Low carb vs low fat diets

       main findings

  • In the short term, weight loss is higher at 3-6 months in the low carb groups, but after 12 months there is no significant difference
  • Triglycerides were lower and HDL higher in low carb group in these short term studies, LDL trends towards higher
  • No significant differences in blood pressure, glycemic control, insulin sensitivity















what the research tells us

low carb vs low fat

There are a lot of books, products, and posts on social media telling us what to eat, when to eat it, and what to avoid to obtain the best health. But what are the facts? There are a multitude of well designed studies that have been conducted to determine what, and even if, there is an ideal eating pattern. This is always going to depend on underlying medical issues and individual goals, but here is a breakdown of some of the trials with links to the full studies provided in the references section. The number preceding the synopsis corresponds to the study in the references. Just remember, when you read something and they say  “fill in the blank diet” is linked with “fill in the blank benefit” you really should evaluate the study yourself. I’m not saying become an expert at reading scientific papers, but if you just check out the abstract you will find that sure, the benefit was there but it may have been either: A) an insignificant change; B) over a short period of time; C) only tested in a small group of people. Also it is important to know what they were comparing the diet to and if the subjects had any underlying medical issues. So without further ado, a breakdown of some of the studies out there today on low carb vs low fat diets. There is a lot of research out there on other eating patterns, such as the Mediterranean Diet, and I will break down some of those studies in a future post.



1. The first trial I will discuss came out of the New England Journal of Medicine in 2003. This study was done around the time that the Atkins Diet was hitting a real surge in the late 90’s an early 2000’s. This study randomized 63 obese male and female subjects to either a low carbohydrate diet or low fat diet for the span of 12 months. Participants were excluded if they had Type 2 diabetes, were taking lipid lowering medications, were pregnant or lactating, or were taking weight loss medications. For the 33 subjects who were assigned to the low-carbohydrate, high-protein, high-fat diet carbohydrate intake was to be limited to 20 g per day  for the first two weeks and then gradually increased until a stable and desired weight achieved. Each subject was given a copy of Dr. Atkins’ New Diet Revolution,  which details the Atkins diet program. Subjects were instructed to read the book and follow the diet as described. The 30 subjects who were assigned to the conventional diet were instructed to follow diets that included about 60% of calories from carbs, 25% from fat, and 15% from protein, with 1200 to 1500 calories per day for women and 1500 to 1800 calories per day for men.

In regards to weight loss, those in the low carb group lost significantly more weight than the group on the conventional diet at 3 months and 6 months, but the difference in weight loss was not statistically significant at 12 months.

Health biomarkers were also measured. The low carb group did see a statistical improvement in triglycerides and HDL cholesterol, however there were no significant differences in blood pressure, total cholesterol, LDL cholesterol, insulin sensitivity.

In conclusion, there was more weight loss in the low-carb group, significant at 3 and 6 months, but not at 12 months.  The low-carb group had greater improvements in blood triglycerides and HDL, but other biomarkers were similar between groups, and actually a trend toward higher LDL in the low carb group.


2. Another study from the New England Journal in 2003 compared low carb vs low fat diets, however this time in severely obese individuals with a BMI above 43. 132 individuals with severe obesity (mean BMI of 43) were randomized to either a low-fat or a low-carb diet, and many of the subjects had metabolic syndrome or type II diabetes. The subjects assigned to the low-carbohydrate diet were instructed to restrict carbohydrate intake to 30 g per day or less. No instruction on restricting total fat intake was provided. Vegetables and fruits with high ratios of fiber to carbohydrate were recommended.The subjects assigned to the low-fat diet received instructions on diet including caloric restriction sufficient to create a deficit of 500 calories per day, with 30 percent or less of total calories derived from fat.

When caloric intake was compared after 6 months, it was found that those on the low carb diet had an overall lower intake of calories compared to the low fat group. There was thought that perhaps this was due to greater satiety. Again, triglycerides were lower in the low carb group, with no change in total, HDL, or LDL cholesterol. 

In regards to weight loss, the low-carb group lost an average of 12.8 lbs while the low-fat group lost only 4.2 lbs. The difference was statistically significant. However, this trial was stopped at 6 months. As we saw in the previous study, at 3 and 6 months weight loss was greater in the low carb group as well, however at 12 months no significant difference was found. Is 6 months really enough time to call this a study in favor of low carbs? Also, was the difference more due to decreased caloric intake or due to ratio of macronutrient intake? There also seemed to be a trend towards better glycemic control (in diabetics only) in the low carb group. However, when broken down this again appeared to be more linked to decreased caloric intake. 

