Two posts ago, we made the rounds of the commonly measured blood lipids (total cholesterol, LDL, HDL, triglycerides) and how they associate with cardiac risk. It's important to keep in mind that many things associate with cardiac risk, not just blood lipids. For example, men with low serum vitamin D are at a 2.4-fold greater risk of heart attack than men with higher D levels. That alone is roughly equivalent to the predictive power of the blood lipids you get measured at the doctor's office. Coronary calcium scans (a measure of blood vessel calcification) also associate with cardiac risk better than the most commonly measured blood lipids.
Lipoproteins Can be Subdivided into Several Subcategories
In the continual search for better measures of cardiac risk, researchers in the 1980s decided to break down lipoprotein particles into sub-categories. One of these researchers is Dr. Ronald M. Krauss. Krauss published extensively on the association between lipoprotein size and cardiac risk, eventually concluding (source): The plasma lipoprotein profile accompanying a preponderance of small, dense LDL particles (specifically LDL-III) is associated with up to a threefold increase in the susceptibility of developing [coronary artery disease]. This has been demonstrated in case-control studies of myocardial infarction and angiographically documented coronary disease.
Krauss found that small, dense LDL (sdLDL) doesn't travel alone: it typically comes along with low HDL and high triglycerides*. He called this combination of factors "lipoprotein pattern B"; its opposite is "lipoprotein pattern A": large, buoyant LDL, high HDL and low triglycerides. Incidentally, low HDL and high triglycerides are hallmarks of the metabolic syndrome, the quintessential modern metabolic disorder.
Krauss and his colleagues went on to hypothesize that sdLDL promotes atherosclerosis because of its ability to penetrate the artery wall more easily than large LDL. He and others subsequently showed that sdLDL are also more prone to oxidation than large LDL (1, 2).
Diet Affects LDL Subcategories
The next step in Krauss's research was to see how diet affects lipoprotein patterns. In 1994, he published a study comparing the effects of a low-fat (24%), high-carbohydrate (56%) diet to a "high-fat" (46%), "low-carbohydrate" (34%) diet on lipoprotein patterns. The high-fat diet also happened to be high in saturated fat-- 18% of calories. He found that (quote source): Out of the 87 men with pattern A on the high-fat diet, 36 converted to pattern B on the low-fat diet... Taken together, these results indicate that in the majority of men, the reduction in LDL cholesterol seen on a low-fat, high-carbohydrate diet is mainly because of a shift from larger, more cholesterol-enriched LDL to smaller, cholesterol-depleted LDL [sdLDL].
In other words, in the majority of people, high-carbohydrate diets lower LDL cholesterol not by decreasing LDL particle count (which might be good), but by decreasing LDL size and increasing sdLDL (probably not good). This has been shown repeatedly, including with a 10% fat diet and in children. However, in people who already exhibit pattern B, reducing fat does reduce LDL particle number. Keep in mind that the majority of carbohydrate in modern America comes from wheat and sugar.
Krauss then specifically explored the effect of saturated fat on LDL size (free full text). He re-analyzed the data from the study above, and found that: In summary, the present study showed that changes in dietary saturated fat are associated with changes in LDL subclasses in healthy men. An increase in saturated fat, and in particular, myristic acid [as well as palmitic acid], was associated with increases in larger LDL particles (and decreases in smaller LDL particles). LDL particle diameter and peak flotation rate [density] were also positively associated with saturated fat, indicating shifts in LDL-particle distribution toward larger, cholesterol-enriched LDL.
Participants who ate the most saturated fat had the largest LDL, and vice versa. Kudos to Dr. Krauss for publishing these provocative data. It's not an isolated finding. He noted in 1994 that: Cross-sectional population analyses have suggested an association between reduced LDL particle size and relatively reduced dietary animal-fat intake, and increased consumption of carbohydrates.
