The Alimentary System (MAIDAT)
FOOD AS RIZQ
Basic Definitions: Nutrition is defined as utilization of food by living organisms. Food is a solid or liquid consumed for the purposes of providing energy, growth, repair, and maintenance of the body in the best health. Human food consists of proteins, fat, carbohydrate, vitamins, minerals, and indigestible fiber. The Qur’anic term rizq translates better as sustenance because it is wider in scope than food. As is known biologically some of the food ingested is not utilized by the body and does not contribute to sustenance of the body. Sustenance of the body is also possible without solid food in some situations that Allah chooses. The Prophet Muhammad (PBUH) for example said that Allah fed him so that he was able to work with very little food intake. Thus rizq should refer only to the food that is useful to the body. It excludes useless food, food that is not absorbed, or food that is not metabolized. Rizq is from Allah alone (p 707-9 11:6, 23:64).
Human nutrition: Humans eat a variety of foods. Perhaps no animal eats as wide a range of food as the omnivorous humans. They must therefore have an alimentary system that can deal with many types of food and reduce them to simple forms: glucose, fatty acids, and amino acids. Thus humans have the most sophisticated alimentary system. Only a proportion of the amount ingested is absorbed and is used by the body. The rest is eliminated from the large intestine. The amount absorbed is determined by: age, weight, height, health, and activity. Some food is harmful and does not fulfil the purposes of human nutrition explained above. Generally food considered haram is dangerous.
Appetite for food: Humans have an appetite for food that is a very strong instinct. The appetite for food is very strong; food is needed for survival. The appetite and hence the search for food start even before the body is in physical need of nourishment. The feeling of hunger is a warning sign of nutritional depletion in the next few hours and does not signal actual starvation. People would starve to death if the feeling of hunger coincided with actual depletion of food reserves. Human appetite is under the control of the hunger center, the thirst center, and the limbic system. Anoretic drugs suppress appetite. Appetite stimulants increase the desire for food. The human appetite for food is so strong that prayer is delayed when food is presented (Muslim # 1134, 1137, MB # 403 p 227). It is conceivable that a hungry person will not concentrate sufficiently in salat. The concept of hunger and thirst are sometimes psychological. A person will feel hungry at a time when he normally takes his meals or when food is smelled. Giving water to the thirsty, human or animal, is emphasised (MB#1094). Feeding the hungry is one of the best virtues.
Food and human behavior: The type of food and nutritional habits reflect underlying visions, beliefs, and cultural experience of individuals and societies. On the other hand one’s nutritional habits may affect the way of behavior and approach to life. For example many daily activities revolve around meal routines and there is a big difference observed in Ramadhan when those routines are interrupted. There is a difference in attitude to feeding between the believer and non-believer (KS p. 331). The etiquette of eating is determined by the underlying vision. The believer eats to get energy for ibadat. The non-believer may eat for enjoyment or to get energy for evil. There is blessing in the food of the believer; he gets satisfied easily. The non-believer has to eat more food to get satisfaction. The Prophet Muhammad (PBUH) in a very revealing hadith mentioned that a believer eats in one stomach whereas a non-believer eats in 7 stomachs. This means that a believer is satisfied with less food than a non-believer. Research is called for to be able to interpret this hadith in physiological terms.
Search for food: Lower animals spend all their waking life either eating or looking for food. Primitive man also spent much time gathering plant food or hunting animals. With growth of human civilization, humans have spent less time looking for food. They learned to domesticate plants and animals. They have also specialized in their division of labor. Some specialize in food production whereas others are engaged in other activities needed for survival of the society. The search for food and its ingestion is part of ibadat when done according to the law. The human is rewarded for efforts to feed himself or herself. The ibadat aspect is emphasized by the fact the feeding starts with the basmalah and end with the hamd (Ibn al Qayim p. 238).
