This new study published in JAMA is being reported as evidence that a low (20% energy) fat diet suppresses metabolism, increases heart disease risk, and predisposes people to weight regain, and that a higher (40% energy) fat, low-glycemic diet is superior.
After reading the article in full, I have some questions about it.
According to the full text, "The low-fat diet, which had a high glycemic load, was designed to reflect conventional recommendations to reduce dietary fat, emphasize whole grain products, and include a variety of vegetables and fruits," while "The low–glycemic index diet aimed to achieve a moderate glycemic load by replacing some grain products and starchy vegetables with sources of healthful fat and low–glycemic index vegetables, legumes, and fruits."
So the low-fat diet both "had a high glycemic load" and was designed to "emphasize whole grain products"? Since whole grains have a relatively low glycemic load, I was curious to see how they managed to put together a diet that achieved two opposite goals.
According to Table 1 of this paper, the low (20%) fat diet, supposedly emphasizing whole grains, vegetables, and fruits, supplied only 30 g of fiber per 2000 kcal, but the low glycemic diet, containing fiber-free olive oil and more fiber-free animal products (evidenced by twice as much cholesterol) to boost fat level to 40% of energy, supplied 33 g of fiber per 2000 kcal.
So I wondered, why does the LF diet, supposed to "emphasize" fiber-rich whole grains, vegetables, and fruits, have less fiber that the LG diet, which included fiber-free oils and animal products to double the fat and cholesterol content while keeping caloric content constant?
Does a low fat diet have to be low in fiber and high in glycemic load?
If not, why call it a "low fat" diet and not call it a "high glycemic load" diet?
Notably, the authors fail to provide example menus, instead opting to provide only the claimed nutrient analysis of each diet prescription.
Here's about half a 2000 kcal diet, emphasizing commonly available and consumed whole grains, vegetables, and fruits:
And here is the macronutrient and fiber analysis:
So 1100 kcal coming from four slices of whole wheat bread, one cup of cooked oatmeal, one cup of cooked lentils, one white potato, one cup of broccoli, one apple, and one banana supplies ~39 grams of fiber. That is about twice the fiber density of the low-fat diet prescribed by the authors of this study.
This selection of foods supplies less than 15% of calories from fat. This means that a 2000 kcal low-fat diet composed of whole grains, legumes, vegetables, and fruits could supply about 80 grams of fiber. Yet theirs supplies only 30 g of fiber, indicating that it doesn't even have the amount of whole grains, vegetables, and fruits that I have in this 1100 kcal menu, despite supposedly "emphasizing" whole grains, vegetables, and fruits.
They state that their low-fat diet supplied about 105 g protein and 140 mg of cholesterol per 2000 kcal, while the LG diet had 280 mg cholesterol. As shown above, a reasonable selection of whole plant foods supplies about 40 g protein per 1000 kcal. Animal flesh supplies 30 to 60 mg cholesterol per 100 kcal, eggs supply about 250 mg cholesterol per 100 kcal, and one percent and non-fat milk supply 0-10 mg cholesterol per 100 kcal. So their 2000 kcal LF diet could have supplied only about 250 to 460 kcal from animal flesh and essentially no eggs.
That means that they could have included at least 1500 kcal worth of low-fat whole plant foods, and if they did, they would have had a diet supplying more than 50 grams of fiber, yet still very low in fat. Instead they chose to include low-fiber high-glycemic load foods, most likely refined carbohydrates. Again, in their publication, presss release, and interviews they should have called this a high-glycemic load diet, not a low-fat diet, because the defining feature is the high glycemic load, not the low fat content.
As they did it, they imply and give the public the impression that any harms that came from their low fat diet were due to the low fat content, rather than from the low fiber, refined character of the carbohydrate-rich foods they included. This makes it look, to me, like they had an axe to grind against any diet low in fats; or, conversely, that they had a bias in favor of high-fat pseudo-Mediterranean diets.
Next, looking at Table 3 I find more curious data. When they measured resting energy expenditure during each of the three diets (low-fat, low-glycemic, and very low carb; LF, LG, and VLC respectively), they found no significant difference. During the LF, LG, and VLC phases, the average REEs were 1576, 1614, and 1643 kcal, respectively. The VLC and the LF numbers differ by 67 kcal, which is only four percent (relative to the LF). The ranges also overlap significantly:
LF: 1528 to 1624
LG: 1566 to 1662
VLC: 1595 to 1691
Since the final study included only 21 people, and the low-fat arm is clearly rigged to have a low fiber content, and the ranges of REE in each phase significantly overlapped, I don't think we can make very much of this four percent difference in REE.
