I recently received a critical email regarding this post I wrote about eating well on a budget. The critique focused on my statements about the benefits of carbohydrate-dense staple foods such as rice, beans, and whole grains. The reader's basic contention was that carbohydrates have no place in a healthy diet, period, and that extremely low-carb diets are the most sensible and healthy. Rather than responding to the email privately, I decided to post a response publicly with the hope that it will benefit others too.
Up to this point, I've avoided getting into specific biochemistry because I am not an expert in the field, only an interested lay person. I think I should probably describe a few basic concepts to frame my earlier statements, though. (My apologies to the biochem community for any oversimplifications.) First is the concept of bio-individuality, which is the idea that people have unique nutritional requirements. These differences are caused by genetics, past history of food consumption, environmental factors, and overall state of health (characterized by stress level, activity level, amount of sleep, chronic conditions, etc). I think some nutritional principles are universally true, but I certainly do not advocate a one-size-fits-all approach to nutrition. There are many healthy people throughout the world who subsist on very different foods. Clearly, humans can thrive on a variety of diets (including high carb diets -- see the Japanese for an example).
The majority of the reader's criticism was leveled at my inclusion of carbohydrates in a "healthy" diet. In case I didn't make it clear before, I do not advocate overconsumption of carbohydrates. That behavior leads to weight gain, diabetes, metabolic syndrome, and degradation of health. For best health, I believe carbohydrates should be eaten in their natural whole format. Americans aren't fat and sick because they over-consume rice and oatmeal. In my opinion, they are fat and sick because they drink too much soda and eat too much processed junk. In the hierarchy of dietary sins, eating processed carbs is near the top.
Current scientific research has zeroed in on leptin disruption as a root cause of the obesity epidemic. Leptin is the "master" hormone that regulates energy intake. If leptin signaling becomes disrupted, the appetite becomes unregulated, leaving a person vulnerable to constant hunger and prone to regular overeating. This often leads to gross overconsumption of carbohydrates. Carbivores have chronically high blood sugar and high insulin levels. Leptin disruption and insulin resistance usually result from constant carbohydrate overconsumption. Once these conditions develop, it becomes exceedingly difficult to regulate the appetite and prevent a downward spiral into obesity. Chronically high insulin levels cause the entire metabolic system to go haywire, leading to dramatic weight gain over time, a damaged immune system, and a cascade of other health problems. This condition is called metabolic syndrome.
Recent scientific research suggests that constant overconsumption of fructose (a specific type of carbohydrate) disrupts leptin signaling. There may be other root causes of leptin disruption as well; research on this subject is ongoing. (Note that sucrose, or table sugar, is 50% glucose and 50% fructose.) When fructose is consumed only in its natural packaging, the associated fiber helps prevent excessive intake. The small amount of fructose found in whole natural sources does not seem to interfere with normal leptin signaling, and is handled effectively by the liver. In the modern world, people usually don't consume fructose in its natural format. Instead, we often throw away all the fiber and concentrate fructose in juices and sweeteners. This means we can easily consume fructose in excessive amounts, causing leptin disruption and liver damage. The number one source of calories in the modern American diet is high fructose corn syrup. This added sweetener accounts for approximately 25% of the total daily calorie intake for the average American. It's no wonder we have an obesity epidemic in this country.
Though excess fructose consumption is a serious problem, I do not believe it is correct to claim that all carbohydrate consumption is unhealthy. Whole food carbohydrates (which are primarily glucose) are not currently believed to disrupt leptin or damage the liver. When leptin regulates consumption to moderate amounts, complex carbs will not produce the chronically high blood sugar levels that eventually lead to insulin resistance and metabolic syndrome. I would not advise elimination of all carbohydrates from the diet lest serious micronutrient deficiencies emerge. However, starchy root vegetables, grains, and particularly high-fructose fruits may be problematic for people whose leptin signaling is disrupted, or who have chronically elevated triglyceride levels. Restricted carb intake may also be advisable for people with a personal history of weight battles.