Diet Affects HDL Subcategories
Krauss also tested the effect of his dietary intervention on HDL. Several studies have found that the largest HDL particles, HDL2b, associate most strongly with HDL's protective effects (more HDL2b = fewer heart attacks). Compared to the diet high in total fat and saturated fat, the low-fat diet decreased HDL2b significantly. A separate study found that the effect persists at one year. Berglund et al. independently confirmed the finding using the low-fat American Heart Association diet in men and women of diverse racial backgrounds. Here's what they had to say about it:
The results indicate that dietary changes suggested to be prudent for a large segment of the population will primarily affect [i.e., reduce] the concentrations of the most prominent antiatherogenic [anti-heart attack] HDL subpopulation.
Saturated and omega-3 fats selectively increase large HDL. Dr. B. G. of Animal Pharm has written about this a number of times.
Wrapping it Up
Contrary to the simplistic idea that saturated fat increases LDL and thus cardiac risk, total fat and saturated fat have a complex influence on blood lipids, the net effect of which is unclear, but is associated with a lower risk of heart attacks. These blood lipid changes persist for at least one year, so they may represent a long-term effect. It's important to remember that the primary sources of carbohydrate in the modern Western diet are wheat and sugar. Are the blood lipid patterns that associate with heart attack risk in Western countries partially acting as markers of wheat and sugar intake?
* This is why you may read that small, dense LDL is not an "independent predictor" of heart attack risk. Since it travels along with a particular pattern of HDL and triglycerides, in most studies it does not give information on cardiac risk beyond what you can get by measuring other lipoproteins.
The Multiple Risk Factor Intervention trial was a very large controlled diet trial conducted in the 1980s. It involved an initial phase in which investigators screened over 350,000 men age 35-57 for cardiovascular risk factors including total blood cholesterol. 12,866 participants with major cardiovascular risk factors were selected for the diet intervention trial, while the rest were followed for six years. I discussed the intervention trial here.During the six years of the observational arm of MRFIT, investigators kept track of deaths in the patients they had screened. They compared the occurrence of deaths from multiple causes to the blood cholesterol values they had measured at the beginning of the study. Here's a graph of the results (source):
Click on the graph for a larger image. Coronary heart disease does indeed rise with increasing total cholesterol in American men of this age group. But total mortality is nearly as high at low cholesterol levels as at high cholesterol levels. What accounts for the increase in mortality at low cholesterol levels, if not coronary heart disease? Stroke is part of the explanation. It was twice as prevalent in the lowest-cholesterol group as it was in other participants. But that hardly explains the large increase in mortality. Possible explanations from other studies include higher cancer rates and higher rates of accidents and suicide. But the study didn't provide those statistics so I'm only guessing.The MRFIT study cannot be replicated, because it was conducted at a time when fewer people were taking cholesterol-lowering drugs. In 2009, a 50-year old whose doctor discovers he has high cholesterol will likely be prescribed a statin, after which he will probably no longer have high cholesterol. This will confound studies examining the association between blood cholesterol and disease outcomes.
Thanks to The Great Cholesterol Con by Anthony Colpo for the MRFIT reference.
Now that we've seen that the first half of the diet-heart hypothesis-- that dietary saturated fat and cholesterol elevate serum cholesterol and low-density lipoprotein (LDL)-- is false, let's take a look at the second half. This is the idea that elevated serum cholesterol causes cardiovascular disease, also called the "lipid hypothesis".
Heart Attack Mortality vs. Total Mortality
We've been sternly warned that high serum cholesterol leads to heart attacks and that it should be reduced by any means necessary, including powerful cholesterol-lowering drugs. We've been assailed by scientific articles and media reports showing associations between cholesterol and heart disease. What I'm going to show you is a single graph that puts this whole issue into perspective.
The following is drawn from the Framingham Heart study (via the book Prevention of Coronary Heart Disease, by Dr. Harumi Okuyama et al.), which is one of the longest-running observational studies ever conducted. The study subjects are fairly representative of the general population, although less racially diverse (largely Caucasian). The graph is of total mortality (vertical axis) by total cholesterol level (horizontal axis), for different age groups: 
If you're 80 or older, and you have low cholesterol, it's time to get your affairs in order. Between the age of 50 and 80, when most heart attacks occur, there's no association between cholesterol level and total mortality. At age 50 and below, men with higher cholesterol die more often. In the youngest age group, the percent increase in mortality between low and high cholesterol is fairly large, but the absolute risk of death at that age is still low. There is no positive association between total cholesterol and mortality in women at any age, only a negative association in the oldest age group.
