The Skeptic’s Diet

January 2005 (updated August 2010)
by Steven Novella, MD

An obese friend of mine commented on how well his new diet was going, as he absentmindedly devoured an entire low-carb cheesecake while happily engaged in his sedentary pastime. He, like many of my friends, family, and patients, truly desire to lose weight and be healthy but seem to be wrapped in an endless cycle of failure, not to mention excess adipose tissue.

Like so many aspects of our society, the average citizen is overwhelmed with weight-loss misinformation, pseudoscience, and a burgeoning industry of dieting fraud, quackery, and bad advice. The correct information is mixed in there too, if you know where to look and how to recognize it when you see it, but sadly, for the average person, it is lost in the noise.

You have probably heard the statistics before, as they have become the standard preface to any article or tome on diet and weight loss. America is fat. The 2001 statistics from the center for disease control (CDC) indicate that in 2001 the prevalence of obesity (body mass index or BMI of 30 or greater) was 20.9% (Mokdad 2003).The numbers for 2007-2008 were 33.8% overall (Flegal 2010). The most recent data does indicate, however, that the trend is leveling off in the last decade.

The National Center for Health Statistics estimates that 65% of adults in the United States are overweight (BMI of 25 or greater) or obese. Children and adolescents are increasingly overweight as well, and the degree to which Americans are overweight is increasing. The prevalence of obesity is about the same across age, sex, race, and level of education, although it tends to be higher at lower socioeconomic levels. The estimated direct annual healthcare costs of obesity is $90 billion, and it is estimated that soon obesity will overtake smoking as the number one preventable health risk.

There’s plenty of blame to go around for what is being called the 21st century pandemic of obesity. The western lifestyle is always the first target of blame, for good reason. Americans eat too much and exercise too little. We have an increasingly sedentary lifestyle, with desk-jobs and video games taking up an increasing portion of our time, displacing manual labor and sporting activity. The American diet also tends to be dense in calories and large in portion. A recent CDC report indicates that Americans are eating more. Between 1971 and 2000 the average American’s daily calorie intake increased by 168 calories for men (from 2,450 to 2,618) and 335 calories for women (from 1,542 to 1,877) (CDC 2004).

The same report also indicates that the portion of calories from carbohydrates increased during the same period, while fat intake increased slightly and protein intake decreased slightly. The additional calories in the American diet was largely from increased carbohydrates.

While increased calorie intake is certainly to blame,  I place a significant portion of the blame for America’s dieting woes on nutritional nonsense itself. Fad diets and misinformation give people false hope, failed strategies, and distract them from the real causes of their obesity, and the real solutions.

Fad Diets

Charlatans will happily flock to any realm where there is a significant gap between the public’s desires and their knowledge. In the area of dieting, there is a vibrant industry of weight-loss gurus and fad diets, in a seemingly never-ending cycle. A casual perusal of any large bookstore chain’s shelves will reveal dozens of popular titles. Most readers will recognize the Atkin’s diet, now enjoying the top spot in popularity, but many others are still popular, such as the South Beach diet, the Pritikin diet, and the Zone.

Fad diets, as a common theme, usually focus on the proportion of various macronutrients in one’s diet. Calories (in physics, the basic unit of energy is the calorie, but in nutrition a thousand such calories, or a kilocalorie, is also referred to as a Calorie, sometimes distinguished with a big “C”) in our diet come from either protein, fat, or carbohydrates – the macronutrients. Alcohol also contains calories, but otherwise there is no other source of energy in our diet.

The basis of most fad diets is to claim that weight loss (and other health benefits) can be achieved by altering one’s diet so that it contains an optimal proportion of the macronutrients. This usually takes the form of labeling one of the macronutrients as “bad” and minimizing it in the diet. In the 1970’s and 80’s the bad macronutrient was fat, spawning a low-fat craze in the food industry which still lingers today. In the 1990’s and the new century carbohydrates have clearly displaced fat as the bad boy on the block, and a low-carb craze is in full swing. Enter the Zone, although in the low-carb camp, went one step further to claim that there was a magical zone of proportions of the three macronutrients which would produce optimal health and weight loss.

The problem will all of the popular diets is that they followed the short and quick path to popular appeal and bypassed the more arduous path of scientific validity. In short, the proponents of such diets never did valid research to establish their premises or their clinical claims. Meanwhile, legitimate researchers have been slowly and quietly advancing our knowledge of nutrition and weight loss, but their findings have been largely disconnected from the cycle of popular marketing and belief. Occasionally, a guru will cherry-pick a study or two which appears to support their position, but largely only useless anecdotes and hand-waving explanations are offered to support claims.

