Low-carbohydrate diets increase LDL: debunking the myth
This week sees the publication of yet another study showing the superiority of the low-carbohydrate diet as compared to the low-fat diet. This study, published in the prestigious American Journal of Clinical Nutrition, demonstrates that subjects following the low-carb diet experience a decrease in triglyceride levels and an increase in HDL-cholesterol (HDL) levels; and that these changes are accompanied by a minor increase in LDL-cholesterol (LDL), which prompts the authors to issue a caveat.
Yes, although just about all the parameters that lipophobes worry about improved with the low-carb diet, the small increase in LDL has caused great concern and has prompted the authors to gravely announce that this small increase is troublesome and should be monitored closely in anyone who may be at risk for heart disease. Since most people who go on low-carb diets do so to deal with obesity issues, and since obesity is a risk factor for heart disease, it would appear that this small increase in LDL often seen in those following a low-carb diet could put these dieters at risk. Does it? We’ll see.
Let’s take a look at the study. But before we do, let’s digress for just a bit and look at low-carb diet studies in general.
As we’ve discussed in these pages before, there are a couple of ways to do dietary studies in which on diet is compared to another. You can compare a low-carb diet to a low-fat diet in a way that reflects what happens in real life. For example, you could randomize your study subjects into two groups, then give those in one group a low-carb diet book (Protein Power, maybe) and those in the other a low-fat diet book (an Ornish or McDougal book, perhaps). You would instruct both groups to follow their respective diets and come back periodically for evaluation. When these kinds of studies are done, the low-carb diet invariably brings about more weight loss and greater changes for the better in just about all parameters. But the folks who are proponents of low-fat diet cry foul. Why? Because in virtually all of these studies the subjects on the low-carb diet consume fewer calories than those on the low-fat diets. Lower-carb, higher-fat diets are satisfying, and it has been shown over and over that those following such diets actually consume fewer calories while still feeling full than do those following ad libitum (eat all you want) low-fat diets.
So, the low-fatters attribute all the improvement in those on the low-carb diets as simply a result of their lower caloric intake.
If you want to eliminate this caloric-deficit difference from your study, then you design a protocol in which calories are the same in both the low-carb and the low-fat arms of the study. This strays from the real-life way of looking at what is likely to happen when people buy diet books and follow them, but it does offer the advantage of getting rid of the calorie issue.
In these kinds of studies you randomize your subjects into either a low-carb or a low-fat diet group and put both groups on the same number of calories. At the end of your study, you can see the differences between the two diets – if any – that are brought about without calories being an issue.
The study under our consideration today is of the latter type; it’s one in which both groups were kept on an equal number of calories, a so-called isocaloric diet.
Here’s the setup for the study titled Long-term effects of a very-low-carbohydrate weight loss diet compared with an isocaloric low-fat diet after 12 mo.
The researchers recruited 118 subjects who had abdominal obesity and at least one other metabolic syndrome risk factor and randomized them to either a low-carb or a low-fat diet for one year.
The diets were designed to be isocaloric with moderate energy restriction (≈6000 kJ/d [1433 kcal] for women, ≈7000 kJ/d [1672 kcal] for men). The planned macronutrient profile of the LC diet was 4% of total energy as carbohydrate, 35% as protein, 61% as total fat (20% saturated fat) with the objective to restrict carbohydrate intake to <20 g/d for the first 8 wk and to <40g/d (with the inclusion of an approved 20-g carbohydrate exchange) for the remainder of the study. The target profile for the LF diet was 46% of total energy as carbohydrate, 24% as protein, and 30% as total fat with the objective to restrict saturated fat intake to <10 g/d and <8% of total energy, with the inclusion of an approved food exchange (equivalent to the energy content of 20g of carbohydrate;) between weeks 9 and 52, so that the diets remained isocaloric.
Sixty nine subjects completed the study, and, fortunately, all the results reported in the paper were for the 69 completers, so we don’t have to worry about data contamination we would have gotten had the researchers done an intention-to-treat analysis. We know how the people fared who actually hung in there for the entire study period, which is what we want to know.
And how did they fare?
Those on the low-carb diet lost 26 percent more weight than those on the low-fat diet (14.5 kg vs 11.5 kg), but the difference wasn’t statistically significant. As you can see from the graph below of the weight loss between the two groups over time, the difference was widening, and we can extrapolate that the difference would have become statistically significant had the study gone on longer, but we can’t say for sure.
As for the other parameters, blood pressure, glucose, insulin, insulin resistance and C-reactive protein were the same for both groups. There was a difference in lipid outcomes, however.