Here's more data from the Framingham study, this time heart attack deaths rather than total mortality (from the book Prevention of Coronary Heart Disease, by Dr. Harumi Okuyama et al.): 
Up to age 47, men with higher cholesterol have more heart attacks. At ages above 47, cholesterol does not associate with heart attacks or total mortality. Since the frequency of heart attacks and total mortality are low before the age of 47, it follows that total cholesterol isn't a great predictor of heart attacks in the general population.
These findings are consistent with other studies that looked at the relationship between total cholesterol and heart attacks in Western populations. For example, the observational arm of the massive MRFIT study found that higher cholesterol predicted a higher risk of heart attack in men age 35-57, but total mortality was highest both at low and high cholesterol levels. The "ideal" cholesterol range for total mortality was between 140 and 260 mg/dL (reference). Quite a range. That encompasses the large majority of the American public.
The Association Between Blood Cholesterol and Heart Attacks is Not Universal
The association between total cholesterol and heart attacks has generally not been observed in Japanese studies that did not pre-select for participants with cardiovascular risk factors (Prevention of Coronary Heart Disease, by Dr. Harumi Okuyama et al.). They also aren't observed on Kitava, where no one seems to have heart attacks or stroke regardless of cholesterol. This suggests that total blood cholesterol as a marker of heart attack risk is not universal. I suspect it would not necessarily apply to someone eating an atypical diet.
Subdividing Cholesterol into Different Lipoprotein Particles Improves its Predictive Value
So far, this probably hasn't shocked anyone. Even entrenched proponents of the lipid hypothesis admit that total cholesterol isn't a great marker. Researchers long ago sliced up total cholesterol into several more specific categories, the most discussed being low-density lipoprotein (LDL) and high-density lipoprotein (HDL). These are tiny fatty droplets containing fats, cholesterol and proteins. They transport cholesterol, fats, and fat-soluble vitamins between tissues via the blood.
The LDL and HDL numbers you get back from the doctor's office typically refer to the amount of cholesterol contained in LDL or HDL per unit blood serum, but you can get the actual particle number measured as well. One can also measure the level of triglyceride (a type of fat) in the blood. Triglycerides are absorbed from the digestive tract and manufactured by the liver in response to carbohydrate, then sent to other organs via lipoproteins.
The level of LDL in the blood gives a better approximation of heart attack risk than total cholesterol. If you're living the average Western lifestyle and you have high LDL, your risk of heart attack is up to twice the risk of someone who has low LDL. LDL particle number has more predictive value than LDL cholesterol concentration. The latter is what's typically measured at the doctor's office. For example, in the EPIC-Norfolk study (free full text), patients with high LDL cholesterol concentration had a 73% higher risk of heart attack than patients with low LDL. Participants with high LDL particle number had exactly twice the risk of those with low LDL number. We'll get back to this phenomenon in a future post.
In the same study, participants with low HDL had twice the heart attack risk of participants with high HDL. That's why HDL is called "good cholesterol". This finding is fairly consistent throughout the medical literature. HDL is probably the main reason why total cholesterol doesn't associate very tightly with heart attack risk. High total cholesterol doesn't tell you if you have high LDL, high HDL or both (LDL and HDL are the predominant cholesterol-carrying lipoproteins). Also from the EPIC-Norfolk study, participants with high triglycerides had twice the risk of heart attack as participants with low triglycerides. Triglycerides and HDL are inversely related to one another, that is, if a person has high HDL, they're likely to have low triglycerides, and vice versa. This has also been consistent between studies.
Together, this suggests that the commonly measured lipoprotein pattern that associates most tightly with heart attack risk in typical Western populations is high LDL (particularly LDL particle number), low HDL and high triglycerides.
