The Body Fat Setpoint, Part III: Dietary Causes of Obesity

What Caused the Setpoint to Change?

We have two criteria to narrow our search for the cause of modern fat gain:

1. It has to be new to the human environment
   
2. It has to cause leptin resistance or otherwise disturb the setpoint
   

Although I believe that exercise is part of a healthy lifestyle, it probably can't explain the increase in fat mass in modern nations. I've written about that here and here. There are various other possible explanations, such as industrial pollutants, a lack of sleep and psychological stress, which may play a role. But I feel that diet is likely to be the primary cause. When you're drinking 20 oz Cokes, bisphenol-A contamination is the least of your worries.

In the last post, I described two mechanisms that may contribute to elevating the body fat set point by causing leptin resistance: inflammation in the hypothalamus, and impaired leptin transport into the brain due to elevated triglycerides. After more reading and discussing it with my mentor, I've decided that the triglyceride hypothesis is on shaky ground*. Nevertheless, it is consistent with certain observations:

• Fibrate drugs that lower triglycerides can lower fat mass in rodents and humans
  
• Low-carbohydrate diets are effective for fat loss and lower triglycerides
• Fructose can cause leptin resistance in rodents and it elevates triglycerides (1)
• Fish oil reduces triglycerides. Some but not all studies have shown that fish oil aids fat loss (2)
  

Inflammation in the hypothalamus, with accompanying resistance to leptin signaling, has been reported in a number of animal studies of diet-induced obesity. I feel it's likely to occur in humans as well, although the dietary causes are probably different for humans. The hypothalamus is the primary site where leptin acts to regulate fat mass (3). Importantly, preventing inflammation in the brain prevents leptin resistance and obesity in diet-induced obese mice (3.1). The hypothalamus is likely to be the most important site of action. Research is underway on this.

The Role of Digestive Health

What causes inflammation in the hypothalamus? One of the most interesting hypotheses is that increased intestinal permeability allows inflammatory substances to cross into the circulation from the gut, irritating a number of tissues including the hypothalamus.

Dr. Remy Burcelin and his group have spearheaded this research. They've shown that high-fat diets cause obesity in mice, and that they also increase the level of an inflammatory substance called lipopolysaccharide (LPS) in the blood. LPS is produced by gram-negative bacteria in the gut and is one of the main factors that activates the immune system during an infection. Antibiotics that kill gram-negative bacteria in the gut prevent the negative consequences of high-fat feeding in mice.

Burcelin's group showed that infusing LPS into mice on a low-fat chow diet causes them to become obese and insulin resistant just like high-fat fed mice (4). Furthermore, adding 10% of the soluble fiber oligofructose to the high-fat diet prevented the increase in intestinal permeability and also largely prevented the body fat gain and insulin resistance from high-fat feeding (5). Oligofructose is food for friendly gut bacteria and ends up being converted to butyrate and other short-chain fatty acids in the colon. This results in lower intestinal permeability to toxins such as LPS. This is particularly interesting because oligofructose supplements cause fat loss in humans (6).

A recent study showed that blood LPS levels are correlated with body fat, elevated cholesterol and triglycerides, and insulin resistance in humans (7). However, a separate study didn't come to the same conclusion (8). The discrepancy may be due to the fact that LPS isn't the only inflammatory substance to cross the gut lining-- other substances may also be involved. Anything in the blood that shouldn't be there is potentially inflammatory.

Overall, I think gut dysfunction probably plays a major role in obesity and other modern metabolic problems. Insufficient dietary fiber, micronutrient deficiencies, excessive gut irritating substances such as gluten, abnormal bacterial growth due to refined carbohydrates (particularly sugar), and omega-6:3 imbalance may all contribute to abnormal gut bacteria and increased gut permeability.

The Role of Fatty Acids and Micronutrients

Any time a disease involves inflammation, the first thing that comes to my mind is the balance between omega-6 and omega-3 fats. The modern Western diet is heavily weighted toward omega-6, which are the precursors to some very inflammatory substances (as well as a few that are anti-inflammatory). These substances are essential for health in the correct amounts, but they need to be balanced with omega-3 to prevent excessive and uncontrolled inflammatory responses. Animal models have repeatedly shown that omega-3 deficiency contributes to the fat gain and insulin resistance they develop when fed high-fat diets (9, 10, 11).