Fasting: A human can survive for long periods without food. Usually nights are periods of fasting. There are periods of fasting during the day between meals or during Ramadhan. Humans can ingest 100 times more food than their immediate caloric needs. This food is stored for later use. The storage capacity is however limited. Glucose storage is only 100g in the liver and 250 g. in muscles. Muscles are a source of protein that can be metabolized for energy in cases of deficient dietary intake. Lipid deposits store fats that can be metabolized for energy. Food storage enables humans to fast from meal to meal. However fasting can not be prolonged beyond a certain period because of the critical need of the brain for glucose. At rest the brain consumes 66% of the circulating glucose and requires 100-150g of glucose per day. Unlike other tissues it can not utilize fatty acids. Lipid deposits and muscle protein can be mobilized when needed but for a limited duration. Fasting of Ramadhan is one the major acts of obligatory physical ibadat. Fasting cleanses the body (al siyam zakat al jism) (KS p. 325). The prophet (PBUH) taught measures to ensure that fasting does not cause physiologic damage: late suhur and early break-fast (KS p. 330, KS p. 331, Muslim #2412, 2414, 2415, 2417, 2419, 2420), prohibiting un-interrupted fasting (wisaal) (KS p. 325, Muslim #2426, 2427, 2429, 2430, 2433, 2434, 2435), exemption of the traveler from fasting (Muslim # 2467, 2468, 2470, 2471, 2472, 2474, 2475, 2476, 2477, 2479, 2480, 2481, 2482, 2483), and exemption of menstruating women from fasting.
Internal and external environments: The alimentary canal is strictly not part of the body. It is a tube covered with epithelium and in direct contact with the external environment. A lot of what is ingested from the mouth exits at the anus without having ever entered the body. It is only those food components absorbed from the intestinal tract that are considered to have entered the internal environment. The alimentary tract must be able to stand up to various types of irritation that humans choose to eat. It is also an excretory pathway for body wastes that are entering the external environment. It thus can play a role in the balance between the internal and external environments. The alimentary system also has a role in the control of the ecosystem since it takes material in the form of food from the environment, acts on it in the alimentary tract and returns it to the ecosystem.
Nutritional assessment: All down the ages, humans have experienced states of bad nutrition. Famines and starvation still occur in the world today despite its technological achievements that have taken man to the moon. At no time in human history was food security assured for all. There is always concern to prevent malnutrition at both individual and community levels. There is therefore a need to assess the nutritional status. Before scientific medicine, nutritional assessment was subjective. It was generally thought that fat and heavy people were well nourished and the thin ones were not well nourished. Now we have more scientific and objective assessment of malnutrition: dietary survey using 24-hour recall, biochemical assays, and clinical assessment. Malnutrition could be under-nutrition or over-nutrition. The prophet Muhammad (PBUH) taught about both under and over-nutrition. He condemned obesity as a sign of social degeneration. Malnutrition could also involve macro-nutrients and micro-nutrients either quantitatively or qualitatively.
Macro-nutrients: carbohydrates, lipids, and proteins are generally referred to as macro-nutrients. Their calorie content in kilocalories per gram are: carbohydrates 4, protein 4, and fat 9. The amount of energy needed by an individual is affected by: surface area, age, sex, and level of physical activity. Big and active people need more energy. Children and the elderly need less energy. Animal foods contain all the essential amino acids. Vegetable foods lack some amino acids. Carbohydrates can spare protein by being a source of energy. Diabetes mellitus and lactase deficiency are situations of carbohydrate intolerance. Periods of rapid growth, pregnancy, lactation, and illness result in higher protein requirements. Lipids play two major roles: energy and structural. Lipids are very efficient energy sources. Some fatty acids, called essential fatty acids, can not be synthesized by the body and have to be ingested in food. They include linoleic acid, linolenic acid, and arachinoid acid. Eight amino acids are essential; they can not be synthesized by the body. Eating animal protein is not efficient; it may be better to eat vegetable protein directly. Food of vegetable origin has a special advantage in that it contains fiber which plays various roles. Cellulose and hemicellulose provide bulk to the stools and decrease intestinal transit time. Lignin provides stool bulk in addition to binding cholesterol. Pectin decreases gastric transit and emptying.
