5.1 Introduction
Digestion of fats is comparatively simpler and more uncomplicated than the absorption of carbohydrates and proteins. The pancreas and the liver play major roles in the digestion of fats by secreting fat-digesting enzymes including lipase. The liver secretes bile that contains bile salts.
Fats are emulsified before they are broken down in the small intestine. Bile and pancreatic lipase help to break down emulsified fats into fatty acids and glycerol. The broken-down fats enter the lymphatic system, from where they are distributed to tissues. Excess fat is stored in the liver and triggers formation of insulin.
Fats are crucial for performing certain functions in the body including cushioning of organs, preventing heat loss and improving joint lubrication. Important organs like the brain and the retina are made up of fats. A diet that is inadequate in fats is likely to lead to adverse effects. Excessive intake of fats is dangerous and results in the increase of health risks for stroke, heart disease and high blood pressure. High levels of fats in the body also reduce sensitivity to insulin.
Micronutrients are required in minute quantities on a daily basis as they play a crucial role in health and wellbeing. Vitamins and minerals are the main micronutrients, and you will learn about the different types of vitamins and minerals as well as their functions. Inadequate intake of micronutrients often leads to deβiciency diseases.
Fact
Research studies indicate that adults carry roughly 50 billion fat cells in the body and they can expand up to ten times their size. 1 gram of fat provides 9K calories of energy.
Source: www.weightlossresources.co.uk
5.2 Step by Step Fat Digestion and Absorption
One of the key challenges in the digestion of fats is the issue of solubility. Fats are insoluble in water (hydrophobic) and consequently, they are not absorbed in the aqueous environment of the stomach. The digestion and absorption of fats can be classified into three major
processes.
Emulsification of Fats
Fats are churned in the stomach into a rough emulsion (minute liquid droplets suspended in another liquid is known as an emulsion). This rough emulsion is then emptied into the small intestine (also known as the duodenum). Certain hormones called 'enterogastrones' inhibit the process of emptying of fats into the duodenum and slow down the process. This is the reason that we tend to feel satiated or full for longer hours after consuming fatty foods.
Emulsification is the process of breakdown of fats into smaller emulsion droplets which are suspended in liquid form. The liquids involved in this process are the acids of the small intestine and the two major organs involved in the digestion of fats are the liver and the pancreas.Pancreatic lipase is a water-soluble enzyme that helps in digestion of fats but only at the surface level. This is due to the aqueous environment of the small intestine which prevents further breakdown of the fat molecules (fats are insoluble in water). The pancreas, located just below the stomach, also secretes bicarbonate ions that neutralise the pH of the chyme that enters the small intestine from the stomach.
The liver secretes bile and it is then stored in the gall bladder. Once the fat travels down to the small intestine, it is acted upon by bile first and then by pancreatic lipase which is able to act on smaller particles of fats. The biggest advantage of fat emulsification is the increase in surface area which makes absorption easier.
Without the effective action of bile, fat molecules would simply aggregate (clump together) and form one large molecule. Please note that bile acids are made from cholesterol and released into the small intestine through the bile duct. When fat and bile molecules combine, they form spherical structures known as 'micelles'.
Lipase acts on micelles and breaks them down into fatty acids and monoglycerides. As the fatty acids and monoglycerides pass through the small intestine, they combine with phospholipids, proteins and cholesterol to form structures known as 'chylomicrons'. The coating of the chylomicrons also makes them water-soluble and this helps them get absorbed through lacteals (lymphatic capillaries) and finally enter the blood stream. Chylomicrons also contain other by-products such as fat-soluble vitamins. The lymphatic system, along with plasma, is responsible for transporting absorbed fats through the body.
Enzymatic Breakdown of Fats
Once the fats are emulsified into small particles suspended in a seamless liquid, lipase is able to deconstruct almost all the fat into a form that is easily absorbed into the blood. 'Colipase' is the name of a protein that helps bind lipase together for easier breakdown action. Bile salts are produced in the liver and help in the digestion of fats and elimination of toxins.