As a matter of fact, most of the papers claiming "saturated fat causes this or that in rodents" are actually studying omega-3 deficiency. The "saturated fats" that are typically used in high-fat rodent diets are refined fats from conventionally raised animals, which are very low in omega-3. If you add a bit of omega-3 to these diets, suddenly they don't cause the same metabolic problems, and are generally superior to refined seed oils, even in rodents (12, 13).

I believe that micronutrient deficiency also plays a role. Inadequate vitamin and mineral status can contribute to inflammation and weight gain. Obese people typically show deficiencies in several vitamins and minerals. The problem is that we don't know whether the deficiencies caused the obesity or vice versa. Refined carbohydrates and refined oils are the worst offenders because they're almost completely devoid of micronutrients.

Vitamin D in particular plays an important role in immune responses (including inflammation), and also appears to influence body fat mass. Vitamin D status is associated with body fat and insulin sensitivity in humans (14, 15, 16). More convincingly, genetic differences in the vitamin D receptor gene are also associated with body fat mass (17, 18), and vitamin D intake predicts future fat gain (19).

Exiting the Niche

I believe that we have strayed too far from our species' ecological niche, and our health is suffering. One manifestation of that is body fat gain. Many factors probably contribute, but I believe that diet is the most important. A diet heavy in nutrient-poor refined carbohydrates and industrial omega-6 oils, high in gut irritating substances such as gluten and sugar, and a lack of direct sunlight, have caused us to lose the robust digestion and good micronutrient status that characterized our distant ancestors. I believe that one consequence has been the dysregulation of the system that maintains the fat mass "setpoint". This has resulted in an increase in body fat in 20th century affluent nations, and other cultures eating our industrial food products.

In the next post, I'll discuss my thoughts on how to reset the body fat setpoint.


* The ratio of leptin in the serum to leptin in the brain is diminished in obesity, but given that serum leptin is very high in the obese, the absolute level of leptin in the brain is typically not lower than a lean person. Leptin is transported into the brain by a transport mechanism that saturates when serum leptin is not that much higher than the normal level for a lean person. Therefore, the fact that the ratio of serum to brain leptin is higher in the obese does not necessarily reflect a defect in transport, but rather the fact that the mechanism that transports leptin is already at full capacity.

Rabbits on a High-Saturated Fat Diet Without Added Cholesterol

I just saw another study that supports my previous post Animal Models of Atherosclerosis: LDL. The hypothesis is that in the absence of excessive added dietary cholesterol, saturated fat does not influence LDL or atherosclerosis in animal models, relative to other fats (although omega-6 polyunsaturated oils do lower LDL in some animal models). This appears to be consistent with what we see in humans.

In this study, they fed four groups of rabbits different diets:

1. Regular low-fat rabbit chow
2. Regular low-fat rabbit chow plus 0.5 g cholesterol per day
3. High-fat diet with 30% calories as coconut oil (saturated) and no added cholesterol
   
4. High-fat diet with 30% calories as sunflower oil (polyunsaturated) and no added cholesterol
   

LDL at 6 months was the same in groups 1, 3 and 4, but was increased more than 20-fold in group 2. It's not the fat, it's the fact that they're overloading herbivores with dietary cholesterol!

Total cholesterol was also the same between all groups except the cholesterol-fed group. TBARS, a measure of lipid oxidation in the blood, was elevated in the cholesterol and sunflower oil groups but not in the chow or coconut groups. Oxidation of blood lipids is one of the major factors in atherosclerosis, the vascular disease that narrows arteries and increases the risk of having a heart attack. Serum vitamin C was lower in the cholesterol-fed groups but not the others.

This supports the idea that saturated fat does not inherently increase LDL, and in fact in most animals it does not. This appears to be the case in humans as well, where long-term trials have shown no difference in LDL between people eating more saturated fat and people eating less, on timescales of one year or more (some short trials show a modest LDL-raising effect, but even this appears to be due to an increase in particle size rather than particle number). Since these trials represent the average of many people, they may hide some individual variability: it may actually increase LDL in some people and decrease it in others.