Low-Carb Diets

You can’t go to a food store or restaurant these days without being inundated with low-carb, or Atkins-friendly, options. Atkins has been making his low-carb claims since the 1970’s, but he was largely eclipsed by the low-fat craze of the 70’s and 80’s. In the 1990’s, however, other dieting gurus, like Spears of Enter the Zone fame, started pushing the low-carb claims, and the momentum has clearly shifted. Interestingly, despite decades of time and millions of books sold, Atkins claimed rather lamely that he did not have the resources to do proper clinical studies of his claims.

The low-carb diet philosophy has several key components. One claim that may have some legitimacy is that carbohydrates stimulate the release of insulin, which then drives down the blood sugar level, which in turn may drive rebound hunger causing further calorie intake. So far the studies have shown that low carbohydrate diets may reduce hunger and lead to decreased caloric intake, but they also show that any such advantage only lasts for 6-12 months (Nordmann 2006). Long term, there does not appear to be any advantage to low-carb diets. Perhaps our bodies adjust to our new diets and we reach a new equilibrium in terms of hunger. Regardless, it is important to note that there does not appear to be any long term advantage to decreased carbohydrates.

It is also important to point out that in clinical trials comparing various diets in obese subjects, overall weight loss is modest, and subjects are usually gaining weight back by the end of the study. In the review cited above, for example, at 6 months subjects on a low carb diet lost on average 3.3 kg more than subjects on a low fat diet. By 12, months, however, the difference was only 1.0 kg, with the 95% confidence interval being between -3.5 to +1.5 kg (meaning that as far as these data can tell low fat diet may be superior to low carb by 12 months). And – on average subjects were gaining weight toward the end of the study, with total weight loss being negligible.

The South Beach diet focuses on glycemic index rather than total carbohydrates, and this does make more sense, although supportive data is still lacking. Glycemic index refers to the rapidity with which a carbohydrate is broken down into glucose, the form of sugar that is used by cells to make energy and which stimulates insulin release. Most people are surprised to learn that potatoes, which are high in starch (starch is basically a long chain of glucose molecules strung together), is metabolized into glucose more quickly than table sugar (which is sucrose and needs to be broken down into glucose and fructose). Therefore, it may be more important to focus on the kinds of carbohydrates we eat rather than on the total amount. Nutritionists now do recommend substituting whole grains for processed white bread, brown rice for white, whole-wheat pasta for white, and avoiding too much sugar. It is not clear if this aids in weight loss, but there may be other health benefits, for example avoiding diabetes (more on this below).

However, most of the fad diets either strongly suggest or directly claim that you can lose weight without reducing calories simply by avoiding carbohydrate calories. This is where the fad diets clearly depart from the scientific evidence, which overwhelmingly supports the idea that a calorie is a calorie. People lose weight because they burn more calories than they consume, and all weight loss diets work by reducing calories.

Another aspect of the low-carb diets worth commenting on is the phenomenon known as ketosis. Although our bodies can burn fat and protein for energy, glucose is the primary fuel of cells. Our brains are especially dependent upon a steady supply of glucose for energy, and this is why we need to maintain a certain blood glucose level for optimal health. We cannot convert fat or protein into glucose, and therefore must consume a certain amount of carbohydrates in order to meet the body’s needs. If the cells in our body are starved of carbohydrates then they produce proteins known as ketones and burn the ketones as an emergency substitute for glucose. This leads to a build up of ketones in the blood, a metabolic state known as ketosis.

Diabetics can go into ketosis, not because they are starved of glucose but because of insulin dysfunction, preventing adequate transport of glucose from the blood into cells. So even though there is plenty of glucose around, the cells can’t get access to it and must rely upon ketones for quick energy. Another way to achieve ketosis, however, is to simply deprive the body of carbohydrates. The Atkins diet recommends decreasing carbohydrate intake to less than 20% of total calories, which is low enough to cause ketosis, and this is, in fact, the goal. Some more extreme low-carb diets call for total carbohydrate intake of 5% or less.

Ketosis is generally considered to be an unhealthy metabolic state. Ketones are acidic, and high levels of ketones in the blood therefore lead to another metabolic state known as acidosis. One side effect of ketoacidosis is a decrease in hunger, and that is likely a major contributor to the apparent short-term weight loss that low-carb dieters experience.

Again, I will emphasize that long term health and weight control cannot be achieved through short-term strategies that result in unhealthy metabolic states.

