The Omentum and Abdominal Fat: Health Benefits and Problems
Subcutaneous and Visceral Fat
The greater omentum is a fatty membrane that covers the small and large intestine. Until recently its only function was thought to be the storage of fat. Now researchers have discovered that it not only has other functions but also has some important health benefits. It seems to be a "Jekyll and Hyde" structure, however. If it contains too much fat, it has the potential to cause serious health problems.
There are two types of fat in the abdominal area—subcutaneous and visceral. Subcutaneous fat is located under the skin and above the muscles. It's the soft fat that we can feel when we grab hold of our skin. Visceral fat lies below the muscles in the omentum and around the organs.
Excess visceral fat can have a more serious effect on our health than excess subcutaneous fat. Health experts say that those of us who have an apple-shaped body (one with excess fat in the abdomen) have a higher risk for certain health problems than those of us with a pear-shaped body (one which collects fat in the hips and thighs). An omentum containing a lot of fat contributes to the apple shape.
The peritoneum and the greater omentum are membranous structures in the abdomen. The peritoneum is a smooth, glistening membrane on top of connective tissue. It lines the abdominal cavity and covers the organs in the abdomen. It's a continuous sheet but is given different names according to its location. The parietal peritoneum lines the inside of the abdomen and the visceral peritoneum covers the organs. The omentum is made of peritoneum.
Parts of the Stomach
The Greater and Lesser Omentum
An omentum is a sheet-like structure made of a double layer of peritoneum. It contains a variable amount of fat. There are actually two omenta—the greater one and the lesser one.
The greater omentum is attached to the greater curvature of the stomach, which is the outer curve furthest away from the midline of the body. (The curvature is shown in the illustrations above and below.) The greater omentum hangs over the small and large intestine, resembling an apron, and then folds back on itself to attach to the transverse colon. This is a horizontal section of the colon below the stomach. The greater omentum is often referred to as simply "the omentum".
The lesser omentum is much smaller than the greater one. It's attached to the lesser curvature of the stomach, which is the inner curve nearest to the midline of the body, and extends over the first part of the small intestine and the edge of the liver.
In most illustrations of the abdominal cavity, the omenta and peritoneum are removed in order to show the organs clearly. Some people may be surprised to learn that the structures and the membrane exist. This article focuses on the greater omentum.
A healthy omentum is a thin, pale yellow sheet that contains fat and often has a lacy appearance. Fat is an essential substance in our body. It only becomes dangerous when it's present in an excessive amount or in the wrong place.
If the omentum absorbs extra fat it becomes thicker and harder. An enlarged omentum may push the front of the abdomen outwards, producing a beer belly or potbelly.
For a long time, it was thought that the omentum wasn't important, except as a minor fat storage depot. Now there is evidence that it does more than just store fat and actually has some important functions.
Dr. Oz Describes and Shows the Omentum
The Immune System
The omentum contains "milky spots", which are collections of macrophages. Macrophages are a type of white blood cell and fight bacteria and viruses. The action of the macrophages in the omentum may help the immune system. This system protects us from disease.
Research suggests that the omentum may also suppress certain aspects of the immune system, at least in mouse cells studied in lab equipment. The discovery may or may not apply to the structure inside our body. The researchers examined the effect of mouse omentum cells on T cells (or T lymphocytes) from mice. T cells are a very important part of the immune system. They attack and destroy invaders such as bacteria, viruses, and cells from other people.
T cells become activated in order to do their job. The researchers mixed omentum cells with activated T cells in laboratory equipment and found that the T cells died. T cells that hadn't been activated were unaffected by the omentum cells. The cells apparently produced a substance that killed the activated T cells.
If this reaction occurs in humans it could be very significant. It may sound bad that T cells are killed, but in some conditions—such as organ transplants and autoimmune diseases—the immune system needs to be suppressed. An autoimmune disease is one in which a person's immune system attacks their own body. The discovery that the omentum may have the ability to dampen immune system activity could lead to improved methods for treating autoimmune diseases.
More and more researchers are finding evidence that the omentum functions in tissue repair and regeneration. In fact, some believe that this is the structure's primary job. Surgeons sometimes attach bits of omentum to damaged tissues in the body. They know from experience that this process stimulates tissue repair, but how it does this is uncertain.
The omentum may stimulate tissue regeneration because it contains mesenchymal stem cells, or MSCs. Stem cells are very important because they are capable of forming other cell types. This isn't true for the other cells in our body. In lab flasks, MSCs from the omentum have produced bone cells and cells that resemble lung cells.
Finding good sources of stem cells is important because they have the ability to repair damage. Human embryos are a good source of the cells, but the use of embryos is controversial. Obtaining the stem cells kills the embryos. Obtaining stem cells from adults (harmlessly) avoids this problem.
Visceral fat is stored around our organs and in our omentum. Although all excess fat can be dangerous, visceral fat may be especially so.
