb diet for 6 months. The low fat group restricted their calorie intake.

Weight loss: Those in the low carb group lost an average of 18.7 pounds (8.5 kg), while those on the low fat diet lost an average of 8.6 pounds (3.9 kg). The difference was statistically significant at 6 months.

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Conclusion: The low carb group lost 2.2 times as much weight as the low fat group. Blood lipids improved significantly for each group, but there was no significant difference between the groups.

5. Aude, Y. W. et al. The National Cholesterol Education Program Diet vs a diet lower in carbohydrates and higher in protein and monounsaturated fat: A randomized trial.Archives of Internal Medicine, 2004.

Details: Sixty individuals with overweight followed either a low carb diet that was high in monounsaturated fat, or a low fat diet based on the National Cholesterol Education Program (NCEP). They followed the diet for 12 weeks

Both groups restricted their calorie intake.

Weight loss: The low carb group lost an average of 13.6 pounds (6.2 kg), while the low fat group lost 7.5 pounds (3.4 kg). The difference was statistically significant.

Conclusion: The low carb group lost 1.8 times as much weight, and several changes occurred in biomarkers:

  • Waist-to-hip ratio is a marker for abdominal fat. This marker improved slightly in the low carb but not in the low fat group.
  • Total cholesterol improved in both groups.
  • Triglycerides fell by 42 mg/dL in the low carb group, compared with 15.3 mg/dL in the low fat group. However, the difference wasn’t statistically significant between groups.
  • LDL particle size increased by 4.8 nm, and the percentage of small, dense LDL particles decreased by 6.1% in the low carb group. There was no significant difference in the low fat group, and the changes weren’t statistically significant between the groups.

Overall, the low carb group lost more weight and had some improvement in several important risk factors for cardiovascular disease.

6. Yancy, W. S. Jr. et al. A low-carbohydrate, ketogenic diet versus a low-fat diet to treat obesity and hyperlipidemia.Annals of Internal Medicine, 2004.

Details: In this study, 120 individuals with overweight and high blood lipids followed either a low carb or a low fat diet for 24 weeks. The low fat group restricted their calorie intake.

Weight loss: People in the low carb group lost 20.7 pounds (9.4 kg) of their total body weight, compared with 10.6 pounds (4.8 kg) in the low fat group.

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Conclusion: People in the low carb group lost significantly more weight and had greater improvements in blood triglycerides and HDL (good) cholesterol.

7. Volek, J. S. et al. Comparison of energy-restricted very low-carbohydrate and low-fat diets on weight loss and body composition in overweight men and women.Nutrition & Metabolism (London), 2004.

Details: In a study involving 28 people with obesity or overweight, females followed either a very low carb or a low fat diet for 30 days, and males followed one of these diets for 50 days. Both diets were calorie restricted.

Weight loss: People in low carb group lost significantly more weight. This was especially true for the men, even though they ate more calories than the low fat group.

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Conclusion: People in the low carb group lost more weight than those in the low fat group. The men on the low carb diet lost three times as much abdominal fat as the men on the low fat diet.

8. Meckling, K. A. et al. Comparison of a low-fat diet to a low-carbohydrate diet on weight loss, body composition, and risk factors for diabetes and cardiovascular disease in free-living, overweight men and women.The Journal of Clinical Endocrinology & Metabolism, 2004.

Details: Forty people with overweight followed either a low carb or a low fat diet for 10 weeks. Each group had the same calorie intake.

Weight loss: The low carb group lost 15.4 pounds (7.0 kg), and the low fat group lost 14.9 pounds (6.8 kg). The difference wasn’t statistically significant.

Conclusion: Both groups lost a similar amount of weight, and the following also occurred:

  • Blood pressure decreased in both groups, both systolic and diastolic.
  • Total and LDL (bad) cholesterol decreased in the low fat group only.
  • Triglycerides fell in both groups.
  • HDL (good) cholesterol rose in the low carb group, but it fell in the low fat group.
  • Blood sugar went down in both groups, but only the low carb group had decreases in insulin levels. This indicates improved insulin sensitivity.

