What You Should Know about Low Carb Diets: 3 Limitations of a Recent High-Profile Study on Weight Loss and Heart Disease

A few weeks ago, the New York Times published an article that highlighted a study that compared the effects of a low carbohydrate and low fat diet on weight loss and cardiovascular disease risk.  The results of this study drew significant media attention, and once again drew attention to the “what-should-i-be-eating-for-optimal-health” debate, which, if you’ve been following dietary trends over the past 20 years, has flip-flopped multiple times.

Still the question remains: Which diet is more optimal for weight loss and heart health? A diet low in carbohydrates or a diet low in fat?

Over the past few weeks, I’ve had a number of people email me with important questions.  The most common question was this: Based on the results of this study, should I eat more fat and protein?  This debate about low-carb vs. low-fat diets is a perfect example of the growing confusion about a simple and yet still unanswered question.

Study Design

The study, published in the Annals of Internal Medicine, enrolled extremely overweight individuals and divided them into two study groups.  One group was instructed to minimize fat intake while the other was instructed to minimize carbohydrate intake.  Neither group was given specific calorie goals, but both were provided with dietary guidance including recipes, shopping guides, meal planners and weekly nutrition consultations.  Subjects were instructed to refrain from changing their physical activity levels during the 12-month study.

Subject Characteristics

Average Body Weight: 213 + 28 lbs.

Average Body Mass Index (BMI): 35.4 (obese)

Average Body Fat Percentage: 40% + 10%


The purpose of the study was to investigate the following two questions:

  • Is a low-carb or low-fat diet is more successful at promoting weight loss?
  • Is a low-carb or low-fat diet better at promoting heart health?


The results of the study are summarized here:

Effects of Diet on Body Composition

  • At the end of the 12 month study, people in both groups lost weight, and subjects in the low-carb group lost more weight on average (12lbs) than the low-fat group (4lbs)
  • There was no difference in waist circumference between groups after 12 months

Effects of Diet on Markers of Cardiovascular Disease Risk

  • Total cholesterol was not different between groups at 12 months
  • LDL cholesterol was not different between groups at 12 months
  • HDL cholesterol was higher in the low-carb group at 12 months
  • Serum triglycerides were reduced in both groups at 12 months, with greater decreases in the low-carb group
  • There were no changes in blood pressure in either group at 12 months

Effects of Diet on Glucose Homeostasis

  • Plasma glucose levels did not differ between groups at 12 months

So while it may appear that the low-fat group experienced significant health benefits beyond the low-carb group, let’s take a closer look at the study to determine how to interpret these results properly.

Study Limitation #1: The Low Fat Diet was Not Low Fat

The two donut charts below represent the relative proportion of carbohydrates, protein and fat in the low-carb and low-fat diets used in this study.  Note that the low-fat diet still contained about 30% calories from fat, which is not technically considered a low-fat diet.  Maintaining a fat intake of less than 20% in most cases is considered low-fat.

Study Limitation #2: The Low Carb Diet Was Not Low Carb

In the donut charts below, you will also see that the low-carb diet contained about 34% calories from carbohydrate energy, equal to about 127 grams of carbohydrate on average.  Diets that are truly low in carbohydrate content maintain a carbohydrate maximum intake of 30-50 grams per day.  The low carbohydrate diet used in this study nearly tripled that guideline.

 Low-Carb low-fat

Study Limitation #3: What Type of Carbs Were They Eating?

Subjects in the low-fat group were instructed to eat more carbohydrates, although we know from previous studies that all carbohydrates are not created equal.  Carbohydrates from grains, breads, cereals, pasta and artificial sweeteners promote more inflammation than do carbohydrates from fruits, vegetables and legumes (1–4).

The effect of high glycemic index (GI) foods on plasma lipids and inflammation have been extensively documented.  Foods with a high GI and low fiber intake cause the following effects:

  • Increased LDL cholesterol (the bad cholesterol)
  • Decreased HDL cholesterol (the good cholesterol)
  • Increased triglycerides
  • Increased blood glucose
  • Increased inflammation

Many studies have shown that high GI carbohydrates such as glucose and fructose-sweetened beverages can significantly impair these metabolic markers, whereas low GI carbohydrates from fruits, vegetables, legumes and grains have antiinflammatory effects (1,5–8).

What You Should Know

This study is not the only study comparing low-fat and low-carb diets.  There have been hundreds of previous studies comparing low-fat and low-carb diets, this study is just the recipient of significant media coverage.

While some previous studies have come to similar conclusions as the current study, others have shown no difference between low-carb and low-fat diets on weight loss (9–11).  More importantly, though, these studies are purely academic and do not faithfully represent conditions of free-living humans in the real world.

In this study, both groups in this study lost only a modest amount of weight, and even with the weight loss are still in an unhealthy body weight and fat mass range.  The study participants reduced their body fat percentage from 40% to 39%.  In order to draw any significant conclusions about the efficacy of either diet, participants should have decreased their body fat percentage by 5-10% or more.

In addition, both groups lost weight in the first 3 months and plateaued or slightly increased weight for the next 9 months.  This tells us that weight loss can result from ANY form of dietary change in the short term.  Long-term weight loss requires a strategic approach.

This study demonstrates that focusing solely on the effect of fat and carbohydrate in isolation does not provide relevant information about what most people care about most: long-term and sustainable weight loss. What we do know from previous studies is that long-term sustainable weight loss is achieved by balancing energy in vs. energy out, and by maintaining a reduced intake of dietary fat in the long term(12,13).


The low-carb vs. low-fat debate presented in this paper is 100% academic. The study design does not teach us what we want to know, namely “What should I eat for long-term sustainable weight loss, improved body composition and high energy?”

My recommendation is simple: forget about this debate and instead focus on eating a diet containing whole, unprocessed plants.

Significantly increasing your consumption of fruits and vegetables helps you achieve and maintain a healthy weight, and promotes significant health benefits beyond what can be described in a controlled research setting.

And remember, before making changes to your own diet it is important to thoroughly investigate the pros and cons of limiting either fat or carbohydrate intake.  There are benefits to restricting both carbohydrate and fat intake, and when making decisions about your own personal diet, be sure to read extensively and see the help of a nutrition expert in order to make educated decisions for your long-term health.


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9.         Fung TT, van Dam RM, Hankinson SE, Stampfer M, Willett WC, Hu FB. Low-carbohydrate diets and all-cause and cause-specific mortality: Two cohort Studies. Ann Intern Med. 2010 Sep 7;153(5):289–98.

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About The Author

Cyrus Khambatta

Diagnosed with type 1 diabetes at the age of 22, I have spent over a decade learning the fundamentals of nutrition at the doctorate level. My goal is to share my knowledge of practical nutrition and fitness with people with prediabetes, type 1 and type 2 diabetes. Diabetes is an OPPORTUNITY to attain excellent health. Reversing the effects of insulin resistance can be a fun and enjoyable process if the right system is in place. That's why I've spent over 10 years developing a rock solid system that can minimize blood glucose variability and insulin resistance.

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