Distribution and Absorption of Fats
Products of fat digestion include fatty acids and cholesterol that are absorbed through the walls of the small intestine. In general, roughly half the cholesterol generated gets passed into the lymphatic system. The remaining half is eliminated through faeces. The fats are introduced into the body through the neck at the thoracic duct. Fatty acids that comprise smaller molecules (chains of 4-10 carbon atoms) are absorbed directly into the bloodstream and transported into the liver. Smaller fatty acids in general are comparatively more water-soluble and are hence not as dependant on the process of emulsification. Our feelings of satiation stem more from the type and quality of food that we eat rather than the volume.
Note: On a minor level, fat digestion begins in the mouth where a small amount of lingual lipase is released to begin the breakdown process. Lingual lipase is produced in a larger amount in infants because it is particularly useful in the digestion of milk fats. Lipase is also secreted in breast milk to enable easy digestion of milk fats.
5.3 Brown Fat and White Fat
Brown fat is essentially derived from muscle tissue and contains a higher number of mitochondria (mitochondria are organelles that are responsible for breaking down nutrients and creating energy for the cell). Infants and hibernating animals have higher levels of brown fat which usually decreases with age.
White fat (also known as adipose tissue) is found in a subcutaneous layer as well as around organs. Insulation and cushioning organs against injury and physical stress are the two main functions of white fat. In addition, white fat also has important hormone sensors and can detect multiple hormones including stress hormones, sex hormones and insulin. White fat is also known to secrete a number of hormones. However, excessive amounts of white fat can result in an increase in the risk of heart disease, cancer and type 2 diabetes. Fats are not always bad - in fact, we need to include a certain amount of fats in our diet. Fats areimportant in several different ways including transport of fat-soluble vitamins and providing support for important physiological processes.
Some Important Functions of Fats
Fats provide a concentrated source of energy per gram which is more than twice the energy provided by carbohydrates and quadruple the amount of energy provided by fibres. However, the energy from fats cannot be metabolised quickly.
For example
When you run a short race, the energy comes from carbohydrates rather than from fats. Cell membranes help give structure and integrity to the cell and control the substances that pass in and out of the cell. The quality of phospholipids and triglycerides help determine the fluidity of the cell membrane and its arrangement. Processed fats (which contain a high percentage of saturated fats) affect the arrangement of phospholipids.
Nutrients are also unable to enter the cells efficiently and the cell is said to be 'clogged up'. Poor permeability leads to inefficient energy utilisation and adversely impacts growth and development. All cell membranes are made of cholesterol although the amount varies according to the type of cell membrane.
Fats are insulators and help prevent loss of body heat (this is why individuals who suffer from anorexia nervosa have very little fat insulation in their bodies and feel cold even on warm days). The brain structure is made from 60% fat and fat is also an important constituent in the structure of the retina. Fats also form a protective sheath for nerve βibres which is known in biological terms as 'myelin'. Fats are important carriers of fat-soluble vitamins including vitamins A, D, E and K.
Cholesterol is essential in the production of vitamin D under the skin as well as for the formation of bile salts used in the breakdown of fats.Fatty acids are important metabolic regulators and act as precursors for a family of bioactive compounds known as 'eicosanoids'. These compounds are responsible for blood clotting, constriction of blood vessels and inflammation. In addition, prostaglandins are hormone-like cyclic fatty compounds that are responsible for crucial functions including aggregation of blood platelets, lowering of blood pressure, a rise in body temperature and stimulation of muscle contractions.
Malabsorption of fats can lead to the problem of fats that are left unabsorbed and this in turn results in a medical condition known as 'steatorrhoea', where abnormal quantities of fat are excreted in faeces.
5.4 A Closer Look at Omega 3-6-9 Fatty Acids
The Omega fatty acids have a designated number after a hyphen that denotes the exact position of the carbon-carbon double bond.
Omega-3 Fatty Acids
Omega 3 fatty acids are considered essential fatty acids (this means that the body cannot make it on its own) and are crucial for a number of biological functions. They are polyunsaturated fats which help reduce the risk of heart disease, enhance the elasticity of blood vessels, reduce cholesterol and minimise the build-up of harmful fatty deposits in arteries.