The press is reporting that the participants burned 300 kcal more per day on VLC diet and 150 kcal more per day on the LG diet compared to the LF diet. In the full text the authors state:
"TEE [total energy expenditure] differed by approximately 300 kcal/d between these 2 diets [LF and VLC], an effect corresponding with the amount of energy typically expended in 1 hour of moderate-intensity physical activity."
The actual figures in the study are 3013 kcal per day expended in the VLC phase, 2926 in the LG phase, and 2767 in the LF phase, resulting in a difference of 246 kcal between VLC and LF, and a difference of 159 between LG and LF.
The difference between 300 and 246 is 54, and 246 is closer to 200 than to 300. It would have been much more scientifically accurate to round 246 to 250, rather than up to the next hundred. Using the figure 300 overstates the supposed increased metabolic effect of the VLC by about 22 percent, which again suggests that the authors have bias against low-fat diets and attempt to put the approach in the worst possible light when opportunity presents.
Of interest, the authors appear somewhat at a loss to explain this large difference in total energy expenditure given the rather small difference in resting energy expenditure, which indicates insignificant differences in basal metabolic rates between the groups. Total energy expenditure consists of basal metabolic rate plus energy expended in activities of daily living, physical activity, and digestion and metabolism (i.e. thermic effect) of food.
The REE of the VLC and LG phases appeared to be 67 kcal and 38 kcal more than the REE of the LF phase, respectively. Since the TEE of the VLC and LG phases appeared to be, respectively, 246 and 159 kcal greater than that of the LF phase, something else must account for the VLC and LG groups burning another 179 and 121 extra kcalories daily. The authors comment thus:
"Triiodothyronine was lowest with the very low-carbohydrate diet, consistent with previously reported effects of carbohydrate restriction23 ; thus, changes in thyroid hormone concentration cannot account for the higher energy expenditure on this diet. The thermic effect of food (the increase in energy expenditure arising from digestive and metabolic processes) dissipates in the late postprandial period and would not affect REE measured in the fasting state. Because the thermic effect of food tends to be greater for carbohydrate than fat,24 - 25 it would also not explain the lower TEE on the low-fat diet. Although protein has a high thermic effect of food,16 the content of this macronutrient was the same for the low-fat and low–glycemic index diets and contributed only 10% more to total energy intake with the very low-carbohydrate diet compared with the other 2 diets. Furthermore, physical activity as assessed by accelerometry did not change throughout the study."
According to their data, thyroid hormone levels were 11 percent higher during the LF phase than during the VLC phase. In short, none of the well-established mechanisms affecting TEE could account for their reported difference; in fact, since, as they admit, the thermic effect of carbohydrate is greater than that for fat, their results contradict the expectation of a higher TEE for the diet highest in carbohydrate, i.e. the low-fat diet. As they state, since the LF and LG diets were 20% energy from protein, and the VLC was 30% energy from protein, differences in protein content were not capable of explaining such a large difference. They offer the following:
"Alternative explanations for the observed differences in REE and TEE may involve intrinsic effects of dietary composition on the availability of metabolic fuels13 - 14 or metabolic efficiency, changes in hormones (other than thyroid) or autonomic tone affecting catabolic or anabolic pathways, and (for TEE) skeletal muscle efficiency as regulated by leptin.26 - 29"The studies to which they refer with footnotes 13 and 14 also compared metabolic effects of low-fat to low-glycemic diets. In one [full text], "The low-fat diet was low in fat, high in carbohydrate and glycemic load, and generally consistent with National Cholesterol Education Program guidelines for a heart healthy diet." In the other, the low-fat diet was also high-glycemic load (i.e. low fiber) and contained white bread, non-fat milk, frozen yogurt, and angel food cake. These studies collectively rest on and promote the misconception that eating a low-fat diet means eating a high-glycemic index diet.
As I have shown above, it is quite easy to create a very high fiber, low glycemic, low fat, high carbohydrate diet, so I wonder why all of these researchers seem fixated on conflating low-fat with high-glycemic, and talking as if their version of low-fat diet is the only possible to compare with their apparently preferred high fat diets?
In any case, all claims these authors make about low-fat diets apply only to their specially selected low-fat diet that is astoundingly low in fiber considering their claim that it emphasized whole grains, vegetables, and fruits.