In the next post, I'll slice up the lipoproteins even further and comment on their association with cardiovascular disease. I'll also begin to delve into how diet affects the lipoproteins.
The diet-heart hypothesis is the idea that (1) dietary saturated fat, and in some versions, dietary cholesterol, raise blood cholesterol in humans and (2) therefore contribute to the risk of heart attack.
I'm not going to spend a lot of time on the theory in relation to dietary cholesterol because the evidence that typical dietary amounts cause heart disease in humans is weak. As far as I can tell, most diet-health researchers don't subscribe to this idea anymore because the evidence has simply failed to materialize. Dr. Walter Willett doesn't believe it, and even Dr. Ancel Keys didn't believe it. Here's a graph from the Framingham Heart study (via the book Prevention of Coronary Heart Disease, by Dr. Harumi Okuyama et al.) to drive home the point. Eggs are the most concentrated source of cholesterol in the American diet. In this graph, the "low" group ate 0-2 eggs per week, the "medium" group ate 3-7, and the "high" group ate 7-14 (click for larger image): 
The distribution of blood cholesterol levels between the three groups was virtually identical. The study also found no association between egg consumption and heart attack risk. Dietary cholesterol does not have a large impact on serum cholesterol in the long term, perhaps because humans are adapted to eating cholesterol. Most people are able to adjust their own cholesterol metabolism to compensate when the amount in the diet increases. Rabbits don't have that feedback mechanism because their natural diet doesn't include cholesterol, so feeding them dietary cholesterol increases blood cholesterol and causes vascular pathology.
The first half of the diet-heart hypothesis states that eating saturated fat raises blood cholesterol. This has been accepted without much challenge by mainstream diet-health authorities for nearly half a century. In 1957, Dr. Ancel Keys proposed a formula (Lancet 2:1959. 1957) to predict changes in total cholesterol based on the amount of saturated and polyunsaturated fat in the diet. This formula, based primarily on short-term trials from the 1950s, stated that saturated fat is the primary dietary influence on blood cholesterol.
According to Keys' interpretation of the trials, saturated fat raised, and to a lesser extent polyunsaturated fat lowered, blood cholesterol. But there were serious flaws in the data from the very beginning, which were pointed out in this searing 1973 literature review in the American Journal of Clinical Nutrition (free full text).
The main problem is that the controlled trials typically compared saturated fats to omega-6 linoleic acid (LA)-rich vegetable oils, and when serum cholesterol was higher in the saturated fat group, this was most often attributed to the saturated fat raising blood cholesterol rather than the LA lowering it. When a diet high in saturated fat was compared to the basal diet without changing LA, often no significant increase in blood cholesterol was observed. Studies claiming to show a cholesterol-raising effect of saturated fat often introduced it after an induction period rich in LA. Thus, the effect sometimes had more to do with LA lowering blood cholesterol than saturated fat raising it. This is not at all what I was expecting to find when I began looking through the short-term trials.I recently read a 2003 study that addresses this point directly. Muller et al. (free full text) compared the effects of three controlled diets on the blood cholesterol of 25 healthy women. The diets were: - High in saturated fat from coconut, low in LA
- Same as #1, with half the saturated fat replaced by carbohydrate
- Low in saturated fat, high in LA, with the same total fat as in #1
The diets were fed to the whole group for three week periods. Investigators found that diet #3 lowered cholesterol and LDL relative to diets #1 and #2. The total cholesterol of women on diets #1 and #2 were not statistically different (p= 0.09), and their LDL was virtually identical. Thus, a very large difference in saturated fat intake didn't affect total cholesterol or LDL when it was replaced by carbohydrate, but it did when it was replaced by LA. The most straightforward explanation is that LA lowers cholesterol and LDL, but saturated fat has little or no effect on either under the conditions of this experiment. From the discussion section:
The most important finding of this study was that lowering total saturated fat in the form of coconut oil, from 22.7 to 10.5 E% without change in the P/S ratio [polyunsaturated to saturated ratio], did not lower total or LDL cholesterol, but significantly reduced HDL cholesterol.