9. Nickols-Richardson, S. M. et al. Perceived hunger is lower and weight loss is greater in overweight premenopausal women consuming a low-carbohydrate/high-protein vs high-carbohydrate/low-fat diet.Journal of the American Dietetic Association, 2005.

Details: Twenty-eight females with overweight, who hadn’t yet reached menopause, consumed either a low carb or a low fat diet for 6 weeks. The low fat diet was calorie restricted.

Weight loss: Those in the low carb group lost 14.1 pounds (6.4 kg), while those in the low fat group lost 9.3 pounds (4.2 kg). The results were statistically significant.

Conclusion: Significantly more weight loss occurred with the low carb diet, and there was also reduced hunger, compared with the low fat diet.

10. Daly, M. E. et al. Short-term effects of severe dietary carbohydrate-restriction advice in Type 2 diabetes.Diabetic Medicine, 2006.

Details: In this study 102 people with type 2 diabetes received either low carb or low fat diet advice for 3 months. Those in the low fat group were advised to reduce portion sizes.

Weight loss: The low carb group lost 7.8 pounds (3.55 kg), while the low fat group lost only 2 pounds (0.92 kg). The difference was statistically significant.

Conclusion: The low carb group lost more weight and had greater improvement in their total cholesterol/HDL ratio. There was no difference in triglycerides, blood pressure, or HbA1c (a marker for blood sugar levels) between the groups.

11. McClernon, F. J. et al. The effects of a low-carbohydrate ketogenic diet and a low-fat diet on mood, hunger, and other self-reported symptoms.Obesity (Silver Spring), 2007.

Details: In this study, 119 people with overweight followed either a low carb, ketogenic diet or a calorie restricted low fat diet for 6 months.

Weight loss: People in the low carb group lost 28.4 pounds (12.9 kg), while those in the low fat group lost 14.7 pounds (6.7 kg).

Conclusion: The low carb group lost almost twice as much weight and experienced less hunger.

12. Gardner, C. D. et al. Comparison of the Atkins, Zone, Ornish, and LEARN diets for change in weight and related risk factors among overweight premenopausal women: the A TO Z Weight Loss Study.The Journal of The American Medical Association, 2007.

Details: In this study, 311 women who hadn’t experienced menopause and who had either overweight or obesity followed one of four diets:

  • a low carb Atkins diet
  • a low fat vegetarian Ornish diet
  • the Zone diet
  • the LEARN diet

Zone and LEARN were calorie restricted.

Weight loss: The Atkins group lost the most weight — 10.3 pounds (4.7 kg) — at 12 months, compared with the Ornish group losing 4.9 pounds (2.2 kg), the Zone group losing 3.5 pounds (1.6 kg), and the LEARN group losing 5.7 pounds (2.6 kg).

However, the difference wasn’t statistically significant at 12 months.

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Conclusion: The Atkins group lost the most weight, although the difference wasn’t statistically significant. The Atkins group had the greatest improvements in blood pressure, triglycerides, and HDL (good) cholesterol levels. Those who followed LEARN or Ornish, which are low fat diets, had decreases in LDL (bad) cholesterol at 2 months, but then the effects diminished.

13. Halyburton, A. K. et al. Low- and high-carbohydrate weight-loss diets have similar effects on mood but not cognitive performance.American Journal of Clinical Nutrition, 2007.

Details: Ninety-three people with either overweight or obesity followed either a low carb, high fat diet or a low fat, high carb diet for 8 weeks. Both groups were calorie restricted.

Weight loss: The low carb group lost 17.2 pounds (7.8 kg), while the low fat group lost 14.1 pounds (6.4 kg). The difference was statistically significant.

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Conclusion: The low carb group lost more weight. Both groups had similar improvements in mood, but speed of processing (a measure of cognitive performance) improved further on the low fat diet.