In the press release announcing the results of the study, the authors claim that the "low-fat" diet caused "an unhealthy lipid pattern and insulin resistance." Looking at the data in the paper, while some signs of insulin resistance did increase during the LF phase, the total cholesterol during low fat phases was 149, a level associated with a very low risk of cardiovascular disease, compared to 156 for the LG phase and 175 for the VLC phase. These data actually provide another study (of hundreds) supporting the notion that dietary cholesterol raises blood cholesterol. The dietary cholesterol for LF, LG, and VLC phases were respectively 140, 280, and 978 mg per day, and the diets with the higher dietary cholesterol produced higher blood cholesterol in a dose-response fashion.
The HDL levels during LF, LG, and VLC phases were 40, 45, and 48 respectively, and this apparently contribute to their claim that the LF diet causes "an unhealthy lipid pattern." However, this reduction of HDL is simply a part of the pattern of lower total cholesterol, as evidenced by the fact that HDL as a percent of total cholesterol did not signficantly differ between phases, at 27%, 29%, and 27% for LF, LG, and VLC phases respectively. Moreover, the idea that a high HDL protects against cardiovascular disease has suffered a major setback with the finding that people who have genetic variations producing naturally higher HDL levels do not enjoy greater protection from cardiovascular disease as a result.
Triglycerides measured 107, 87, and 66 mg/dL in the LF, LG, and VLC phases respectively, apparently also contributing to the authors' idea that the LF diet produced an unhealthy lipid pattern. The American Heart Association recommends people keep their triglyceride levels under 100 mg. Despite feeding these people a low fiber low fat diet, rich in refined carbohydrates, their triglycerides were only slightly elevated above desired levels.
During the LF and LG phases, the subjects had similar C-reactive protein levels, but during the VLC phase C-reactive protein increased about 13 percent, suggesting that the VLC diet supported a higher level of systemic inflammation than either the LF/refined carb diet or the LG diet.
Finally, since the study design involved feeding each type of diet to each individual for only one month at a time, this study does not provide any evidence for long-term effects or adaptations to any of the three diets studied, so the authors really have no warrant to assert that this study shows that any of the diets supports long-term weight stability better than any of the others. However, in their press release they appear bent on implying that "low-fat diets" will cause people to quickly regain lost weight and that their higher fat, low-glycemic diet is easier to sustain. From their press release:
“In addition to the benefits noted in this study, we believe that low-glycemic-index diets are easier to stick to on a day-to-day basis, compared to low-carb and low-fat diets, which many people find limiting,” says Ebbeling. “Unlike low-fat and very- low carbohydrate diets, a low-glycemic-index diet doesn’t eliminate entire classes of food, likely making it easier to follow and more sustainable.”Which makes me wonder who they are referring to when they say ".....which many people find limiting." Perhaps themselves? Also, Ebbeling (one of the authors) implies that a low-fat diet eliminates entire classes of food. Is he referring to vegetable oils and animal fats? Can these really be considered "classes of food"? In any case, his low-fat diet did not eliminate animal products supplying fat. As I indicated above, a selection of low-fat plant foods will typically supply less than 15 percent of energy from fat, so a 20 percent fat allows use of some higher fat foods.
While Ebbeling et al believes that a pseudo-Mediterranean 40 percent fat is "easier to stick to on a day-to-day basis" and "easier to follow and more sustainable" and will produce less or no weight gain compared to a low fat diet. Due et al compared people randomly assigned to a diet providing a moderate amount of fat (35–45% of energy) and about 20% of fat as monounsaturated fatty acids (MUFA diet; n = 54) to a "low-fat" (20–30% of energy) diet (LF diet; n = 51)." Their results? More people dropped out of the MUFA group (28%) than out of the LF group (16%), and the MUFA group regained more weight than the LF group (2.5 vs. 2.2 kg respectively). Apparently the MUFA diet isn't necessarily easier to follow, more sustainable, or more effective at preventing weight regain than a low fat.
In short, this study, touted as showing the deleterious metabolic and cardiovascular effects of a low-fat diet, and the benefits of eating a diet supplying 40% of energy from fat, used a low fat diet unnecessarily low in whole unrefined plant foods,. Since it did not compare the LG diet to a low-glycemic low-fat diet composed of unrefined plant foods, I think it fails to show any long-term metabolic or health advantage of any moderately high fat pseudo-Mediterranean diet laced with oils, fats, and animal products over a low-fat diet composed of unrefined plant foods.