I don't claim that this one study settles the question, but it does illustrate that saturated fat does not necessarily have a large and consistently detectable effect on total or LDL cholesterol. Among the many other studies I examined, I found a well-controlled counterexample: Arterioscler. Thromb. Vasc. Biol. 18:441. 1988. In this 8-week study, increasing saturated fat (at the expense of carbohydrate and with LA constant) increased total cholesterol and LDL, while also increasing HDL, and decreasing Lp(a) and triglycerides (the latter three changes are thought to be protective). Decreasing saturated fat from 15% to 6% of calories (drastic), reduced total cholesterol by 9% and LDL by 11% (calculated by the Friedewald equation). The variation between trials may have to do with the specific saturated fatty acids used in each trial, their duration, or some other unknown confounder.
Reading through the short-term controlled trials, I was struck by the variability and lack of agreement between them. Some of this was probably due to a lack of control over variables and non-optimal study design. But if saturated fat has a dominant effect on serum cholesterol in the short term, it should be readily and consistently demonstrable. It clearly is not.The long-term data are also not kind to the diet-heart hypothesis. Reducing saturated fat while greatly increasing LA certainly does lower blood cholesterol substantially. This was the finding in the well-controlled Minnesota Coronary Survey trial, for example (14% reduction). But in other cases where LA intake changed less, such as MRFIT, the Women's Health Initiative Diet Modification trial and the Lyon Diet-Heart trial, reducing saturated fat intake had little or no effect on total cholesterol or LDL (0-3% reduction). This generally surprised the investigators. The small changes that did occur could easily have been due to other factors, such as increased fiber and phytosterols, since these were multiple-factor interventions.
Another blow to the idea that saturated fat raises cholesterol in the long term comes from observational studies. Here's a graph of data from the Health Professionals Follow-up study, which followed 43,757 health professionals for 6 years (via the book Prevention of Coronary Heart Disease by Dr. Harumi Okuyama et al.): 
What this graph shows is that at a relatively constant LA intake, neither saturated fat intake nor the ratio of LA to saturated fat were related to blood cholesterol in freely living subjects. This was true across a wide range of saturated fat intakes (7-15%). If we can't even find a consistent association between dietary saturated fat and blood cholesterol in observational studies, how can we claim that saturated fat is a dominant influence on blood cholesterol?
There's more. If saturated fat were important in determining the amount of blood cholesterol in the long term, you'd expect populations who eat the most saturated fat to have high blood cholesterol levels. But that's not at all the case. The Masai traditionally get almost 2/3 of their calories from milk fat, half of which is saturated. In 1964, Dr. George V. Mann published a paper showing that traditional Masai warriors eating practically nothing but very fatty milk, blood and meat had an average cholesterol of 115 mg/dL in the 20-24 year age group. For comparison, he published values for American men in the same age range: 198 mg/dL (J. Atherosclerosis Res. 4:289. 1964). Apparently, eating three times the saturated animal fat and several times the cholesterol of the average American wasn't enough to elevate their blood cholesterol. What does elevate the cholesterol of a Masai man? Junk food.
Now let's swim over to the island of Tokelau, where the traditional diet includes nearly 50% of calories from saturated fat from coconut. This is the highest saturated fat intake of any population I'm aware of. How's their cholesterol? Men in the age group 20-24 had a concentration of 168 mg/dL in 1976, which was lower than Americans in the same age group despite a four-fold higher saturated fat intake. Tokelauans who migrated to New Zealand, eating half the saturated fat of their island relatives, had a total cholesterol of 191 mg/dL in the same age group and time period, and substantially higher LDL (J. Chron. Dis. 34:45. 1981). Sucrose consumption was 2% on Tokelau and 13% in New Zealand. Saturated fat seems to take a backseat to some other diet/lifestyle factor(s). Body fatness and excess calorie intake are good candidates, since they influence circulating lipoproteins.
I have to conclude that if dietary saturated fat influences total cholesterol or LDL concentration at all in the long term, the effect is modest and is secondary to other factors. That being said, it's clear that linoleic acid, in large amount, reduces circulating total cholesterol and LDL.