Micro-nutrients: Micro-nutrients refers to vitamins, macro minerals, and trace elements. The most important vitamins are: A, B, C, D, E, and K. Vitamins A, D, E, and K are fat soluble. Vitamins B and C are water-soluble. Vitamin A, of plant origin, is needed for maintenance of epithelial tissues and is protective against cancer and infection. Vitamin D is synthesized by the body using sunlight and is needed for calcium metabolism. Too much sunlight exposure can lead to hypervitaminosis D. Polluted air reduces sunlight exposure and therefore synthesis of vitamin D. Vitamin E is an anti-oxidant. Vitamin K is involved in the coagulation process. Water-soluble vitamins play three main roles: (a) energy-releasing reactions: thiamine B1, riboflavin, niacin, pyridoxine B6, pantothenic acid, and liotin. (b) hematopoietic: folic acid, cobalamine B12 (c) others: ascorbic acid vitamin C. Vitamin C is protective against the common cold and some cancers. The macro minerals are magnesium, phosphorus, and calcium. Calcium in involved in enzymatic and hormone reactions, coagulation, and muscle contraction. Trace elements or micro-minerals are: iron, iodine, zinc, chromium, serium, and manganese. Chlorine, fluorine, and iodine are components of needed mineral salts.
DIGESTION AND ABSORPTION:
Anatomy and functions of the alimentary canal: The alimentary canal starts at the mouth and ends at the anus. It is a tube lying outside the body with several major parts: mouth, esophagus, stomach, small intestine, large intestine, rectum, and anus. The upper GIT (mouth, esophagus, and stomach) is involved in preparing food for digestion. The lower GIT (small intestine, large intestine and rectum) is involved in absorption. The small intestine is involved in both digestion and absorption. The large intestine is mainly absorptive. The general functions of the alimentary tract are: digestion, absorption, and elimination. Digestion is preceded by mastication and swallowing. Humans have less powerful teeth than many animals. The process of swallowing shows the co-ordination of many muscles. Enzymes act on food to cause hydrolysis that breaks down food into small absorbable particles. Food of different types and methods of preparation is reduced in the end to glucose, amino acids, and fatty acids. The alimentary canal absorbs 90% of digested proteins, fats, and carbohydrates.
The mouth (buccal) cavity: The mouth was described in several verses of the Qur’an (fam, p 282 3: 118, 3:167, 5:41, 9:8, 9:30, 9:32, 13:14, 14:9, 18:5, 24:1, 33:4, 36:65, 61:8). In only one verse was it mentioned as an organ involved in feeding (13:14). It is also one of the 2 dangerous holes that a human has, the other hole refers to the genitals. Both are involved in the biggest sins that humans commit: eating the prohibited, talking falsehoods, and adultery. The mouth is the final pathway for most worldly gains which end up as food in one way or another. Money is literally eaten because it is used to buy food. The struggle to accumulate wealth even at the expense of others’ lives has in the background assuring food security. The mouth is the entry to the alimentary system. The oral cavity is the site of break-down of food and start of digestion. The teeth, the tongue, and buccal muscles help in the mastication and swallowing of food. Salivary glands have both a digestive and lubricative function. Salivary glands secrete 0.5-1.5 liters/day, the maximal rate being 4ml/min. The mouth is wont to be infected because of its constant exposure to many types of food material that may stay in the mouth and provide a suitable substrate for bacterial action. The prophet recommended rinsing the mouth after a meal (MB # 158 p 126-7 & # 160 p 127) in order to clear away retained particles. Additional protection to the soft tissues of the mouth is by the normal flora and secretory IgA. The lip (shafat, 90: 8-9) plays a role in the initial mastication of food and is the actual gate-keeper of the buccal cavity. The teeth (sinn 5:45) are involved in both mastication and chewing of food. Because humans eat both soft and hard food, Allah created the tooth enamel as the hardest substance in the body to be able to stand constant attrition. The prophet recommended use of the tooth-pick to make sure that gaps between the teeth do not harbor food material that could be a substrate for infective bacteria (MB # 181-182 p 134, MB# 498 p 265, # 499 p 265-266, Muslim #487, 488, 489, 490, 491, 493, 494). Human jaw muscles can exert a lot of force but come nowhere the force of meat eating animals like the lion or the tiger. The force of biting is 120 N for incisors and 900N for molars. The tongue, lisaan (5:45) helps push food for chewing. It also helps in swallowing. Its taste buds enable discrimination and selection among foods.
Swallowing: The process of swallowing (deglutition) proceeds in a very coordinated and synchronized way. The mouth is closed and the tongue is raised against the hard palate forcing the bolus of food back. The nasal passage and trachea are closed. Breathing is inhibited during swallowing (deglutition apnea). The Prophet used to stop to breathe in the midst of drinking (Ibn al Qayim p. 236). He forbade drinking in one gulp (Ibn al Qayim 237) because that would prolong the deglutition apnea.