Dangers of Excess Visceral Fat
While the omentum seems to have some impressive and helpful functions, there's no doubt that excess visceral fat in the structure is dangerous. The fat cells are metabolically active and release a variety of chemicals. Some serve as messengers in the body and others trigger harmful inflammation. Fatty acids are also released from omental fat. These reach the hepatic portal vein, which transports them to the liver.
An excessive amount of visceral fat increases the risk of cardiovascular disease, hypertension (high blood pressure), high fasting blood sugar, insulin resistance, and type 2 diabetes. It also raises the blood level of LDL cholesterol (the bad cholesterol) and increases the risk of certain types of cancer. In addition, it seems to increase the chance of developing non-alcoholic liver disease.
Men generally have a larger tendency to develop visceral fat in the abdomen than women. After menopause, a woman's tendency to store visceral fat increases, however. The tendency is also influenced by genetic factors.
Detecting the Presence of Hidden Fat
If we don't have a beer belly, how do we know that we have too much visceral fat? Health professionals say that our waist size is an indication that we're likely to have excess visceral fat and may be a more accurate indicator of potential health problems than the body mass index, or BMI.
For a person of average height, a waist size over thirty-five inches in females and over forty inches in males may be a danger sign. Another guideline states that a person's waist size should be no more than half their height.
People who have serious health, weight, or nutritional issues should visit their doctor to get lifestyle advice appropriate for their situation.
Lose Abdominal Fat: Follow a Healthy Diet
Reducing calorie intake and getting regular, moderately intense exercise is the best way to lose abdominal fat, according to many health experts. Severely restricting calories will probably make a diet very hard to maintain, however. The best plan is to make permanent and healthy lifestyle changes instead of "going on a diet".
Nutritionists generally recommend a diet that emphasizes unprocessed or minimally processed foods from plants and consists of vegetables, fruits, whole grains, legumes or pulses, lean protein, low-fat dairy, and moderate amounts of monounsaturated and polyunsaturated fats. Foods that contain healthy fats include nuts and seeds, some fish, and certain vegetable oils.
Some "healthy" foods may be high in calories, so it's important to be aware of the calorie content of foods. For example, fruit smoothies may be loaded with calories as well as nutrients.
Scientists have found that even a small amount of weight loss— five to ten percent of a person's weight—is likely to have substantial health benefits. It's a great goal to aim for even if it doesn't have a huge effect on a person's appearance. Some research indicates that visceral fat is lost before subcutaneous fat when we reduce our calorie intake.
Get Regular Exercise and Adequate Sleep
Anyone starting a new exercise program should begin with relatively easy exercise sessions that last for a short period. This will reduce the chance of injury. The intensity and duration of the sessions should increase slowly over time. Caution is also necessary when someone who already performs one type of exercise decides to start another type. If someone is very overweight, they should see a doctor before they start exercising.
Getting an adequate amount of sleep on a regular basis is also important in a healthy lifestyle. Interestingly, scientists have discovered that lack of sleep is linked to an increased amount of visceral and subcutaneous fat.
The Definition of Obesity
Some scientists have suggested that we need a new definition of obesity that is based on the location of body fat. A person who looks overweight may have a lot of subcutaneous fat but not much visceral fat. This person may be healthy and have a low risk of many serious health problems. On the other hand, someone who looks thin may actually be unhealthy because they have a lot of visceral fat around their organs.
Sumo wrestlers are an example of people who look obese but are relatively healthy. Most of their fat is subcutaneous, not visceral. However, when they retire from wrestling and regular exercise, the amount of visceral fat in their body increases.
Specialized medical scans can detect visceral fat, but most of us don't have access to these scans. The best that we can do is to follow a healthy lifestyle and monitor our waist size in relation to our height. Even if our waist size is appropriate, we don't know how much fat is hiding deep inside our abdomen below the omentum. A healthy lifestyle is therefore important for everyone.
References and Resources
- Fatty membrane helps to regulate the immune system from the Medical Xpress news service
- Tissue regeneration by the omentum from PLOS One (an open access and peer-reviewed science journal published by the Public Library of Science)
- Beer belly facts from WebMD
- Body mass index problems from the BBC (British Broadcasting Corporation)
- A healthy lifestyle and a healthy weight from the CDC (Centers for Disease Control and Prevention)
- Sleep disturbances, body fat distribution, food intake, and/or energy expenditure from the NIH (National Institutes of Health)
- Health tips for adults from the NIH (The tips are related to diet, exercise, and stress reduction.)
This content is accurate and true to the best of the author’s knowledge and does not substitute for diagnosis, prognosis, treatment, prescription, and/or dietary advice from a licensed health professional. Drugs, supplements, and natural remedies may have dangerous side effects. If pregnant or nursing, consult with a qualified provider on an individual basis. Seek immediate help if you are experiencing a medical emergency.
Questions & Answers
© 2013 Linda Crampton