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Low-Fat vs. Low-Carb Weight Loss Debate: Which Diet Comes Out On Top?

If you’re reading this article, then you’re no doubt well aware of the countless diets claiming to be the magic bullet for permanent weight loss. The lasting low-fat dogma of the 80’s and 90’s was met with the lower-carb South Beach and Atkins crazes that have continued with the latest fat-loss darling – ketogenic diets. Despite there being a multitude of other markers of good health beyond just the number of pounds lost, the number on the scale continues to be the main marker of diet success, and with that continues the obsession with finding the perfect diet.

We at Kion stress the importance of customizing your diet to fit your individual needs, and we give you all of the tools necessary to do just that in the article Why Diets Fail & How To Determine The Perfect Diet For You, but we also recognize the need for less involved and more immediately applicable dietary programs and scientific research around their effectiveness.

The latest study that aimed to compare the success of low-fat versus low-carb dietary approaches is notable because it overcame the common hurdles that prevent diet research from being relevant.

Traditionally, some of the bigger issues with nutrition studies comparing diets are that they are:

  • Too short in duration
  • Using a sample size that is not big enough
  • Not enforcing the prescribed diet

The study that was just published, 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, released by Stanford and costing a cool $8 million, aimed to take care of these issues by being:

  • 12 months long
  • Enlisting 600+ participants
  • Adding 22 nutrition coaching sessions with a multi-pass 24-hour dietary recall

Furthermore, the focus of the study was to test the potential link between genotype pattern and diet type for weight loss success, going beyond the conventional BMI and pounds lost data.

Why Was This Study Conducted?

Previous studies comparing a diet’s macronutrient composition on weight loss have continually produced conflicting results, with some studies concluding that a low-carb, Atkins approach leads to greater weight loss, while others indicate that calorie restriction causes weight loss, regardless of the amount of fat or carbs.

These varying and inconclusive results are not well understood by scientists, leading to hypotheses that certain genotypes or insulin sensitivity may be better predictors of dietary weight loss success.

This study included genetic markers and insulin production as a way to predict successful weight loss from a low-fat or low-carb diet.

What Was The Research Testing?

This study was a randomized clinical trial (RCT).

The primary purpose of the study was to test a potential link between an individual’s genotype and diet type for weight loss.

All participants were screened for 15 genotypes, including 5 which arguably would do better on a low-fat diet and 9 that would do better on a low-carb diet.

Furthermore, the secondary purpose was to test a potential link between insulin secretion and diet type. At the start of the trial, and at months 3, 6, and 12 months, all participants completed an oral glucose tolerance test (OGTT) to measure insulin production.

Other measurements included body composition (measured by DEXA), cholesterol levels, blood pressure, fasting glucose, insulin levels, resting energy expenditure, and total energy expenditure.

While blinded studies are ideal, it is near impossible to have individuals consume food they have no knowledge on, much less for 12 months in duration.

What Did The Research Involve?

The study enrolled 609 participants (236 males and 346 premenopausal females) free of major health conditions. The average BMI was 33 (obese), and the average age was 40 years old.

Over the course the of the study, each subject was told to attend 22 dietary counseling sessions with a dietitian.

The low-carb group was initially told to target 20 g of carbs per day and the low-fat group was told to target 20 g of fat per day for the first two months, after which they were instructed to adjust these amounts to a level they could maintain with their lifestyle.

The goal here was to also test the sustainability and adherence to these diets. As such, the participants were strongly encouraged to consume high-quality whole foods and drinks. They were told to maximize vegetable intake and minimize added sugars, refined flours, and trans fats. Furthermore, while the goal was to focus on either a low-carb or low-fat diet, participants were told to add the other macronutrient to find the right balance for themselves.

While a small minority were able to achieve a low enough intake of carbs to achieve ketosis, that was not the goal of this study, so the data really reflects that of a low-carb diet.

Throughout the trial, twelve random and unannounced multi-pass 24-hour dietary recalls were given where the participants were asked to recall everything they ate and drank over the last 24 hours.

Lab results were also taken at months 3, 6, and 12 during the study.

What Were The Results?

At the end of the study, 481 participants completed the study. This 21% dropout rate is not unexpected for a trial this long.

While the two groups had similar diets before the study started, there were significant differences at months 3, 6, and 12 when it came to carbs, fat, fiber, and added sugars consumed. Both groups added additional fat and carbs to their diet after the 2-month mark, and by month 3, the low-fat group was consuming an average of 42 g of fat per day, and the low-carb group was consuming an average of 96.6 g of carbs per day.

Via’s in-depth analysis, you can see the change over time in Figure 1:

You can also see each individual’s weight loss/gain at the end of 12 months via Figure 2:

If we were to quickly summarize the basic results:

  • Weight loss between the low-carb (LC) and low-fat (LF) diet were similar
  • Fat loss between LC and LF were similar
  • The different genotypes had no difference in weight loss
  • Insulin secretion made no difference in weight loss
  • Cholesterol:
    • Low-Fat:
      HDL: ↑ (+0.40)
      LDL: ↓ (-2.12)
      Triglycerides: ↓ (-9.95)
    • Low-Carb:
      HDL: ↑ (+2.64)
      LDL: ↑ (+3.62)
      Triglycerides: ↓ (-28.20)
  • There was no difference in resting energy expenditure between the two groups
  • By the end of 12 months, it had decreased for both groups, likely as a consequence of weight loss
  • 10% of each group saw improvements in their metabolic syndrome

What Should I Take Away From This?