Low-carb, high-fat diets work for fat loss. Severe carbohydrate restriction deprives the body of quick energy, depletes the "emergency stores" of glycogen, causes insulin levels to fall, and starts a process known as ketogenesis in which the body extracts fatty acids from body fat. If few carbohydrates are taken in and insulin levels remain low, a person will transition to a state known as ketosis. Ketosis simply means that fatty acids are being burned for fuel because no glucose is present. Absent any significant calorie intake, a metabolic downshift will also happen during ketosis to protect the body's vital organs from being burned as fuel. Unfortunately, that metabolic downshift also limits total fat loss. When trying to lose excess weight, high-quality dietary fat should be consumed to keep the metabolic engine running. Fat does not trigger an insulin response (which would shut down ketosis), and fat consumption prevents the severe metabolic downshift characteristic of starvation diets. It also helps leptin function more normally, effectively suppressing the appetite. This low-carb high-fat approach allows a person to remain in a state of ketosis for a prolonged period of time. This leads to significant body fat reduction without destruction of lean tissue and without constant feelings of deprivation. This is why low-carb high-fat diets are effective for the vast majority of dieters, even ones who otherwise have chronically elevated insulin levels.
Aside from helping a person lose weight and moderate insulin response, low carb diets may help prevent unwanted reactions between glucose and excess dietary protein. Obviously some protein is needed to promote healthy growth and tissue maintenance, but excess protein molecules can bond to glucose molecules in a process called glycation. (Hint: Skip that protein shake - it's a waste of money.) Sometimes glycated proteins react with oxygen molecules (also known as "free radicals") in a reaction called glycoxidation, which produces several undesirable chemical end products. Advanced glycoxidation end products (AGEs) have been implicated in dementia, tissue degeneration, and a host of other serious ills. Personally, I regard the cause of this problem as excessive protein intake, but I readily acknowledge that this judgment is just my personal opinion. Other people might interpret this situation as a fundamental problem with glucose. Scientific research on AGEs is ongoing.
Restricted calorie diets are thought to prolong life and improve health specifically because harmful processes like glycoxidation are not widespread in the body. When the body has just enough energy and just enough protein to live, grow, and regenerate, health is optimum and signs of aging are minimized. Most metabolic disorders and severe health problems seem to result from sugar excesses, toxins, micronutrient deficiencies, and macronutrient imbalances. So, I feel a sane response is to limit sugar intake, eliminate as many toxins as possible, eat richly nutritious whole foods, and generally avoid overconsumption of any single class of macronutrients.
I hesitate to eliminate all carbohydrates from my diet simply because they can cause problems when used incautiously. I think serious carb restriction is sensible for people who struggle with their weight, but only if approached cautiously. I would never undertake a low-carb low-fat diet under any circumstances, period. This is tantamount to starvation and can cause serious damage. Dietary fat from high-quality sources (such as avocados, olives, the byproducts and flesh of healthy pastured animals, wild game, and wild cold water fish) should be used to curb appetite when undertaking a carb-restricted diet to encourage healthy ketosis. Vegetables and low-sugar fruits must be eaten regularly to maintain proper micronutrient balance. These points are key for preservation of lean tissues and proper function of vital biological processes during fat loss.
It is extremely important to choose only high-quality natural fat sources if eating a high-fat diet. Processed fats are nutritionally worthless to the body, and may be damaging in their own right. Hydrogenated franken-fats should not be consumed, period. Among other things, oxidized cooking oils and trans fats have been implicated in heart disease, heart attacks, strokes, and DNA damage. The less processed a fat is, the better it is to eat.
I sincerely hope I addressed the concerns raised in the email critique, and also that I managed to convey these rather complex nutritional principles clearly. Nutrition is a complicated subject. New research continuously challenges existing assumptions and raises new questions. I am always open to reader questions and comments!