14. Dyson, P. A. et al. A low-carbohydrate diet is more effective in reducing body weight than healthy eating in both diabetic and non-diabetic subjects.Diabetic Medicine, 2007.

Details: Thirteen people with diabetes and 13 without diabetes followed either a low carb diet or a “healthy eating” diet. This was a calorie restricted, low fat diet recommended by Diabetes UK. The study lasted 3 months.

Weight loss: People in the low carb group lost an average of 15.2 pounds (6.9 kg), compared with 4.6 pounds (2.1 kg) in the low fat group.

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Conclusion: The low carb group lost about three times as much weight as the low fat group. There was no difference in any other marker between groups.

15. Westman, E. C. et al. The effect of a low-carbohydrate, ketogenic diet versus a low-glycemic index diet on glycemic control in type 2 diabetes mellitus.Nutrion & Metabolism (London), 2008.

Details: Eighty-four individuals with obesity and type 2 diabetes followed a low carb, ketogenic diet or a calorie restricted low glycemic diet for 24 weeks.

Weight loss: The low carb group lost more weight — 24.4 pounds (11.1 kg) — than the low glycemic group — 15.2 pounds (6.9 kg).

Conclusion: People in the low carb group lost significantly more weight than the low glycemic group. In addition:

  • Hemoglobin A1c went down by 1.5% in the low carb group, compared to 0.5% in the low glycemic group.
  • HDL (good) cholesterol increased in the low carb group only, by 5.6 mg/dL.
  • Diabetes medications were either reduced or eliminated in 95.2% of the low carb group, compared to 62% in the low glycemic group.
  • Blood pressure, triglycerides, and other markers improved in both groups, but the difference between groups wasn’t statistically significant.

16. Shai, I. et al. Weight loss with a low-carbohydrate, Mediterranean, or low-fat diet.New England Journal of Medicine, 2008.

Details: In this study, 322 people with obesity followed one of three diets:

  • a low carb diet
  • a calorie restricted low fat diet
  • a calorie restricted Mediterranean diet

They followed the diet for 2 years.

Weight loss: The low carb group lost 10.4 pounds (4.7 kg), the low fat group lost 6.4 pounds (2.9 kg), and the Mediterranean diet group lost 9.7 pounds (4.4 kg).

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Conclusion: The low carb group lost more weight than the low fat group and had greater improvement in HDL (good) cholesterol and triglycerides.

17. Keogh, J. B. et al. Effects of weight loss from a very-low-carbohydrate diet on endothelial function and markers of cardiovascular disease risk in subjects with abdominal obesity.American Journal of Clinical Nutrition, 2008.

Details: In this study, 107 individuals with abdominal obesity followed either a low carb or a low fat diet, both with calorie restrictions, for 8 weeks.

Weight loss: The low carb group lost 7.9% of their body weight, compared with 6.5% in the low fat group.

Conclusion: The low carb group lost more weight. There was also no difference in common markers or risk factors between groups.

18. Tay, J. et al. Metabolic effects of weight loss on a very-low-carbohydrate diet compared with an isocaloric high-carbohydrate diet in abdominally obese subjects.Journal of The American College of Cardiology, 2008.

Details: Eighty-eight people with abdominal obesity followed either a very low carb or a low fat diet for 24 weeks. Both diets were calorie restricted.

Weight loss: People in the low carb group lost an average of 26.2 pounds (11.9 kg), while those in the low fat group lost 22.3 pounds (10.1 kg). However, the difference wasn’t statistically significant.

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Conclusion: Both diets led to similar weight loss results and improvements in triglycerides, HDL (good) cholesterol, C-reactive protein, insulin, insulin sensitivity, and blood pressure. Total and LDL (bad) cholesterol improved in the low fat group only.

19. Volek, J. S. et al. Carbohydrate restriction has a more favorable impact on the metabolic syndrome than a low fat diet.Lipids, 2009.

Details: Forty people with high risk factors for cardiovascular disease followed either a low carb or a low fat diet for 12 weeks, both with calorie restrictions.