Low-Fat Diets

Just like with carbs, you can’t lose weight by reducing fat unless you also reduce total caloric intake. But the food industry aggressively marketed low-fat foods to dieters, without significantly reducing or disclosing total calories. In order to keep food appealing calories were typically replaced with more protein or carbohydrate calories. In fact, many dieters were lulled into eating more calories because they thought low-fat foods would be healthy.

It is true that fat contains more calories per gram (9) than protein or carbohydrates (which contain 4). Therefore, fat is more calorie dense and foods high is fat are typically also high in calories. However, just counting calories from fat can be deceiving, and still dieters are better off just counting total calories.

Another aspect of the low-fat diet claims is that they are more heart healthy, whether or not they are an advantage for dieting. This story, however, turned out to be more complex and interesting than was at first thought, and the low-fat diets of the 70’s and 80’s may have actually contributed to increased heart disease. I discuss this more thoroughly below.

East Less, Exercise More

That, in a nutshell, is the formula for weight loss and general nutritional health. The magic formula of calories in vs calories out still holds sway over our weight, despite the aspirations of frustrated dieters. Many times have I seen knowing resignation on people’s faces when I tell them that the evidence still strongly supports this basic fact. We can’t help wishing there were a magic bullet, a way to lose weight without (as copious ads claim) dieting and exercise. But down deep, we all know the unfortunate truth.

There is also no magical way to achieve the goals of reduced calorie intake and increased expenditure, but there are some common-sense recommendations that are worth keeping in mind. It is difficult to estimate how many calories we are and should be eating day to day. Overeating by even a small amount, the equivalent of a slice a bread, for example, can result in a slow and steady weight gain adding up to several pounds per year. Therefore, there are many legitimate dieting aids which essentially help the dieter count calories. Some systems use actually calorie counting – keeping track of every morsel and looking up the caloric content in a table or booklet – but these tend to be tedious. Some systems, like Weight Watchers, use points, others use cards, or some other proxy for tracking calories that is a bit easier. Some systems, like Jenny Craig or Nutri System, require you to buy their prepackaged food with pre-measured caloric content. These systems are generally more expensive once you count the cost of the food, and don’t teach people to fend for themselves in the real world.

The research does show that dieters have a hard time estimating calories, and most people tend to grossly underestimate the total number of calories they eat in a day. People tend to specifically underestimate the calorie content of diet meals (Carels 2007) and large meals (Wansink 2006).  The research also shows that structured meal plans (eating prepared foods with known caloric content) and partial food substitution (diet shakes and bars also with fixed calorie content) do result in more weight loss and greater long term success (Vázquez 2009, Davis 2010). Whatever method you use, consider if it is affordable and convenient in the long term. There is no point in using a system that you do not feel you can do for the rest of your life.

The goal should be to change your eating habits for life, which means adopting eating habits that are realistic and tolerable. Cumbersome or expensive systems will likely be abandoned eventually, and studies indicate that the weight will likely just come back on as old habits are resumed. Also, highly restrictive diets are monotonous and difficult to maintain. So keep it simple, keep it tolerable. This brings up another major recommendation for weight loss – think long term. Short term strategies are by definition doomed to failure.

Unfortunately, many dieters fall into the trap of thinking short term. “If I can just get rid of these 20 pounds quickly, then I will change my habits to keep it off.”

Lose-weight-quick schemes are like get-rich-quick schemes – they seem to appeal to our natural tendency to desire instant gratification and avoid long-term effort. But the data are clear, people who keep weight off long term adopt healthy eating and exercise habits and stick with them. Rapid weight loss programs almost always fail.

Reducing calories can be achieved without spending money on a specific weight loss program. The most important strategy is simply portion control. Put less food on the plate and don’t take seconds. When eating out, if served a large portion then only eat part of it and take the rest home. Don’t order appetizers and additional side dishes.

Another strategy to reduce calories is to avoid snacking between meals. This is something that everyone knows, but also seems to be one of the more difficult things to do. Part of the reason for this is that we snack more out of habit than out of hunger, and habits are hard to break. Let’s face it, eating is fun, and we have cheap access to an abundance of tasty snacks which are enjoyable to eat. There is also often a social dimension to eating that further reinforces the behavior. The common sense recommendation for snacking is to do it in moderation. It’s OK to have an occasional treat, but avoid making it a regular ritual. Also, keep low-calorie healthy snacks close at hand (I prefer baby carrots), and try not to fill the cupboards with high-calorie snacks. Some people might find it easier to comply with some hard-and-fast rules, like no snacking after 7pm, but have some low-cal snacks on hand for emergencies.

