23 September 2014 New Data on Low Carb vs Low Fat Diets

The Low Carb vs Low Fat diet debate just never goes away. And for good reason. It’s always possible to design a study that can be interpreted as favoring one over the other. Even when the study is designed to be neutral, critics will find some design flaw that biased the results. Take Away Message: definitive proof that one or the other is the “best” diet is not going to happen.

Yet researchers keep on trying. Yet another study published this summer seemed to show that low carb was more effective than low fat for weight loss and reduced heart disease risk. But is that really what the data said?

The study went like this:

  • The subjects were obese men and women, who didn’t have existing medical problems like Type 2 diabetes or heart disease.
  • They were divided into two groups. One group was put on a low carb (less than 40 grams/day) diet, one a low fat (less than 30% of calories) diet.
  • The subjects were not told to cut calories, just to stick to the limits on carbs or fat.
  • They were assessed periodically throughout the 12 month study for weight, blood pressure, waist circumference and lab values for heart disease risk.
  • Diet was assessed periodically and analyzed for calories, fat, protein and many other nutrients.

At the end of 12 months, this is what the researchers found:

  1. Everyone cut calories. The low carb group cut calories slightly more than the low fat group.
  2. Everyone lost weight. The low carb group lost somewhat more.
  3. The low carb group lost more body fat.
  4. While both groups saw improvement in many heart disease risk factors, the low carb group saw somewhat more improvement.

Journalists and dedicated low carb dieters were quick to tout these results as proving that low carb was more effective. But there are some problems with that conclusion:

  • The low carb dieters ate fewer calories on average, so no surprise they lost slightly more weight. While the researchers speculated that a low carb diet might have some metabolic effect that enhanced weight loss, beyond the effect of calorie deficit, they didn’t have any actual evidence of that.
  • The low carb dieters had more reduction of heart disease risk factors. But they lost more weight, and more of that weight loss was body fat. At the end of the day, loss of weight/excess body fat impacts heart disease risk factors.

But there are two things that make these results a bit more interesting:

  1. The low carb group ate fewer calories and less than the low fat group, even though they were not told to cut calories. Is there something about cutting carbohydrates that makes it easier to stick to the plan?
  2. The low carb group lost more body fat mass. This isn’t explained completely by simple calorie deficit.

One potential plus for low (or lower) carb for weight loss is that all those tempting, high carb, high calorie foods that are to easy to overeat are not options. And for some dieters, restricted choice is helpful. Many foods are off the table, literally. If you can’t have them, you can’t over-eat them. The list can range from reasonable foods like pasta, potatoes and cereal to ice cream, pastries, cookies and candy.

One major drawback: Food choices are so limited that you could end up with nutritional deficiencies if you don’t eat enough of the nutrient dense foods like vegetables.

Another major drawback: it’s hard (maybe impossible) to stick to low carb indefinitely. Eventually higher carb foods start sneaking back into your food choices. If the low carb diet didn’t teach you anything about portion control or mindful eating, you are likely to go right back to over-indulging on those foods, regaining weight. The study above didn’t follow the subjects for more than 12 months, and by the end of the study everyone was eating more than at the beginning. Calorie creep.

Eventually that’s the problem with every weight loss diet: calorie creep and falling off the diet wagon. A recent analysis of almost 50 diet studies concluded that it's not the specific diet, it’s the sticking-to-it that counts. If you can’t stick to a diet, then it’s not going to work long term at keeping the weight off. According to the study authors, key components of long term diet success: exercise and smart food choices. Not low carb; not low fat.

Donna P. Feldman MS RDN

Nutrition journalist at Radio Nutrition

Co-host: Walk Talk Nutrition podcast.

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Disclaimer: The information provided here does not constitute medical advice. If you are seeking medical advice, please visit your healthcare provider or medical professional.

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Meal Planning & Diets/Paleo & Low Carb

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