3. A very small study was done in adolescents comparing a low fat to low carb diet. This study only included 30 adolescents, all who were overweight. They were not instructed to decrease calorie intake. Those in the low carb group did lose a larger amount of weight than those in the low fat group, 21.8 pounds compared to 9 pounds. The low carb group again had decreases in triglycerides, but total cholesterol and LDL only decreased in the low fat group. Not only was this study small in size, it was also only conducted over a 12 week period.

4. A study in obese women without other medical comorbidities compared a very low carb diet that was also low in calories to a low fat diet. This trial only lasted 6 months and included 53 participants with moderate obesity, which was a BMI between 30-35. It excluded patients with medical issues such as hypertension, diabetes, and cardiovascular disease. Subjects were randomized to 6 months of either a very low carbohydrate diet or a calorie-restricted diet with 30% of the calories as fat.  The women in the very low carbohydrate group lost an average of 7.6 ± 0.7 kg after 3 months and 8.5 ± 1.0 kg after 6 months of diet. Women following the low fat diet lost 4.2 ± 0.8 and 3.9 ± 1.0 kg at 3 and 6 months, respectively. There were no differences in blood pressure, lipids, glycemic control, or insulin levels between groups. 

5. Another 6 months trial again compared low carb to low fat diets in treating patients with obesity and high cholesterol. It was conducted on 120 overweight subjects with a BMI between 30-60 and high cholesterol, however subjects had no other medical issues. The results did find a higher weight loss in the low carb group at 20.7 pounds on average compared to 10.6 pounds on average in the low fat group.  However, the percentage of total weight loss that was fat mass was similar in the 2 groups (78% in the low-carbohydrate diet group and 74% in the low-fat diet group). As would be expected, the low-carbohydrate diet group lost a greater amount of total body water in the first 2 weeks of the study than did the low-fat diet group (−1.1 kg versus −0.5 kg).  As in the above studies, the low carb group had lower levels of triglycerides and higher HDL cholesterol but no difference in LDL or total cholesterol. Symptomatic adverse effects occurred more frequently in the low-carbohydrate diet group than in the low-fat diet group, including constipation, headache, halitosis,  muscle cramps, diarrhea, general weakness. While not statistically significant, LDL did increase 10% from baseline in the low carb group which was consistent with increasing LDL levels in other studies.

6. A trial that looked at a few different diets randomized 311 overweight/obese premenopausal, nondiabetic women to 4 diets: A low-carb Atkins diet, a low-fat vegetarian Ornish diet, the Zone diet and the LEARN diet. The Atkins diet was the very low carb diet starting with 20 g of carbs a day and working up to about 50 g of carbs a day. The Zone diet was moderately low carb (about 40% of calories from carbs). The LEARN diet (Lifestyle, Exercise, Attitudes, Relationships, and Nutrition) was low in fat, high in carbohydrate (55-60% calories from carbs). The Ornish diet was very high in carbohydrates, with the emphasis being no more than 10% of calories from fat. Again, the very low carb Atkins group lost the most weight at 12 months (10.3 lbs) compared to Ornish (5.7 lbs),  LEARN (4.9 lbs), and Zone diets (3.5 lbs). The weight change at 2 and 6 months was statistically significant. However, (and again), the difference was not statistically significant at 12 months except for when the low carb diet was compared to the Zone diet. 

7. A longer study lasting 2 years was done in 61 individuals with type 2 diabetes. They again were randomized to low carb or low fat diets, along with calorie restriction for both groups. Patients on the low fat diet aimed for 55–60% of calories from carbohydrates and those on low carb diet aimed for 20% calories from carbohydrates. Over the 24 month period there was no difference in weight loss or common risk factors. At 6 months there was improved glycemic control in the low carb group, but this was not seen long term at 2 years due to noncompliance in the low carb group.



overall conclusions

The above studies compared low carbohydrate to low fat diets. This is by no means all of the studies out there, but there is a similar trend in all of them. As you can see, the main finding was that more weight loss was seen in the low carb diets compared to the low fat diets, however this was only in the first 3-6 months. Many studies were stopped after 3-6 months which would lead many to believe that low carb diets were indeed better if they simply looked at the results and ignored the time frame. However, studies extending out to >6 months showed that there was no long-term advantage in weight loss. In addition, there was really no benefit in other health parameters. The main health benefit was in decreased triglyceride levels and improved HDL. However, total cholesterol, LDL cholesterol, blood pressure, and insulin sensitivity were not statistically different between groups. In fact, there tended to be a trend toward increased LDL in the low carb groups which makes sense as the fat intake is much higher in these diets. Compliance in the low carbohydrate group was also low.  Unfortunately, these trials all compared low carb to low fat diets, and there are so many different diets and eating patterns out there which are not just simply “low fat.” Also, I am not saying that this shows low fat diets are by any means better than low carb, but there really does not seem to be a major benefit from low carbohydrate diets in regards to long term weight loss or health parameters.If you’re only looking for weight loss for 6 months, and then regaining it back, then I suppose low carb is for you.