Weight loss: The low carb group lost 22.3 pounds (10.1 kg), while the low fat group lost 11.5 pounds (5.2 kg).

Conclusion: People in the low carb group lost almost twice as much weight as those in the low fat group, although their calorie intake was the same.

In addition:

  • Triglycerides fell by 107 mg/dL on the low carb diet, but it only fell 36 mg/dL on the low fat diet.
  • HDL (good) cholesterol rose by 4 mg/dL on the low carb diet, but it fell by 1 mg/dL on the low fat diet.
  • Apolipoprotein B went down by 11 points on the low carb diet, but it only went down 2 points on the low fat diet.
  • LDL particles size increased on the low carb diet, but it stayed the same on the low fat diet.

On the low carb diet, the LDL particles partly shifted from small to large, which is good. However, on the low fat diet, they partially shifted from large to small, which is less healthy.

20. Brinkworth, G. D. et al. Long-term effects of a very-low-carbohydrate weight loss diet compared with an isocaloric low-fat diet after 12 months.American Journal of Clinical Nutrition, 2009.

Details: In this study, 118 individuals with abdominal obesity followed either a low carb or a low fat diet for 1 year. Both diets were calorie restricted.

Weight loss: People in the low carb group lost 32 pounds (14.5 kg), while those in the low fat group lost 25.3 pounds (11.5 kg). The difference wasn’t statistically significant.

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Conclusion: The low carb group experienced greater decreases in triglycerides and greater increases in both HDL (good) and LDL (bad) cholesterol, compared with the low fat group.

21. Hernandez, T. L. et al. Lack of suppression of circulating free fatty acids and hypercholesterolemia during weight loss on a high-fat, low-carbohydrate diet.American Journal of Clinical Nutrition, 2010.

Details: Thirty-two adults with obesity followed either a low carb or a calorie restricted, low fat diet for 6 weeks.

Weight loss: The low carb group lost 13.7 pounds (6.2 kg), while the low fat group lost 13.2 pounds (6.0 kg). The difference wasn’t statistically significant.

Conclusion: The low carb group saw a greater decrease in triglycerides (43.6 mg/dL) than the low fat group (26.9 mg/dL). Both LDL (bad) and HDL (good) cholesterol decreased in the low fat group only.

22. Krebs, N. F. et al. Efficacy and safety of a high protein, low carbohydrate diet for weight loss in severely obese adolescents.Journal of Pediatrics, 2010.

Details: Forty-six individuals followed either a low carb or a low fat diet for 36 weeks. People in the low fat group restricted their calorie intake.

Weight loss: Those in the low carb group had a greater decrease in body mass index (BMI) Z-scores than the low fat group, but weight loss didn’t differ between groups.

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Conclusion: The low carb group had a greater reduction in BMI Z-scores, but weight loss was similar between groups. Various biomarkers improved in both groups, but there was no significant difference between them.

23. Guldbrand H. et al. In type 2 diabetes, randomization to advice to follow a low-carbohydrate diet transiently improves glycaemic control compared with advice to follow a low-fat diet producing a similar weight loss.Diabetologia, 2012.

Details: Sixty-one individuals with type 2 diabetes followed either a low carb or a low fat diet for 2 years, both with calorie restrictions.

Weight loss: Those in the low carb group lost 6.8 pounds (3.1 kg), while those in the low fat group lost 7.9 pounds (3.6 kg). The difference wasn’t statistically significant.

Conclusion: There was no difference in weight loss or common risk factors between groups. There was a significant improvement in glycemic control at 6 months for the low carb group. However, compliance was poor, and the effects diminished at 24 months as people started to consume more carbs.

Weight loss

The following graph shows how weight loss compared between the 23 studies. People lost weight in 21 of the studies.

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Most of the studies found a significant difference in weight loss, in favor of the low carb diet.