The esophagus: The esophagus is 23-25 cm long. It is a highly distensible muscular tube. It has three narrowings: proximal, mid-way, and cardiac. It is highly specialized in pushing food towards the stomach and prevent its regurgitation. Esophageal control is achieved in 4 ways: extrinsic innervation, intrinsic innervation, myogenic properties, and humoral substances. The maintenance of vagal tone is necessary for prevention of reflux.
Stomach (batn): The stomach was mentioned a lot in the Qur’an in an anatomical sense (2:174, 4:10, 22:20, 37:66, 44:45, 56:53). The Qur’anic term batn refers to the whole gastro-intestinal tract and not the anatomical stomach. The stomach can store 1.5 liters of food. A mixed diet takes 2-6 hours to transit the stomach. Humans unlike some animals like cows and camels can store food for only a few hours and have to replenish by feeding regularly. They are however better than birds that have to eat almost continuously. The stomach contains swallowed air. This could have been the air that the prophet referred to when he recommended that a person’s stomach should be divided into 3 thirds: solid food, water, and air. Besides its digestive function, the stomach acts to warn humans of impending starvation by its hunger pangs. Hunger pangs are due to powerful contractions of stomach muscles during fasting. The digestive function of the stomach is both mechanical (churning of food) and chemical (enzymatic digestion). The stomach wall is lined by 2 types of cells: chief cells that secrete pepsinogen and parietal cells that secrete hydrochloric acid. The stomach wall is lined by 1 billion parietal cells that are replaced every 2-6 days by rapid mitotic processes since they are very active and age rapidly. These cells release hydrochloric acid used in digestion on neurological vagal stimulation, endocrine gastrin stimulation, and local stimulation due to histamine released by mast cells. The phases of acid secretion mirror the three types of stimulation. In the cephalic phase, vagal stimulation leads to gastrin secretion due to sight, smell, taste, and swallowing of food. In the gastric phase, stimulation is mechanical or due to contact of food with receptors in the stomach wall. In the intestinal phase, digested food in the intestine leads to release of a stimulatory polypeptide. Gastric mucosa is protected from acid auto-digestion by mucosal secretions, bicarbonate, the epithelial barrier, and the rich mucosal blood flow that removes acid.
Intestine (am’a): The Qur’an has mentioned the intestine as being destroyed by bad food (47:15). Most digestion and absorption of nutrients take place in the small intestine. The small intestine is 6 meters long when fully relaxed. In the usual state of tonic contraction it is 2.5 meters long. The large intestine, 1.5 meters long, is largely absorptive. Both intestines have a large surface area to facilitate absorption. The intestines also secrete and excrete some products. Absorption is both passive and active. The active depends on the concentration and electrical gradient. The intestinal mucosa has a high regeneration potential given its active contact with food and other environmental irritants. The regeneration time for the small intestine is 4-6 days and for the large intestine 3-8 days. This high growth is needed to replace aged or damaged epithelium but also renders the intestinal epithelium liable to carcinogenesis. The appendix is developmentally part of the caecum. It has lymphoid tissue that atrophies progressively and finally becomes fibrous. The intestines are rich in normal flora consisting of bacteria. There is a well-maintained equilibrium between harm and benefit. It is human interference with this equilibrium that leads to infections. The intestinal lumen like the stomach is protected from auto-digestion. The splanchnic circulation has fine adjustments during digestion and absorption. Blood is diverted to the splanchnic bed during digestion and absorption. This explains the rest recommended after a meal because of decreased perfusion of muscles that undertake physical activities. The intestinal lymphoid tissue protects against infection.
Detoxification: The alimentary system is exposed to many toxic substances that humans take knowingly or not knowingly. It separates the nutritious food from the non-nutritious one. The former is absorbed into the blood stream whereas the latter is eliminated to the outside. The system protects the environment in 2 ways. Ingested toxic substances are treated before elimination so that they are no longer toxic. Some metabolic products of the body excreted via the alimentary canal are detoxified before elimination.
Feces: are indigestible cell debris, mucin, and bacteria (dead and alive). 50% of the mass of feces is due to bacteria. The color of feces is due to bilirubin. Feces are considered filth (najasat) largely because of their content of infective and toxic material. Proper disposal of excreta at both the individual and community levels is a mark of civilizational development.