The final strategy for reducing caloric intake is to alter the kinds of food that you eat, but this is much less important than portion control. Some foods are denser in calories than other foods, and eating calorie-dense food is likely to result in greater overall caloric intake. It is therefore helpful to familiarize yourself with the approximate caloric content of the common foods you enjoy eating.

Exercise, Exercise, Exercise

Don’t forget the second half of the “calories in vs calories out” equation. Long term weight loss can also be achieved through increased exercise. Some health professionals blame America’s weight problem more on our sedentary lifestyle than our eating habits, even though both are clearly playing a role. Recent data, however, seem to favor overeating as the inciting problem, not lack of exercise (Metcalf 2010).

Even though it is difficult to burn off large numbers of calories through exercise, unless you are a marathon runner, steady modest exercise can burn off hundreds of excess calories per week, which add up over the long term. Exercise also appears to be key to long term maintenance of weight.

As with dieting, knowing a few basic commonsense rules are all that is needed. Regardless of your age, start slow and build up steadily over time. Avoid beginning a strenuous exercise program that will simply discourage you and burn you out quickly. Pick exercises that you find fun and are convenient and you are more likely to continue them long term. The research suggests that for weight loss and cardiac health it is more important to exercise for a longer period of time rather than at a greater intensity (Chambliss 2005). If you are older or if you have any serious medical problems, it is probably best to consult your primary care doctor before starting an exercise program. For those with physical limitations, a referral to a physical therapist may be helpful in outlining a program. For many people, swimming (what therapists call “aquatherapy”) may be an excellent option because the buoyancy of the water decreasing any weight-bearing and therefore is much easier on the joints.

There are many benefits to regular exercise other than just weight control. Cardiovascular health is also improved, muscle strength and endurance increase, and exercise even increases the level of HDL (the good cholesterol – see below) in our blood.

There is another way to increase calorie output other than exercising – by increasing the basal metabolic rate of your body with stimulants. Many diet pills, even the “all natural” pills favored by the supplement industry today, promise weight loss without diet and exercise by offering stimulants. Caffeine is a popular choice, but also substances that many people would not recognize as stimulants are used. Ephedra was a popular herbal stimulant used in diet pills, until the cases of sudden death piled up high enough so that the FDA could finally force a ban of this chemical.

Although stimulants can cause you to lose weight in your sleep, this is not a healthy or even successful strategy. Stimulants can be risky to use, especially for those with heart disease, but even young healthy people are taking a chance. Long term stimulant use can also cause osteoporosis and sleep disturbance. Once the stimulants are stopped, metabolism will decrease again, perhaps even lower than before, and the weight will quickly come back on. My advice for stimulants – just say no.

Overall Health

There are reasons to change your diet and adopt regular exercise other than weight control. I reviewed the benefits of exercise above. For diet, there are several health concerns in addition to weight control. The most basic one is simply overall nutrition. Even though there is no magic formula of macronutrients, we do need to get a reasonable mix of all three macronutrients in our diet. In addition, there are several “essential” fatty acids (fats) and amino acids (proteins) that we must get in our diet. By definition, “essential” means that we cannot make the fats or amino acids from other substances, we must consume them directly. There are also numerous micronutrients, including vitamins and minerals, that we must get from our diet.

The issue of routine supplementation is complex and beyond the scope of this article. Many nutritionists feel that complete nutrition can be achieved by food alone, without the need for supplementation. Others, however, feel that a multi-vitamin is a safe and reasonable measure to insure proper nutrition. Also, some populations, such as women of child-bearing age, have specific nutritional requirements and should have routine supplementation.

Regardless of whether or not you choose to take supplementation, it is a good idea to get as much quality nutrition as possible from your food. The number one rule for this is to simply eat a varied diet – another reason why fad diets with restrictive food choices are a bad idea. In terms of what food to eat, it is a good idea to get as much fruits and vegetables as possible, since they are an excellent source of vitamins and minerals. If you are trying to lose weight, however, be careful with fruits, as they can be calorie rich. Whole grains are another staple, and the diet should be rounded out with nuts and legumes and protein from fish or chicken. Red meat should be consumed only occasionally. Dairy is still a bit controversial, but moderate consumption is reasonable.