In addition:

  • The low carb groups often lost 2–3 times as much weight as the low fat groups. In a few instances, there was no significant difference.
  • In most cases, the low fat groups followed calorie restrictions, while the low carb groups ate as many calories as they wanted.
  • When both groups restricted calories, the low carb dieters still lost more weight (1, 2, 3), although it wasn’t always significant (4, 5, 6).
  • In only one study, the low fat group lost more weight (7), but the difference was small— 1.1 pound (0.5 kg) — and not statistically significant.
  • In several of the studies, weight loss was greatest in the beginning. Then people started regaining the weight over time as they abandoned the diet.
  • The low carb diets were more effective in reducing abdominal fat, a type of fat that researchers have linked to various health conditions. (8, 1, 3).

Two reasons why low carb diets may be more effective for weight loss are:

  • the high protein content
  • the appetite-suppressing effects of the diet

These factors can help reduce a person’s calorie intake.

You can read more about why this diet works here: Why do Low Carb Diets Work? The Mechanism Explained.

LDL (bad) cholesterol

Low carb diets generally don’t appear to raise total and LDL (bad) cholesterol levels.

Low fat diets can lower total and LDL (bad) cholesterol, but this is usually only temporary. After 6–12 months, the difference isn’t usually statistically significant.

Some healthcare providers have reported that low carb diets can cause LDL (bad) cholesterol and other lipid markers to increase in a few people.

However, the authors of the above studies didn’t note these adverse effects. The studies that looked at advanced lipid markers (8, 3) only showed improvements.

HDL (good) cholesterol

One way to raise HDL (good) cholesterol levels is to eat more fat. For this reason, it’s not surprising to see that low carb diets, being higher in fat, are more likely to raise HDL (good) cholesterol than low fat diets.

Higher HDL (good) levels may help improve metabolic health and reduce the risk of cardiovascular disease. People with metabolic syndrome often have low HDL (good) levels.

Eighteen of the 23 studies reported changes in HDL (good) cholesterol levels.

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Low carb diets generally raise HDL (good) levels, but these levels appear to change less on low fat diets. In some cases, they go down.

Triglycerides

Triglycerides are an important cardiovascular risk factor and other key symptoms of metabolic syndrome.

The best way to reduce triglycerides is to eat fewer carbohydrates, and especially eat less sugar.

Nineteen of 23 studies reported changes in blood triglyceride levels.

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Both low carb and low fat diets can help reduce triglycerides, but the effect is stronger in the low carb groups.

Blood sugar, insulin levels and type II diabetes

People without diabetes saw their blood sugar and insulin levels improve on both the low carb and low fat diets. The difference between the groups was usually small.

Three studies compared how the diets affected people with type 2 diabetes.

Only one study managed to reduce carbohydrates sufficiently.

In this study various improvements occurred, including a drastic fall in HbA1c, a marker for blood sugar levels (9). In addition, over 90% of the individuals in the low carb group managed to reduce or eliminate their diabetes medications.

However, the difference was small or nonexistent in the other two studies, because compliance was poor. The participants ended up eating around 30% of their calories as carbs. (10, 7).

Blood pressure

When measured, blood pressure tended to decrease on both types of diet.

How many people finished?

A common problem in weight loss studies is that people often abandon the diet before the study is complete.

Nineteen of the 23 studies reported the number of people who completed the study.

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The average percentage of people who followed the diet throughout was:

  • low carb groups: 79.51%
  • low fat groups: 77.72%

This suggests that a low carb diet is no more difficult to stick to than other types of diet.

The reason may be that low carb diets appear to reduce hunger (11, 12), and participants can eat until they’re full. Low fat diets, meanwhile, are often calorie restricted. The person needs to weigh their food and count calories, which can be onerous.

Individuals also lose more weight, and lose it faster, on a low carb diet. This may improve their motivation to continue the diet.

Adverse effects

The participants in these studies didn’t report any serious adverse effects due to either diet.

Overall, the low carb diet appears to be well tolerated and safe.

The bottom line

Many people have traditionally opted for a low fat diet and counting calories to lose weight.

However, the findings of these studies suggest that a low carb diet may be just as effective, and perhaps more so, than a low fat diet.