Also, since the studies really don’t go out to more than 12 months (other than the one that was 24 months) we don’t really know the long term safety of low carb diets. Most people do not adhere to low carb diets in the long term, but if they could would there be any long term metabolic effects due to reliance on ketones for energy? Would this even be something sustainable over several years? The decreased triglyceride and increased HDL levels are important, but this can be achieved through other dietary and fitness interventions. Also, as a low carb diet essentially leads to higher fat intake, LDL and total cholesterol levels showed higher trends in the low carb group. Could this have significant impacts in the long run? Since these trials did not look at cardiovascular outcomes and only on biomarkers this is also something that is unknown. 

The above studies were all randomized controlled trials. I do want to bring up a recent prospective study that was presented at the European Society of Cardiology this year that looked at he relationship between low carbohydrate diets, all-cause death, and deaths from coronary heart disease, cerebrovascular disease (including stroke), and cancer in a nationally representative sample of 24,825 participants of the US National Health and Nutrition Examination Survey (NHANES) during 1999 to 2010. Compared to individuals with the highest carbohydrate consumption, those with the lowest intake of carbs had a 32% higher risk of all-cause death over an average 6.4-year follow-up. The risks of death from coronary heart disease, cerebrovascular disease, and cancer were increased by 51%, 50%, and 35%, respectively (8). This was a prospective study and there are many variables that are not accounted for between participants. Additionally, these studies can only find associations, not causation, and rely heavily on participants food journals. While prospective studies are not ideal, a six year randomized controlled study would not be feasible, so this may at least shed some light onto the long term effects of low carb diets. Another meta-analysis of eight prospective studies showed a U-shaped trend of mortality based on carb intake. Those in the lowest (<40% of calories) and highest (>70% of calories) ranges of carb intake had the highest mortality, with intakes of 55-60% being the most ideal range (9). (Disclaimer: This does not mean I think that everyone should have this percentage of intake).


What should we be eating then?

I think the overall thinking shouldn’t be “low carb” or “low fat” because you need to look at how all the macro-nutrients are altered in these diets. For low carb diets are the carbs being replaced more with fat or protein? And vice versa in the low fat diet-are these people increasing protein or increasing carbs to make up the difference in energy consumption? It doesn’t need to be an all or nothing approach to eating, which is why I think breaking down your nutrition into a ratio of protein, carbs, and fats seems to be the most beneficial way to approach nutrition. You have heard this as “Counting Macros”. This is not just something for elite athletes and body builders. I believe that anyone looking to live a healthy lifestyle can benefit from approaching nutrition in this way instead of arbitrarily cutting out food groups. It’s basically just eating a balanced diet that is customized to your fitness goals, activity level, and also to the types of sports or training in which you are involved, and will change on a daily basis. I will discuss this in future posts


  1. Foster GD, et al. A randomized trial of a low-carbohydrate diet for obesity. New England Journal of Medicine, 2003.
  2. Samaha FF, et al. A low-carbohydrate as compared with a low-fat diet in severe obesity.New England Journal of Medicine, 2003.
  3. Sondike SB, et al. Effects of a low-carbohydrate diet on weight loss and cardiovascular risk factor in overweight adolescents. The Journal of Pediatrics, 2003.
  4. Brehm BJ, 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.
  5. Yancy WS Jr, et al. A low-carbohydrate, ketogenic diet versus a low-fat diet to treat obesity and hyperlipidemia. Annals of Internal Medicine, 2004.
  6. Gardner CD, 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.
  7. Guldbrand, 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.
  8. M Mazidi, N Katsiki, D P Mikhailidis, M Banach, International Lipid Expert Panel (ILEP); P5409
    Low-carbohydrate diets and all-cause and cause-specific mortality: a population-based cohort study and pooling prospective studiesEuropean Heart Journal, Volume 39, Issue suppl_1, 1 August 2018.
  9. Siedelmann SB, et al “Dietary carbohydrate intake and mortality: A prospective cohort study and meta-analysis”The Lancet Public Health 2018;