Merry Christmas!

The Dirty Little Secret of the Diet-Heart Hypothesis

The diet-heart hypothesis is the idea that saturated fat, and in some versions cholesterol, raises blood cholesterol and contributes to the risk of having a heart attack. To test this hypothesis, scientists have been studying the relationship between saturated fat consumption and heart attack risk for more than half a century. To judge by the grave pronouncements of our most visible experts, you would think these studies had found an association between the two. It turns out, they haven't.

The fact is, the vast majority of high-quality observational studies have found no connection whatsoever between saturated fat consumption and heart attack risk. The scientific literature contains dozens of these studies, so let's narrow the field to prospective studies only, because they are considered the most reliable. In this study design, investigators find a group of initially healthy people, record information about them (in this case what they eat), and watch who gets sick over the years.

A Sampling of Unsupportive Studies

Here are references to ten high-impact prospective studies, spanning half a century, showing no association between saturated fat consumption and heart attack risk. Ignore the squirming about saturated-to-polyunsaturated ratios, Keys/Hegsted scores, etc. What we're concerned with is the straightforward question: do people who eat more saturated fat have more heart attacks? Many of these papers allow free access to the full text, so have a look for yourselves if you want:

A Longitudinal Study of Coronary Heart Disease. Circulation. 1963.

Diet and Heart: a Postscript. British Medical Journal. 1977. Saturated fat was unrelated to heart attack risk, but fiber was protective.

Dietary Intake and the Risk of Coronary Heart Disease in Japanese Men Living in Hawaii. American Journal of Clinical Nutrition. 1978.

Relationship of Dietary Intake to Subsequent Coronary Heart Disease Incidence: the Puerto Rico Heart Health Program. American Journal of Clinical Nutrition. 1980.

Diet, Serum Cholesterol, and Death From Coronary Heart Disease: The Western Electric Study. New England Journal of Medicine. 1981.

Diet and 20-year Mortality in Two Rural Population Groups of Middle-Aged Men in Italy. American Journal of Clinical Nutrition. 1989. Men who died of CHD ate significantly less saturated fat than men who didn't.

Diet and Incident Ischaemic Heart Disease: the Caerphilly Study. British Journal of Nutrition. 1993. They measured animal fat intake rather than saturated fat in this study.

Dietary Fat and Risk of Coronary Heart Disease in Men: Cohort Follow-up Study in the United States. British Medical Journal. 1996. This is the massive Physicians Health Study. Don't let the abstract fool you! Scroll down to table 2 and see for yourself that the association between saturated fat intake and heart attack risk disappears after adjustment for several factors including family history of heart attack, smoking and fiber intake. That's because, as in most modern studies, people who eat steak are also more likely to smoke, avoid vegetables, eat fast food, etc.

Dietary Fat Intake and the Risk of Coronary Heart Disease in Women. New England Journal of Medicine. 1997. From the massive Nurse's Health study. This one fooled me for a long time because the abstract is misleading. It claims that saturated fat was associated with heart attack risk. However, the association disappeared without a trace when they adjusted for monounsaturated and polyunsaturated fat intake. Have a look at table 3.

Dietary Fat Intake and Early Mortality Patterns-- Data from the Malmo Diet and Cancer Study. Journal of Internal Medicine. 2005.

I just listed 10 prospective studies published in top peer-reviewed journals that found no association between saturated fat and heart disease risk. This is less than half of the prospective studies that have come to the same conclusion, representing by far the majority of studies to date. If saturated fat is anywhere near as harmful as we're told, why are its effects essentially undetectable in the best studies we can muster?

Studies that Support the Diet-Heart Hypothesis

To be fair, there have been a few that have found an association between saturated fat consumption and heart attack risk. Here's a list of all four that I'm aware of, with comments:

Ten-year Incidence of Coronary Heart Disease in the Honolulu Heart Program: relationship to nutrient intake. American Journal of Epidemiology. 1984. "Men who developed coronary heart disease also had a higher mean intake of percentage of calories from protein, fat, saturated fatty acids, and polyunsaturated fatty acids than men who remained free of coronary heart disease." The difference in saturated fat intake between people who had heart attacks and those who didn't, although statistically significant, was minuscule.

