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A no-carbohydrate diet (no-carb diet) excludes dietary consumption of all carbohydrates and suggests fat as the main source of energy with sufficient protein. A no-carbohydrate diet is ketogenic, which means it causes the body to go into a state of ketosis, converting dietary fat and body fat into ketone bodies and using them to fuel the entire body and up to 95% of the brain. The remaining 5% still runs on glucose which is adequately supplied by converting dietary protein via gluconeogenesis or by converting glycerol from the breakdown of fat. It uses mainly animal source foods and requires a high saturated fat intake.
An early proponent of an all animal-based diet was Icelandic-Canadian explorer Vilhjalmur Stefansson (1879–1962), who lived with the Inuit for some time and witnessed their diet as essentially consisting of meat and fish, with very few carbohydrates—just berries during the summer. However, the accuracy of his analysis has been since called into question, as the Inuit diet has not been shown to be a ketogenic diet and roughly 15-20% of its calories are from carbohydrates, largely from the glycogen found in the raw meats. Stefansson and a friend later volunteered for a one year experiment at Bellevue Hospital Center in New York City to prove he could thrive on a diet of nothing but meat, meat fat, and internal organs of animals. His progress was closely monitored and experiments were done on his health throughout the year. At the end of the year, he did not show any symptoms of ill health; he did not develop scurvy, which many scientists had expected to manifest itself only a few months into the diet due to the lack of vitamin C in muscle meat. However, Stefansson and his partner did not eat just muscle meat but also fat, raw brain, raw liver (a significant source of vitamin C and others), and other varieties of offal.
One of the first registries on low-carbohydrate diets was in 1860 when English casket maker William Banting was prompted to lose weight and decided to write "Letter on Corpulence", which aimed to completely avoid starch and sugar. Banting lost 45 pounds on a diet composed by lean meat, dry toast, soft boiled eggs, and a few drinks a day. Thus, the Banting diet became a very well known method back then in the 19th century, promoted also for weight loss and diabetes control.
More than a century after this, carbohydrate-restricted diets gained great popularity, particularly in the case of the Atkins Diet which emerged in 1972, thanks to Robert Atkins. While his diet is not a zero-carbohydrate diet, it does reduce carb intake to a ketogenic level in its initial stages (20 grams daily in Induction; weekly increase of 5 thereafter), allowing followers to take advantage of the fat-burning mechanism that is ketosis. According to Atkins, this nutritional approach is more effective for weight loss than a low-fat, "high carbohydrate diet," although there has always been much controversy and great dispute amongst healthcare professionals concerning drastic carbohydrate restriction.
Foods that are low in carbohydrates include:
According to observational and prospectively designed studies from physicians and nutrition scientists, impaired physical performance is a common but not an obligate result of a low carbohydrate diet or no-carbohydrate diet. However, therapeutic use of ketogenic diets should not require restriction of any physical labor or recreational activity, with the particularity that only anaerobic performance is limited, such as weight lifting. In this case, due to the low glycogen levels in the ketogenic diet, competitive athletes cannot follow this kind of diet.
In 1939 two Danish scientists, Christensen and Hansen, made a study of low carbohydrate, moderate carbohydrate and high carbohydrate diets, each one lasting at least one week. At the end of each diet, the subjects' endurance time to exhaustion on a stationary bicycle was measured, and they found that with the low carbohydrate they lasted only 81 minutes, while they were able to ride for 206 minutes after the high carbohydrates diet.
In 1946, another experiment was made by Kark, Johnson and Lewis to determine effects of pemmican (a mixture of fat and dry meat) as an emergency ration for infantry troops in winter training in the Canadian Arctic. Results on this study showed that in 3 days, soldiers were unable to complete their assigned tasks. Then, in the 1960s, with the resurgence of biomedical science, new research revealed that fat had limited utility as fuel for high intensity exercise, and that humans are physically impaired if they are given a low carbohydrate or no-carbohydrate diet.
At the Center for Obesity Research and Education at Temple University, Philadelphia, researchers found recently that after a two-year comparison, a low carbohydrate diet is almost similar to low-fat diet in terms of weight loss, but low-carbohydrate improves cardiovascular risk factors more, such as blood pressure and blood lipid levels. This study would suggest that low-carbohydrate diet protects individuals from potential coronary heart diseases in a more effective way. 307 patients were randomly assigned to either one of the two diets and researchers found 2 years later that good cholesterol levels were higher among the low-carbohydrate group compared to the low-fat group, 23% and 11% respectively.
On the other side, a study by the Beth Israel Deaconess Medical Center (BIDMC), a teaching hospital of Harvard Medical School, revealed after a study made on mice with different diets that with a low-carbohydrate there is a significant impact on atherosclerosis, even though it didn't affect cholesterol levels. Anthony Rosenzweig, a professor of Medicine at Harvard Medical School, found that the increase in plaque build-up in the blood vessels and the impaired ability to form new vessels were associated with a reduction in vascular progenitor cells, which some researchers claim could play a protective role in keeping vascular health.
Alexander Ströhle, Maike Wolters and Andreas Hahn, with the Department of Food Science at the University of Hanover, rely on Bjerregaard et al. (2003) to argue that hunters like the Inuit, who traditionally obtain most of their dietary energy from wild animals and therefore eat a low-carbohydrate diet, seem to have a high mortality from coronary heart disease, but the study did not control for carbohydrate consumption or smoking, which is significant, considering it was a "westernized" Inuit population of which 79% were current smokers and more than likely ate a non-traditional diet.
There are still some questions about the long term effects on health adopting a no-carbohydrate diet. In 2005, the British Heart Foundation recommended not to follow diets of this kind, for those individuals who want to lose weight and take care of their heart. Working together with the Oxford University team, they found that the energy stored in the heart was reduced by an average of 16% among those who followed a high-fat, low-carbohydrate diet.
When the Beth Israel Deaconess Medical Center (BIDMC) found in their investigation that this diet is associated with serious artery damage in animals, The Stroke Association in the UK added that foods such as red meat and dairy products, containing high levels of saturated fat, are the ones that cause the buildup in the arteries. Researchers suggested having a moderate and balanced diet, coupled with regular exercise.
The restriction of starchy plants, by definition, severely limits the dietary intake of microbiota-accessible carbohydrates (MACs) and may negatively affect the microbiome in ways that contribute to disease. Starchy plants, in particular, are a main source of resistant starch — an important dietary fiber with strong prebiotic properties. Resistant starches are not digestible by mammals and are fermented and metabolized by gut flora into short chain fatty acids, which are well known to offer a wide range of health benefits. Resistant starch consumption has been shown to improve intestinal/colonic health, blood sugar, glucose tolerance, insulin-sensitivity and satiety. Public health authorities and food organizations such as the Food and Agricultural Organization, the World Health Organization, the British Nutrition Foundation and the U.S. National Academy of Sciences recognize resistant starch as a beneficial carbohydrate. The Joint Food and Agricultural Organization of the United Nations/World Health Organization Expert Consultation on Human Nutrition stated, "One of the major developments in our understanding of the importance of carbohydrates for health in the past twenty years has been the discovery of resistant starch."