Are carbohydrates really fattening? This one question has generated much acrimonious debate over the years. Some believe all that matters is the caloric content of food while others (yours truly, for instance) believe more is at play than simply the number of calories available. A recent study published in the New England Journal of Medicine (NEJM) provides some evidence that carbohydrates are indeed more fattening than fat or protein.
Many years ago, I was taking questions after giving a presentation to a large group of both doctors and laymen. A lady stood up and told me she always lost weight whenever she went on a low-carbohydrate diet, but she also always gained it back more quickly after going off such a diet than she did when she regained weight she had lost on a low-fat diet. As I listened to her question, my brain was busily formatting a smart-assed answer along the lines of, Well, that’s God’s way of telling you not to go off of your low-carb diet. But, looking out over the audience, I noticed a lot of heads nodding in agreement. I realized I needed a reasonable answer.
The truth was, I didn’t really think there was anything to it. I figured it was all in her imagination. I had heard such reports before, and I didn’t think much of them. But as I saw heads nodding throughout the auditorium, it reminded me of the old maxim that one report is anecdotal, but many similar anecdotes become data.
So, I thought about it quickly and concocted and answer on the spot. Most public speakers know the surest way to embarrassment is to engage in original thinking while in a public forum. Though I was in peril, I think in this case I came out okay.
I’ll go through my thought processes, as I remember them well.
My bias was/is that carbohydrates are vastly more fattening than the same number of fat or protein calories, so let’s assume that at the start.
A low-carb diet restricts carbohydrates, and so restricts the most fattening of the three macronutrients. If my assumption is true, then people on low-carb diets should lose more weight faster, which the majority of studies shows to be the case.
A low-fat diet restricts fat and secondarily protein, because most forms of high-quality protein contain fat. Both fat and protein are less fattening per kcal than carbohydrate. If this assumption is true, then people on low-fat diets that restrict fat and/or protein should lose less weight and lose it more slowly, which a majority of studies shows to be the case.
How does one cheat or go off a low-fat diet? By eating more fat and/or protein.
How does one cheat or go off a low-carb diet? By eating more carbohydrate.
Since – in my view – carbs are more fattening than fat and/or protein, it makes sense that adding the more fattening carbs to a low-carb diet would cause more weight gain than adding the less-fattening fat/protein to a low-fat diet.
Ergo, in going off of a low-carb diet one would pack on pounds more quickly as compared to going off a low-fat diet.
Seemed to make sense to me at the time, and it made sense to the audience.
I’ve thought about it a lot since then and have concluded that my on-the-spot thinking was probably correct. And I’ve used it since whenever asked the question.
But unlike the numerous studies showing the low-carb diet to be quicker and more effective for weight loss, there have been no studies (at least not that I’ve seen) showing that people who go off of low-carb diets regain faster.
A recent paper in the NEJM looked at what happens over the four years after subjects go off of different kinds of diets.
Subjects were studied on one of three diets over a two-year period. One group went low-carb, another low-fat and a third group followed a Mediterranean diet. The original two-year study, published in 2008, clearly showed the obvious advantage of a low-carb diet. But, at the time, the media kept misrepresenting what the study really showed, so I corrected the record here and here.
After completion of the study, the researchers kept in touch with 95 percent of the subjects and were able to gather data from them four years after the end of the original two-year study.
What the researchers found is what you would expect if carbs really are more fattening. The low-carb dieters gained more weight after going off their low-carb diets than did the low-fat dieters after going off theirs.
You can see from the charts below (click to enlarge) the outcome of several parameters.
First, in Chart A, you can see that the low-carbers both lost the most weight in the first study then regained the most, which provides some evidence in support of the notion that carbs are more fattening than fat and/or protein.
The study gets really interesting when you start looking at what happens to lipid levels when people go off of either low-fat or a low-carb diets.
Chart B looks at the LDL cholesterol to HDL cholesterol ratio. Low fat diets lower LDL cholesterol but they also lower HDL cholesterol levels, and, consequently, end up not driving a lot of change in this ratio. The LDL/HDL ratio typically improves slightly with low-fat dieting because there is generally a greater drop in the LDL cholesterol level than there is in the HDL level, driving the ratio down a bit. (Lower is better.)
Low-carb diets tend to keep the LDL levels the same or elevate them slightly (in some cases, though, low-carb diets can also lower LDL levels as well). And low-carb diets typically raise HDL levels. So when the LDL stays the same or goes up a little (or even falls) while the HDL goes up, the LDL/HDL ratio goes down.
As you can see in Chart B, both the low-carb diet and the low-fat diet brought about positive changes in the LDL/HDL ratio (the low-carb was slightly better), but look what happened after the subjects went off their respective diets. Those on the low-fat diet saw LDL/HDL ratios going the wrong way. They experienced a substantial increase in their LDL/HDL ratio. The low-carb dieters, on the other hand, found their LDL/HDL ratios refusing to budge much despite adding more carb to their diets.
Why did this happen? As far as I know, no one knows. But it would seem that two years on a low-carb diet appears to confer some protection against increases in the LDL/HDL ratio despite the increase in carb intake. And this protection lasts at least four years.
Looking at Chart C, we find some interesting corroborating data. It is well known that carbs tend to raise triglyceride levels while restricting carbs lowers them. Chart C shows that those on the low-fat diet didn’t experience much change in their triglyceride levels, which would be expected. Those on the low-carb diet saw substantial reductions. After the two year study period the original low-carbers shot their triglycerides up when they went back to eating carbs, which confirms the message that chowing down on carbs increases triglyceride levels.
And Chart D shows that reductions in total cholesterol brought about by low-carb dieting seem to be maintained despite the increase in carb intake after the end of the two year study. Once again, it appears that restricting carbs for a couple of years provides some protection for at least the next four.
So what does it all mean?
Well, I think one of the take home messages from this study is that following a low-carb diet for a couple of years brings about improvements in lipids that last for a least four more years – even if you go off the low-carb diet. Which, to my way of thinking at least, would be reason enough for following a low-carb diet instead of a low-fat one.
And it’s pretty clear that going off the low-carb diet will result in more weight gain than going off of a low-fat diet. Which would have to at least imply that carbohydrates are more fattening than are fat and protein. We can see from the length of this second follow-up – four years – we’re not talking about the immediate water gain that comes from going off a low-carb diet for just a few days, but the long-term weight gain.
I have to issue my standard caveat here. This is a single study. I don’t know of any others like it. So, repeat after me, we can’t draw certain conclusions from a single study like this. Other studies may come along and show differing results.
But having said that, this study along with the enormous mass of anecdotal data seems to indicate that weight gain is more rapid after bolting from a low-carb diet as compared to straying from a low-fat diet. If this proves to be true, then it really is indicative that carbohydrates are more fattening than fat or protein.
I would love to learn of your experiences in going off various kinds of diets. Did you gain more after a low-carb diet or after a low-fat diet? Let me hear from you in the comments.