The Science of Low-Carb Diets

Some Clues from the Research

Directly Above Shot Of Lunch Served On Table
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You've probably heard these things said:

"Any diet will work as long as you stick to it."

"It all comes down to lowering calories; it doesn't matter how you do it."

"Eating in moderation is the best way. Don't 'cut out a food group'."

All of these statements have at least a bit truth to them but are also misleading, and for many people have aspects that are outright wrong. Yet these "truths" are repeated continually.

Not only do we have quite a lot of evidence now that different diets work better for different people, we even have some very strong clues about who can most benefit from reducing the amount of carbohydrate they eat. 

Why doesn't this information show up in news reports or even research reports? There are a number of reasons, and after reading the research for many years I think two of the most important ones are:

  1. People often don't stay on the diet they were supposed to be following during many studies, especially large ones. So guess what? A bunch of people who didn't change their way of eating much didn't get much of a result, and the results of the "different" diet groups all looked very similar.
  2. Often the average change of the diet groups is similar, but within the group, there may be a lot of variation. So the people who lost various amounts of weight are averaged in with the people who didn't lose much weight or even gained. In this case, just talking about the group average loses a lot of information, yet this is what is most commonly done.

    In fact, the two may be linked.

    Searching for an Answer: The A-Z Diet Study

    The A-Z Diet Study, first published in 2007, compared four popular weight-loss diets with different amounts of carbohydrate. After 12 months, those on the Atkins Diet, on average, lost the most weight - 10.5 pounds. Although this was 2-3 times more than the other groups, it's obviously not a whole lot of weight.

    However, the researchers noticed something interesting that they did not discuss in the original report of the study. It turned out that there was a lot of variation within the groups - as much as a 66-pound difference from the least to the most weight lost. Some people even gained weight, and this was averaged in with those who lost weight. So what was the difference between the people who gained and the people who lost a lot? One thing that jumped out was how well people stuck to the diet they were assigned to. But why would some adhere to the diet better than others? The researchers had a hunch they wanted to test.

    They knew from other research that people who are insulin resistant may respond better to low-carb diets than low-fat diets (low-fat diets are almost always high in carbs). One of the differences that had been noted was the amount of hunger those people felt eating a low-carb vs high-carb diet. Could it be that people were not sticking to the low-fat diet because it wasn't working well for them? They decided to go back and re-analyze part of their original data, looking at the highest and lowest-carb diets (Ornish vs Atkins), and the people who were the most and least insulin resistant.

    What they found was fairly dramatic: The people who were the most insulin resistant basically did not stick to the Ornish diet but stuck to the Atkins diet just as well as their insulin-sensitive counterparts. On the other hand, people who were insulin-sensitive (the opposite of insulin resistant) adhered to and lost weight on both diets equally well. The researchers are now doing a more definitive study about this, but at this point conclude that, "advice for insulin-resistant individuals to follow a low-fat/high-carbohydrate diet for weight loss may actually undermine the success of insulin resistant individuals." and "it is possible that the growing population of insulin resistant adults might be more successful adhering, losing weight, and improving insulin levels when prescribed a low-carbohydrate diet."

    Note that they are not saying that people who are insulin-sensitive should not follow a low-carb diet, but that in the study those people did well on either a low-carb or low-fat diet, so they may have more options.

    More Evidence

    Generally, studies that look at the effects of diet on the signs of insulin resistance and metabolic syndrome (also called insulin resistance syndrome) find that low-carb diets are most effective at improving these signs. These include blood triglycerides, HDL cholesterol, blood pressure, blood glucose, waist/hip ratio, and blood insulin. Therefore, it has been proposed by some that people with signs of insulin resistance should try a low-carb diet as the first line of treatment. Although this is not yet a mainstream diet, the 2014 American Diabetes Association Nutritional Recommendations recognize low-carb diets as an option for weight loss and blood glucose control and mention that very low-fat diets (such as Ornish advocates) may be a bad idea for diabetics.

    The Bottom Line: Are you Insulin Resistant?

    Here are some signs that you may be insulin resistant (or have metabolic syndrome, which is closely related) and benefit from a low-carb diet. Except for the first one, none of these alone means you are insulin resistant or have metabolic syndrome, but more than two is pretty much the definition of metabolic syndrome. 

    • Type 2 Diabetes 
    • Prediabetes - high fasting blood sugar and/or poor glucose tolerance (high blood glucose after consuming significant carbohydrate)
    • BMI greater than 30 (see calculator)
    • Increased waist to hip ratio - different organizations have different criteria, but more than .9 for women or 1.0 for men would be a sign under any of the systems. 
    • High blood insulin
    • High blood pressure
    • Blood triglycerides over 150 mg/dL
    • HDL Cholesterol less than 40 mg/dL
    • Polycystic Ovarian Syndrome
    • The Endocrine Society has issued a statement including the warning that a fasting blood glucose of 89 or above could signal early damage to the insulin-producing cells of the pancreas (beta cells), a sign of impending problems with insulin.

    Also, any family history of diabetes and its related conditions can be a clue.

    This is actually good news in at least one sense. As someone told me a long time ago when I was depressed over finding out I'm gluten intolerant: "At least you have a condition that can be treated with diet." Also, many people find they get other health benefits from a low-carb diet. This could be the start of a whole new healthy life!

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