Diabetes and Glucose Metabolism

Diabetes is a serious health problem often related to diet. There are two basic types of diabetes: Type I diabetes is an autoimmune disease that kills off the pancreas cells which secrete insulin. This type tends to have its onset early in life and is often referred to as insulin-dependent. Type II diabetes is a metabolic disorder thought to be due primarily to insulin resistance. The cells of the body do not respond as vigorously as they should to the hormone, insulin, which causes glucose in the blood to enter cells. Therefore cells do not get enough glucose to function, and the glucose level in the blood rises. The pancreas then produces more insulin to compensate, but its ability to do so can be exceeded in severe cases. There is also a milder form of diabetes called impaired glucose tolerance (IGT) which is being increasingly recognized.

In addition to genetic predisposition, the primary risk factor for developing Type II diabetes is being overweight. Often the disease can be eliminated by weight loss alone. However, it is also important to avoid overloading the body with a sudden surge of glucose, stressing its ability to secrete insulin in order reduce blood sugar levels. Those with Type II diabetes or IGT, or those at risk for either condition, should therefore avoid foods which are rapidly broken down into glucose.

For control or prevention of diabetes, therefore, it is helpful to have a diet with a moderate amount of glucose. It is also helpful to get carbohydrates in a form which is not rapidly broken down into glucose, but is more slowly broken down over time (a low glycemic index). Basically, avoid sugar and refined white starches.

Here again most low carb diets, such as Atkins, fall down, for they concentrate on the total amount of carbohydrates and ignore the types of carbohydrates. In fact it is the types of carbohydrates that are likely more important, and ultra-low total carbohydrate intake is unnecessary, even for diabetics.

Heart Health

Another aspect of health that is tied closely to diet is heart disease. More specifically, atherosclerosis, which is a build up of cholesterol deposits on the inner surface of arteries. The cholesterol plaques can restrict blood flow, break off and then clog one or more arteries downstream (embolus), or they can cause platelets (sticky cell fragments in blood that are the first line of clotting to stop bleeding) to clump on the plaque (called a thrombus) thereby blocking flow through the artery. Such blockage of arteries that feed the heart cause heart attacks, and when they affect the arteries that feed the brain they cause strokes.

It is well established that the risk of atherosclerosis directly correlates with the level of cholesterol in the blood. People with very high cholesterol are therefore at high risk for strokes and heart attacks. Other causes of high cholesterol are being overweight and lack of exercise.

It was this link between blood cholesterol and heart disease that led to the low fat craze of the last 30 years. There was a brief push for low-cholesterol foods (eggs became the bad-boys of the refrigerator for awhile), but the data soon showed that fat intake was more important than actual cholesterol intake. Ironically, the low fat-diets of the 80’s and 90’s seemed to have lead to an increase in heart disease. One cause likely relates to the fact that during this time Americans continued to grow fatter, supporting the hypothesis that fad diets are a failed strategy. But there is likely a more important explanation as well.

As most people have likely heard by now, there is good cholesterol (HDL or high density lipoprotein) and bad cholesterol (LDL or low density lipoprotein). HDL is good because it carries cholesterol away from the inner lining of arteries to the liver to be metabolized. LDL is bad because it carries cholesterol to the arteries where it is deposited, forming streaks of cholesterol and eventually plaques. The low fat movement did not distinguish between types of fat, but oversimplified by labeling all fat as bad (much as the low-carb diets of today label all carbohydrates as bad). Foods, like nuts and vegetable oils, which increase HDL, were banned from low fat diets. Low fat food products replaced fat (even if it was the good kind) with protein and carbohydrates. The net result of decreasing both good and bad cholesterol was more heart disease, not less.

There are three basic types of dietary fats: saturated, mono and polyunsaturated, and rehydrogenated. Saturated fats (those that have the maximal number of hydrogen atoms on the carbon backbone of the fat molecule) are bad fats because they increase LDL. Saturated fats are found in red meat, dairy products, lard, and therefore fatty deserts. Lean meats, like chicken, have much less.

Unsaturated fats are missing one or more hydrogen atoms. They are found in fish (omega-3 fatty acid), nut and legumes, and vegetables (and therefore vegetable oils). Eating generous amounts of these foods, therefore, is heart healthy.

Hydrogenated fatty acids (also called trans-fatty acids) start as unsaturated fatty acids and are processed to put back the hydrogen atoms. These fatty acids have a very negative effect on lipid profile, worse than saturated fat, and are therefore the worst kind of fat. They are found primarily in solidified fats, such as margarine, and processed foods, primarily baked goods. As much as possible, hydrogenated fats should be eliminated completely from the diet.


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