Diet and 20-Year Mortality From Coronary Heart Disease: the Ireland-Boston Diet-Heart Study. New England Journal of Medicine. 1985. "Overall, these results tend to support the hypothesis that diet is related, albeit weakly, to the development of coronary heart disease."

Relationship Between Dietary Intake and Coronary Heart Disease Mortality: Lipid Research Clinics Prevalence Follow-up Study. Journal of Clinical Epidemiology. 1996. "...increasing percentages of energy intake as total fat (RR 1.04, 95% CI = 1.01 – 1.08), saturated fat (RR 1.11, CI = 1.04 – 1.18), and monounsaturated fat (RR 1.08, CI = 1.01 – 1.16) were significant risk factors for CHD mortality among 30 to 59 year olds... None of the dietary components were significantly associated with CHD mortality among those aged 60–79 years." Note that the associations were very small, also included monounsaturated fat (like in olive oil), and only applied to the age group with the lower risk of heart attack.

The Combination of High Fruit and Vegetable and Low Saturated Fat Intakes is More Protective Against Mortality in Aging Men than is Either Alone. Journal of Nutrition. 2005. Higher saturated fat intake was associated with a higher risk of heart attack; fiber was strongly protective.

The Review Papers

Over 25 high-quality studies conducted, and only 4 support the diet-heart hypothesis. If this substance is truly so fearsome, why don't people who eat more of it have more heart attacks? In case you're concerned that I'm cherry-picking studies that conform to my beliefs, here are links to review papers on the same data that have reached the same conclusion:

The Questionable Role of Saturated and Polyunsaturated Fatty Acids in Cardiovascular Disease. Journal of Clinical Epidemiology. 1998. Dr. Uffe Ravnskov systematically demolishes the diet-heart hypothesis simply by collecting all the relevant studies and summarizing their findings.

A Systematic Review of the Evidence Supporting a Causal Link Between Dietary Factors and Coronary Heart Disease. Archives of Internal Medicine. 2009. "Insufficient evidence (less than or equal to 2 criteria) of association is present for intake of supplementary vitamin E and ascorbic acid (vitamin C); saturated and polyunsaturated fatty acids; total fat; alpha-linolenic acid; meat; eggs; and milk" They analyzed prospective studies representing over 160,000 patients from 11 studies meeting their rigorous inclusion criteria, and found no association whatsoever between saturated fat consumption and heart attack risk.

Where's the Disconnect?

The first part of the diet-heart hypothesis states that dietary saturated fat raises the cholesterol/LDL concentration of the blood. This is held as established fact in the mainstream understanding of nutrition. The second part states that increased blood cholesterol/LDL increases the risk of having a heart attack. What part of this is incorrect?

There's definitely an association between blood cholesterol/LDL level and heart attack risk in certain populations, including Americans. MRFIT, among other studies, showed this definitively, although the lowest risk of all-cause mortality was at an average level of cholesterol. The association between blood cholesterol and heart attack risk does not apply to Japanese populations, as pointed out repeatedly by the erudite Dr. Harumi Okuyama. This seems to be generally true of groups that consume a lot of seafood.

So we're left with the first premise: that saturated fat increases blood cholesterol/LDL. This turns out to be largely a myth, based on a liberal interpretation of short-term feeding studies. In fact, it isn't even true in animal models of heart disease. In the 1950s, the most vigorous proponent of the diet-heart hypothesis, Dr. Ancel Keys, created a formula designed to predict changes in blood cholesterol based on the consumption of dietary saturated and polyunsaturated fats. This formula is extremely inaccurate and has gradually been dropped from the modern medical literature. Yet the idea that saturated fat consumption increases blood cholesterol/LDL lives on...

This is it, folks: the diet-heart hypothesis ends here. It's been kept afloat for decades by wishful thinking, puritan sensibilities and selective citation of the evidence. It's time to put it out of its misery.