The LC [low-carbohydrate] diet also provided greater improvements in triglycerides and HDL cholesterol than did the LF [low-fat] diet, which occurred independently of differences in energy intake and weight loss. This finding is consistent with those of long-term ad libitum studies. High triglyceride and low HDL-cholesterol concentrations are 2 of the MS risk factors, a syndrome that is associated with an increased risk of type 2 diabetes and CVD. Elevated triglyceride concentrations have also been identified as an independent CVD risk factor, and the triglyceride:HDL cholesterol ratio is considered a strong predictor of future cardiac events and is a surrogate measure of insulin resistance. Our data show that the triglyceride:HDL cholesterol ratio was halved after the LC diet and was approximately double the improvement observed with the LF diet. A recent review suggests that biological markers typically associated with the MS are those improved by carbohydrate restriction, which suggests that LC diets may offer the greatest clinical benefits for overweight populations who are insulin resistant and have several metabolic risk factors.
So far, so good. But now the other shoe is ready to drop.
Whereas the LC diet improved a range of cardiometabolic risk factors, greater increases in total and LDL cholesterol also occurred. Other studies that compared LC and LF diets reported similar findings, although the overall magnitude of the differences was smaller: 0.60 and 0.20 mmol/L in favor of the LF diet.
Let’s see how much the total cholesterol and LDL changed.
Those in the low-fat group started with an average total cholesterol of 212 mg/dl (5.5 mmol/L) and ended up a year later at same number. These same subjects also started out with average LDL levels of 131 mg/dl (3.4 mmol/L) and ended up the same at the end of the study. The low-carb dieters began the study with average total cholesterol levels of 209 mg/dl (5.4 mmol/L) and ended the study a year later with average total cholesterol levels of 232 mg/dl (6.0 mmol/L). Their average LDL levels started at 124 mg/dl (3.2 mmol/L) and ended up at 147 mg/dl (3.8 mmol/L).
The authors of this study bestow great significance on this fairly minor increase in LDL levels in those subjects on the low-carb diet. In their summary of the results of this study, they list the many benefits of the low-carb diet, then end on an ominous note:
However, these potential benefits may be counteracted by the detrimental effects of an increase in LDL cholesterol, which should be monitored…
The abstract of the study echoes this warning.
However, the increase in LDL cholesterol with the LC diet suggests that this measure should be monitored.
It was my impression that the tone of the authors was one of a little foreboding. Kind of a ‘this looks too good to be true, and, hey, look at those LDL levels; it is too good to be true’ aura about it. But is it too good to be true? Is the rise in LDL seen in most low-carb diets the hidden stinger? Is what all the lipophobes say true? You know, the old ‘Well you may lose weight on those diets, but you’ll clog your arteries at the same time.’
It’s all hogwash, of course, but before we get to the heart of the explanation as to why, let me remind you that numerous studies have shown that whenever subjects go on low-carb diets, they end up increasing the size of their LDL particles. Large, fluffy LDL particles are not only harmless, but may be protective. If they are protective, what’s wrong with having a bit more of them?
At the same time, numerous studies have shown that low-fat diets usually decrease LDL levels, but do so while reducing the particle size. Followers of such diets end up with lower levels of LDL made of smaller, denser, more atherogenic particles, which, in my mind, isn’t a good trade off.
The authors of our paper acknowledge this fact and cite some of this research, but they are still fixated – as are most lipophobes – on LDL levels. They just can’t get their heads around the notion that there is more to cardiovascular risk and health than LDL-cholesterol.
Since these researchers placed so much emphasis on LDL levels in their interpretation of all the data from their study, I got to wondering how they measured LDL levels. I looked in the Methods section of their paper and found the following:
Plasma glucose, C-reactive protein, serum lipids, and apolipoprotein B (apo B) were also measured by using standard methods (11).
The #11, of course, means that the description was in another paper that I had to go to the trouble of looking up. I always find it annoying when authors do this when they could just as easily stick a short paragraph in their paper and save people who really want to read it critically a lot of trouble.
Tracking down the other paper in the Journal of the American College of Cardiology, I found the following:
The LDL-C was calculated according to the method described by Friedewald et al.
What this means is that the researchers did not measure LDL levels directly in their study subjects, but calculated them using the Friedewald equation.
For reasons we don’t need to go into here, LDL is fairly difficult (as compared to total cholesterol and HDL) to measure. It can be done, but it’s expensive. So instead of measuring it directly, most labs calculate it based on an equation derived by William Friedewald and others in 1972.
Friedewald realized that it was pretty simple to measure total cholesterol, HDL-cholesterol and triglycerides. He knew that total cholesterol was the sum of all the various subfractions of cholesterol, which can be presented by the following equation:
Total cholesterol = HDL-cholesterol + LDL-cholesterol + VLDL-cholesterol
Rearranging this equation to solve for LDL gives us this one.