Toilet hygiene: Islam has emphasized toilet hygiene for this purpose and treats it as part of religious duty. The Prophet took great care to teach his followers the main principles of hygiene. All books of fiqh start with the chapter on cleansing (taharat). Defecation must be in special places to avoid environmental contamination. Istinjah is the process of cleansing oneself of filth and its details have been given by Islam to make sure that there is personal hygiene that prevents infection. Ritual ablution for prayer can not be valid before istinjah (MB # 138 p 119). Hands must be washed after defecation and instinjah to prevent transfer of infected or toxic material to human food (MB # 117 p 112-113 & #121 p 114).
The concept of the final common pathway: The complex human food in all its varieties is in the final analysis reduced to a few chemical compounds after digestion (amino acids, glucose, and fatty acids). They are all finally reduced to Acetyl CoA that enters the Krebs cycle.
Metabolism and energy: The main purpose of the metabolic processes is to generate energy from food. Energy is needed for maintenance of body temperature, muscle activity, and metabolic processes. Energy is obtained from metabolism of carbohydrates, fat, protein, and ethanol. Protein, fat, and carbohydrates are interchangeable as sources of energy. Energy is stored in C-H and C-C bonds. The bonds yield different amounts of energy depending on whether they are single, double, or triple bonds. The potential energy content of foods measured as heat of combustion using the bomb calorimeter is: protein 2.25 mJ/100g, carbohydrate 1.65 mJ/100g, and fat 3.9 mJ/100g. The actual energy available, called metabolic energy or Atwater factors, is: protein 1.7mJ/100g, carbohydrate 1.6 mJ/100g, fat 3.7 mJ/100g.
The liver: The liver is the chemical intermediary between the alimentary system and the rest of the body. It is responsible for maintenance of the body’s chemical and metabolic homeostasis. It is the great factory involved in both anabolism and catabolism. The liver is also the great protector of the body by detoxifying toxic substances from the alimentary tract. The liver is the largest gland in the body. A normal adult liver weighs 1400-1600 grams being 2.5% of body weight. It receives 1.5 liters/minute of blood. Blood flow into the liver is 60-70% from the portal vein and 30-40% from the hepatic artery. The functions of the liver are (a) metabolism of amino acids, carbohydrates, fats, vitamins, and cholesterol (b) mechanical protection by phagocytosis of infective organisms (c) synthesis of proteins (d) bio-transformation of metabolites (e) detoxification (f) excretion of bile, and (g) storage. The detoxifying role of the liver is very important. It seems that it can deal with all toxins whether naturally-occurring or man-made. In carbohydrate metabolism, glucose enters the Krebs cycle as acetyl CoA. The products from the cycle are carbon dioxide and H atoms. In protein metabolism, amino acids are metabolized in many biochemical reactions and the final waste products are excreted via the urea cycle and urine. Essential amino acids are not synthesized by the body.
The gall bladder: This is an excretory organ embedded within the liver. It produces bile whose functions are: a) emulsification of dietary fat to facilitate absorption (b) excretion of excess bilirubin and excess cholesterol.
The pancreas: In the adult the pancreas is 15 cm long and weighs 60-140 gm. About 80-85% of pancreatic secretion is exocrine. The pancreas plays a role in carbohydrate metabolic regulation. It secretes insulin, glucagon, somatostatin, and pancreatic polypeptide.
GIT and NUTRITIONAL DISORDERS
Over-nutrition & Under-nutrition: The diseases of over-nutrition are: obesity, diabetes mellitus, ischemic heart disease, and atherosclerosis. These diseases are more common in the rich countries with a higher prevalence of over-nutrition. The prophet taught the rule of the thirds (musnad Ahmad) as a guide for food intake: one third for solid food, one third for water, and one third for air. He also taught that Muslims are a community who do not eat until they are hungry and when they eat they do not fill their belly. Ibn al Qayim defined three levels of food: necessary (hajat), sufficient (kifayat), and excess (fadhlat). The necessary amount of food is that necessary for maintenance of life and health. The sufficient is more than the necessary and satisfies the psychological desire for food. The excess is what is beyond the body’s needs and is definitely harmful to health. Early man in agricultural societies could use up all the food ingested because of the hard physical work of looking for food and assuring the basic necessities of life. Sedentary man in the modern industrial society still eats the same number of meals as early man without the same amount of physical work with the result that obesity develops. Obesity sumn was mentioned in the Qur’an in relation to animals being fed well and getting fat (p 599 12:43, 12:46, 51:26). Some food is fattening whereas other food is not; for example food of hell is non-fattening. Obesity is deviation from homeostasis. Obesity could be caloric or genetic. Caloric obesity develops when energy intake exceeds energy expenditure. In that situation the equilibrium between lipolysis and lipid storage is shifted in favor of storage. There is accumulation of fat. This affects androgen and estrogen metabolism. Atheroma and physical disabilities also develop. Protein-energy malnutrition is common in children of poor developing countries. Starvation occurs in famine situations (qaht). In the early stages of starvation the body can synthesize 160g of glucose daily from the liver. Muscle protein and fat stores are also mobilized. In later stages more deleterious effects appear leading to death.