The primary hypothesis was to test 15 different gene patterns and insulin levels on weight loss; this study found no difference between the low-fat or low-carb diet when attempting to achieve weight loss.

The real takeaway was that by focusing on consuming whole foods, people ate less. Adherence to a strict low-fat or low-carb diet was incredibly difficult. Furthermore, via Figure 2 above, you can see that while some individuals lost over 25 kg (55 lb), others gained as much as 10 kg (20 lb).

As such, the overall conclusion would be that the best diet is the one you can stick to: choose an eating style that fits your lifestyle and food preferences, and one that you can stick to indefinitely.


Although we can’t argue with the logic of choosing a diet that fits your preferences and lifestyle, we still recommend that you focus on measuring and tracking health parameters that go beyond weight loss, such as inflammation, cholesterol particle count, blood glucose and insulin, vitamin and mineral levels, blood pressure and other important variables. Weight loss alone does not make for a reliable indicator of whether a diet is working for you if you’re looking to optimize health.

To stay up to date on the latest health research and to gain access to one of the most comprehensive exercise, diet, and supplement resources, subscribe to’s monthly publication The Examine Research Digest. It’s critical that one look at the full study, not just the quick headlines.

What do you make of this study? Leave your comments below and let the debate rage on!

7 thoughts on “Low-Fat vs. Low-Carb Weight Loss Debate: Which Diet Comes Out On Top?

  1. Carbs will result in heart disease. This reaseach was flawed. Niether diet was either low carb or low fat. No reliable conclusions can be drawn from it. When you understand how raised blood sugar following a meal damages the protective lining that stops cholesterol build up in the arteries, completely disrupting it for up to 12 hours, you can easily see high carbs/sugar is long term deadly to you.

  2. Definitions define studies and in this study the definition of low-carb was ultimately up to each individual’s application. As a result the low-carb participants defined low-carb as an averaged definition of 96.6 g of carbs per day (which is statistically meaningless not knowing the variance). Therefore this study’s definition of “low-carb” has absolutely no relevance to a Keto diet. At best this is a moderate carb analysis, NOT a low-carb analysis, which makes it once again meaningless for analyzing ketosis.

  3. Some doctors believe the most important cholesterol measure to look at is the Triglycerides to HDL ratio. Going by that, it looks like the Low Carb group had the greatest swing toward a better ratio.

  4. I lost 60 pounds in about nine months by not eating before noon, extending my daily fasting duration to 18 hours. My purpose in doing this was to re-establish sensory connection to my daily metabolic cycle. I am never hungry in the morning or after dark, so this is a no-brainer. For break-fast, I had a shake centered around whey, usually with whole fat not homogenized grass fed milk, coconut and grapeseed oils, raw oatmeal for bulk, acidophilus in some form, frequently Chia, Quinoa, chocolate powder to cut through the sweetener taste added to the whey, coconut flour. I never experienced hypoglycemia or high blood sugar narcosis. Also, my ‘hidden’ hunger went away. Because I was living in the woods out of the back of my truck, cooking was a chore. I frequently had a glass of vegetable juice and dried figs for a midafternoon break. For supper, around 4 when I had hunger pangs, which I hadn’t felt in years, I would occasionally cook meat, or eat canned fish with crackers, or if I felt the need, boiled potatoes or sweet potatoes slathered in butter. My doctor was amazed at my blood tests during my semi-annual, especially the drop in sugar from diabetic to a healthy level. There are a lot of other positive observations, but space precludes my writing on about this.
    The real reason I am writing you is that I read that the tongue is an information ‘sending unit’ to the digestive unit and the rest of the metabolic complex. This is why chewing your food is so important. BTW, my diet contributed to my eating slowly rather than wolfing down food, which I had done my whole life. I can’t stand any more sweetened and ‘flavored’ protein mix. I wonder if the sweet flavor interferes with ketosis, and causes too much insulin production, such as diet drinks do.

  5. It does not appear that either group came even close to the test requirements for LC (20gm) or LF (20gm) according to the graphs. So the LC group was not LC, nor was the LF group LF. This study, like most others, does not appear to meet the test requirements for LC or LF so it would seem difficult to derive conclusions that show the study found anything related to the original goal. It isn’t surprising the weight change graphs are almost identical – they are both restricted calorie diets (-700 cal/day) without an increase in fat (LC) or carbs (LF) to make up for the calorie reduction.

    1. I agree with this comment: I’ve noticed that many of the so-called low-carbohydrate diets studied are not low enough to truly achieve and maintain ketosis. I just spent an hour and a half perusing and wondered if any of them were even measuring whether or not the participants were in a ketogenic state. None seem to mention it. I HAVE read that different people will need a different level of carbohydrate intake to reach this, for example, postmenopausal women often have to keep carb levels below 25 or even 20 grams/day.

      I’m puzzled — if a lay person can find this material, why do the studies not insure that participants are truly reaching a ketogenic state?

  6. I tried the Keto diet in order to lose weight, get rid of gluten, and to increase my veggie intake. But I felt so restricted and did not feel well at all without carbs. I still try not to eat gluten but use organic rice, and other healthier carbs instead, and I’m eating more veggies. I feel much better but have a long way to go to get to my goal weight.

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