LDL = Total cholesterol – HLD – VLDL
Friedewald knew that it was easy to measure total cholesterol and HDL but difficult to measure the others. His insight was that the triglyceride level if divided by five could give a close approximation of VLDL. In running his experiments he also realized that this relationship held only if triglyceride levels were 400 mg/dl or under. If they were over this, all bets were off.
So, Friedewald substituted triglycerides (TGL) divided by 5 for VLDL in the above equations, giving us the so-called Friedewald equation for calculating LDL.
LDL = Total cholesterol – HDL – TGL/5
And this is how it is still done in labs all over the world 27 years after Friedewald’s paper. If you’ve had a lab report showing an LDL figure, I can guarantee it was calculated by the Freidewald equation and not measured directly.
What’s wrong with this if it works? Nothing. If it works. Problem is, it doesn’t always work. Friedewald himself found that in subjects with triglyceride levels greater than 400 mg/dl the equation didn’t hold. Anyone reading this who has had a lipid test showing triglycerides greater than 400 will have note on their lab report saying that LDL couldn’t be calculated because triglycerides were too high.
I’ve always thought the same held true for triglycerides under 100 mg/dl, which would apply to almost everyone who sticks to a low-carb diet for any length of time. Triglyceride levels of 40-90 mg/dl are not uncommon, and are, in fact, typical. When Friedewald did his work, the triglyceride levels were mainly up in the 150 – 250 mg/dl range, and in this range his equations match pretty well to directly measured LDL levels, but all bets are off with triglycerides above 400 mg/dl and, I suspect, triglyceride levels below 100 mg/dl. MD and I did find this ourselves in a few patients that we did direct LDL measurements on in our practice.
A paper published a few years ago in a pathology journal corroborating what we found. (Full text here.)
This paper is basically a case presentation of a 63-year-old man with a total cholesterol level of 263 (all results in mg/dl), an HDL of 85, a triglyceride level of 42, and an LDL level of 170. The LDL level was, of course, calculated using the Friedewald equation.
For some unexplained reason the authors of this paper decided to repeat the lab results and got the same readings. They then wondered if his very low triglyceride readings might be having an effect, so they measured his LDL levels directly and found that instead of the 170 predicted by the Freidewald equation, his actual LDL levels were only 126.
More recently a paper appeared in – of all places – the Archives of Iranian Medicine showing the same phenomenon. These authors tested 115 subjects with low triglyceride levels. You can get the full text of the paper, but a line in the abstract says it all:
Statistical analysis showed that when triglyceride is <100 mg/dl, calculated low-density lipoprotein cholesterol [LDL] is significantly overestimated (average :12.17 mg/dL or 0.31 mmol/L), whereas when triglyceride is between 150 and 300 mg/dL no significant difference between calculated and measured low-density lipoprotein cholesterol is observed.
The authors of this paper derived their own equation to be used in lieu of the Friedewald equation when the triglyceride levels are below 100 mg/dl. I suspect that if we were to apply this equation to the labs of the 33 subjects who finished the low-carb arm of the study we started out discussing in this post, whose average triglyceride levels were under 100, the LDL levels would have averaged much lower than the 147 mg/dl they were calculated to be by the Friedewald equation. If you subtract the 12.17 mg/dl that the Iranian paper estimates as the difference from the average triglycleride levels (an admittedly extremely unscientific and non-statistically valid way to do it), you find that the average drops to 135 mg/dl, which I doubt is significantly different than the 131 average of the low-fat dieters. If you did it the right way – subject by subject and then average – I suspect it would be greater yet.
The moral of this story is that if you have been following a low-carb diet and your triglycerides are low (or if your triglycerides are just low) and your LDL reading comes out a little high – or even a lot high, don’t let anyone mule you into going on a statin or undergoing any therapy for an elevated LDL. Demand to have a direct measurement of your LDL done. Or if you get an insurance physical and your triglycerides are low and your LDL up a little, fight to get a direct measurement so they don’t stick you with higher premiums because they think you’ve got an increased risk for heart disease.
What we do know based on the work of many is that low-carb diets change LDL particles to the large, fluffy, harmless variety. Thanks to these other papers we also know that the LDL levels so many people end up with on their lab reports after being on low-carb diets for a while are artificially high.
Now when you hear people say that low-carb diets may help you lose weight but run your LDL levels up and increase your risk for heart disease, you’ll know this is just so much gibberish. Sadly, your doctor will probably spout the same thing, and it will be up to you – who after reading this post will know more about this point than 99.9 percent of doctors practicing today – to educate your trained professional.
And if you are a researcher studying the effect of the low-carb diet on LDL, for crying out loud, hit your grant up for the extra few bucks it takes to get LDL cholesterol measured directly in your subjects so you won’t be in the embarassing position of having your data become worthless.