Disorders of the upper GIT: Salivary glands may be infected (sialadenitis) or may develop neoplasms, both benign or malignant. The malignant neoplasms are either squamous cell carcinoma or adeno carcinoma. Esophageal congenital anomalies include atresia, fistulas, and diverticula. Chronic inflammation leads to esophageal stenosis. Achalasia is failure of relaxation leading to dilatation of the esophagus. Hiatal hernias occur when the esophagus herniates into the abdomen. Esophageal reflux is a functional disorder due to several causes. Esophageal lacerations may develop due to prolonged vomiting or alcohol. The congenital disorders of the stomach are diaphragmatic hernia and pyloric stenosis. The latter is in new-borns and leads to regurgitation and vomiting. Gastritis may be acute or chronic. Acute gastritis is caused by non-steroid anti-inflammatory medication eg aspirin, excessive alcohol, heavy vomiting, cancer chemotherapy, uremia, systemic infections eg salmonellosis, severe stress due to trauma or burns, ischemia and shock, mechanical trauma as in intubation. Chronic gastritis is associated with immunologic factors like pernicious anemia, chronic infection eg H pylori, alcohol, cigarettes, post-surgery, radiation, and chronic granulomatous conditions. Gastric and duodenal ulcers are increasing in most parts of the world due to increasing stress. An ulcer is defined as breach of the mucosa of the alimentary tract which extends through the muscularis mucosae into the submucosa or deeper. Ulcers occur in the stomach (peptic ulcers) or the duodenum. Gastric carcinoma occurs world-wide and is caused by dietary and genetic factors as well as chronic gastritis.
Disorders of lower GIT: Congenital intestinal disorders are: Merckel’s diverticulum, Hirschprung’s disease (congenital aganglionic megacolon), atresia and stenosis. Vascular disorders of the intestines may be ischemic bowel disease (following thrombosis) or hemorrhoids. Diets poor in roughage or fiber lead to constipation. Low fiber diets are associated with constipation, appendicitis and diverticulitis. Normal healthy people eliminate 3 times a day but elimination once a week is within the normal. Headache in constipation is not due to toxins from feces but due to distention of the colonic wall. Entero-colitis may be: secretory (infection, neoplastic, laxatives, and mal digestion), osmotic (disacharidase deficiency, antacids, drugs, or malabsorption), exudative disease (inflammation bowel disease, infections eg shigella), deranged motility (decreased transit time, diverticula, blind loop syndrome, bacterial overgrowth, neurogenic or irritable colon syndrome), malabsorption. Diarrhoea may also be caused by: eating too much food, food poisoning, change of diet, and emotional changes. Inflammatory bowel diseases are: Chron’s disease and ulcerative colitis. Bowel obstruction may be due to hernia, adhesions, intususception, and volvulus. Benign bowel tumors are polyps and adenomas. Malignant tumors are colorectal adenocarcinoma and lymphomas. The risk factors for colo-rectal cancer are: low fiber diet, high fat and protein intake. Lactose or milk intolerance is due to enzyme deficiency and may manifest as diarrhoea. The sources of gas are from swallowing direct from air or air trapped in food. Air is also from bacterial fermentation. Flatus through the anus is 0.2-4 liters per day. Flatus consists of hydrogen sulphide, methane, hydrogen, and atmospheric gases. Excessive flatus (as sound or smell) requires repeating ablution but not istinjah.
By Prof Omar Hasan Kasule