Implications of carbohydrate metabolism on weight control
Carbohydrates are not fat, but they have a significant effect on fat storage. The monosaccharides, which have been absorbed from the intestinal tract, are carried to the liver by portal circulation. The fructose and galactose are converted into glucose. The liver cells release this glucose into the bloodstream to be distributed to the body tissues as a source of energy. The excess glucose is stored as insoluble glycogen in the liver cells. The conversion of glucose to glycogen is reversible – as the need arises, the liver mobilizes glucose from it’s glycogen stores and releases it into the bloodstream to maintain an optimal blood sugar level.
When glucose are absorbed from digested food in the small intestines, the blood sugar level rises. The liver counteracts this rise in blood sugar levels through glycogenesis (glucose converted into glycogen). Skeletal, cardiac and smooth muscles, also store glycogen from glucose, when at rest or under minor work loads; glycogen is found in small quantities in all organs of the body. The amount of reserve glycogen stored in the liver and muscle depends largely on the nature of the diet and the amount of exercise. Cardiac muscle preserves it’s reserves of glycogen tenaciously, by using blood glucose as an energy source for its muscular contraction. Brain tissue uses blood glucose(carbohydrates) almost exclusively for its energy metabolism, while skeletal muscle utilize carbohydrates or fatty acids to obtain energy for its activities.
The metabolism of carbohydrates is geared to the digestion of complex carbohydrates in the gastrointestinal tract: e.g. digestive enzymes in saliva and pancreatic secretions break down complex polysaccharides into mono- and disaccharides. These disaccharides are further digested by enzymes in the lining of the small intestines into monosaccharides, which are easily absorbed into the bloodstream, from which they pass to the liver. (refined foods bypass these digestive systems!).
Certain hormones are involved in the control of carbohydrate metabolism – insulin, adrenalin and glucagon. They work together to maintain a steady blood glucose level. The major function of glucose in the body is to produce energy, although some glucose is involved with the metabolism of cholesterol, amino acids and fat. If someone starves, the glucose stores lasts for 24-48 hours; after this glucose must be produced from the body’s fat. For glucose to be used, it needs insulin and potassium, to cross the cell membranes into the cell substance itself. Diabetics who have no insulin, or who’s body cells are unresponsive to insulin, have a high blood glucose level as result. When excess insulin is injected, the blood glucose levels falls dramatically, producing a temporary clouding of consciousness or even complete loss of consciousness. This is corrected by giving glucose rapidly by mouth or intravenously.
Therefore, insulin lower blood glucose levels. Adrenalin and glucagon raise blood glucose levels.
Carbohydrate metabolism is also affected by liver disease. Glucose, glucagon and galactose tests are abnormal with a liver dysfunctioning. In liver dysfunction glycogen storage is impaired and the administration of glucagon, leads to hypoglycemia. Galactose tolerance tests are abnormal in liver disease, since the ability to convert galactose to glucose is impaired.
The body has a limited capacity for storing glycogen, but the ability to store fat is not nearly so restricted. When carbohydrates are supplied by the diet in excess of what is needed for energy or glycogen storage, they are converted into fat and stored in the adipose tissues. Therefore, any food (includes fat and protein as well) ingested in excess of caloric expenditure, are stored as fat!. A certain minimal amount of carbohydrate must be supplied by the diet at all times to prevent the use of dietary protein as an energy source from the body’s metabolic pool of structural components (protein sparing). Protein sparing is obtained when carbohydrates and protein are ingested simultaneously.
Weight control and correction of carbohydrate metabolism are important. Excess weight and insulin insensitivity lead to hypercalcuria (abnormally high concentration of calcium in the urine), which can cause stone formation. After sugar is ingested, the calcium levels in the urine is higher, along with decreased phosphate reabsorption; this leads to low plasma phosphate; more active vitamin D is produced; increase in intestinal calcium absorption with simultaneous higher excretion of calcium.
“Carbohydrates are fattening” is a myth. “Fat is more fattening” is the truth! Why?
People mistakenly think of carbohydrates as “fattening” and avoid them when trying to lose weight. Such a strategy may be helpful if the carbohydrates are concentrated sugars (soft drinks, cookies, candies etc), but it is counterproductive if the carbohydrates are from whole grains, vegetables and legumes. Not all carbohydrates are created equal. Scarsdale diet (low carbohydrate and high-protein) and others, lead to nutritional deficiencies and metabolic imbalances. It is doomed to fail, since it leaves the person feeling weak, irritable and hungry; the person fall back to old eating habits and the lost pounds return!.
A high carbohydrate diet, consisting of complex carbohydrates, is effective in controlling weight!. Complex carbohydrates not only fill you up, but the body STORES these carbohydrates LESS EFFICIENTLY as fat tissue than dietary fats; the digestion of carbohydrates itself burns more calories than the same amounts of fat e.g. 75% of the excess carbohydrate calories are stored as fat. 97% of excess fat calories are stored as fat. Fat is more fattening, because fat is converted to body fat MORE EFFECTIVELY; more calories is burned by the body after eating carbohydrates than it is for fat (“thermic effect of carbohydrates”). Men who overate both fats and complex carbohydrate foods, gained weight more slowly than men who overate a high-fat diet alone!
Obesity is a major threat to health and quality of life. It is the largest nutrition-related problem in the developed world. Although it could be genetically determined, it results from an imbalance between food intake and daily physical activity. Three lifestyle factors will improve health: increased physical activity and reducing the intake of sugars (sucrose) and fat. A low carbohydrate diet restrict carbohydrate intake to 20-60g/day, often for the treatment of chronic diseases like obesity, diabetes, cardiovascular disease, high blood pressure and metabolic syndrome. In affluent countries, excess body fat accounts for 30-40% of coronary heart disease; cancers of the colon, breast and endometrium and most cases of diabetes.
What is the fat paradox?:
With the increasing popularity of low-fat products, the dietary fat intake has decreased, but the prevalence of obesity is rising. Why?. Many low-fat foods are based on sugars and their energy density values are similar to those of the high-fat foods. Sugars are therefore the primary nutritional factor behind the increase in obesity.
Research has shown that the addition of excess carbohydrate in a mixed diet (therefore excess energy intake), results in the accumulation of body fat, BUT not by the conversion of carbohydrate to fat. The higher priority of carbohydrate over fat to be oxidized, results in a SUPPRESSION OF DIETARY FAT OXIDATION, which leads to fat storage.
- Following a strict protein diet, causes liver damage and getting energy chiefly from dietary fats rather than carbohydrates, causes heart disease and other health problems. With a person following a low-carbohydrate diet (20-60 g/day) one of the common metabolic changes to take place, is ketosis. A low- carbohydrate diet result in a reduction of insulin, which promotes a high level of fatty acids, used for oxidation and production of ketone bodies. It induces several adaptive responses: low blood glucose causes the pancreas to produce glucagon, which stimulates the liver to convert glycogen into glucose and release it into the blood. When liver glycogen stores are exhausted, the body starts using fatty acids instead of glucose (maximize fat oxidation) for it’s energy needs. The brain cannot use fatty acids for energy and instead uses ketones (produced by the fatty acids from the liver). Scientific literature shows that low-carbohydrate diets have favorable effects: rapid weight loss, decrease of fasting glucose and insulin levels, reduction of circulating triglyceride levels and improvement of blood pressure. By using fatty acids and ketones as energy sources, supplemented by conversion of proteins to glucose (gluconeogenesis), the body can maintain normal levels of blood glucose with very low dietary carbohydrates e.g. the Atkins diet states the human body is adapted to function primarily in ketosis. They argue that high insulin levels cause many health problems, like fat storage and weight gain. They say the dangers of ketosis is unsubstantiated (ketosis is confuse with ketoacidosis, which is mostly a diabetic and life threatening condition unrelated to low-carbohydrates intake). They also argue that fat in the diet only contributes to heart disease in the presence of high insulin levels; by adjusting the diet to induce ketosis, fat and cholesterol in the diet are beneficial. (Most low-carbohydrate diets discourage the consumption of trans fats).
- A study done in Sweden on weight loss diets, concluded that a low-carbohydrate ketogenic diet are more effective as a means to reduce weight than a low-fat diet, over a short period of time (6 months or less). One of the reasons why people also lose weight on a low carbohydrate diet, is related to the phenomenon of spontaneous reduction in food intake. Studies have shown that low-carbohydrate/high protein diets are more effective than low-fat diets in reducing weight and cardiovascular disease risk (HDL cholesterol, triglycerides, systolic blood pressure) up to 1 year.
- In the first week or two of low-carbohydrate diet a great deal of weight loss comes from eliminating water retained in the body. This is a short-term effect and is entirely separate from general weight loss that these diets can produce through eliminating excess body fat. A low carbohydrate diet will not reduce endurance performance after adapting, but there is a deterioration of anaerobic performance e.g. sprinting and strength training, because these performances rely on glycogen for fuel. The Olympic biathlon gold medalist Bjorn Ferry won his gold medal after about 6 months on a low-carbohydrate
- , high fat diet, which proves that a ketogenic diet can be combined with outstanding achievements in physical exercise.
- A ketogenic diet is known to cause dehydration as an early-onset complication.
- Other less desirable immediate effects of low-carbohydrate diets are: loss of lean body mass, increased urinary calcium loss, increased plasma homocysteine levels, increased LDL cholesterol. The long-term effect of these changes is unknown at present.
- However, these undesirable effects may be counteracted with consumption of a LOW—CARBOHYDRATE, HIGH-PROTEIN, LOW-FAT DIET – it induces favorable effects on feelings of satiety; preserve lean body mass; reduce fat mass; have a beneficial impact on insulin sensitivity and blood lipids; supply sufficient calcium for bone mass maintenance