Based on their chemical shape and size, omega-3 fatty acids can be of three types:
EPA (Eicosapentaenoic acid): EPA fatty acids have a chain consisting of 20 carbon atoms that help reduce inflammation and depression. This is due to the fact that EPA inhibits the formation of arachidonic acid that is responsible for cellular inflammation.
DHA (Docosahexaenoic acid): Structurally composed of 22 carbon atoms, DHA comprises about 8% of brain weight and is responsible for normal brain function. It helps improve elasticity of cell membranes and supports the transmission of signals from the membrane to the interior of nerve cells. Moreover the sweeping action of DHA acids helps inhibit the entry of unhealthy LDL cells
into membranes because they are larger in size.
ALA (Alpha-linonelic acid): This is a 20-carbon chain fatty acid that provides energy for the body.
Good sources of Omega-3 fatty acids include flaxseed oil, sardine oil, walnuts, salmon, herrings, canola oil and seaweed. They are also found in pumpkin seeds, Brazilian nuts and mustard seeds.
Omega-6 Fatty Acids
Omega-6 fatty acids are also polyunsaturated fatty acids. Research indicates that they may be helpful in relieving symptoms of rheumatoid arthritis, reducing symptoms of eczema and providing relief for women suffering from the menopause. Omega-6 fatty acids are found in rapeseeds, pistachio, wheatgerm, sesame oil as well as raw nuts and seeds.
Omega-9 Fatty Acids
These are unsaturated fatty acids found in plant and animal foods; they are also synthesised in the human body. They are also known as monounsaturated fats or oleic acids, and are found in foods like canola oil, mustard oil, safβlower oil and almonds. Omega-9 fatty acids are known to streamline cholesterol imbalances in the body by decreasing LDL and increasing HDL cholesterol levels. Studies also suggest that Omega-9 fatty acids may help reduce mood swings and enhance energy levels.
Specific Omega-9 fatty acids like erucic acid may also be helpful in relieving certain symptoms of Alzheimer's disease including cognitive function. Omega-9 fatty acids can be found in almonds, pistachios, avocados and olive oil.
Healthy sources of oil include:
Almonds
Almond butter
Avocado
Egg
Flaxseed oil
Low fat dairy products
Olive oil
Peanut butter
Pistachios
Salmon
Sunβlower seeds
Tofu
Tuna
Walnuts
Unhealthy sources of fat include:
Beef
Butter
Fried food like doughnuts
Full fat dairy products
Lard
Margarine
Pastries/Cakes
Pork
Shortening
Tropical oils like coconut oil and palm oil
Effects of Excess Consumption of Fats
-Excessive intake of fats leads to obesity or an increase in weight, which is known to result in high blood pressure, heart problems, breathing difficulties, diabetes, irregular menstrual flow, and joint problems.
-Obesity is also known to increase the risk of certain types of cancers including cancer of the colon, breast and gall bladder.
-High cholesterol levels can block arteries and blood vessels due to deposition of plaque and result in stroke and heart attacks.
-Cholesterol levels are considered to be high if it is higher than 200 mg/dL. Obesity (which is usually caused due to high consumption of fatty food) forces the heart to work harder to pump blood around the body and this leads to high blood pressure. High blood pressure increases the risk of stroke, heart attacks and kidney failure.
-The higher the level of fat in the body, the lower is the sensitivity to insulin.
-Excessive levels of circulating fat (a condition known as lipemia) results in deposition of fat on the liver and muscle tissue. The increased fat layers around muscle tissue which in turn suppress glucose metabolism and stimulates increased release of insulin from the liver.
Effects of Consuming Inadequate Amounts of Fat
Poor absorption of fat-soluble vitamins: In the absence of adequate fat, vitamins A, D, E and K are not readily absorbed into the body and may be excreted without being ingested. This will result in vitamin-deficiency diseases such as problems with vision, lack of adequate immunity against diseases, poor transmission of nerve impulses, weakened bones and cancer.
Mental health issues: Lack of Omega-3 and Omega-6 acids may lead to the development of mental health issues such as depression. Fatty acids are crucial to the formation of protective sheaths around nerve fibres and lack of fats can lead to bipolar and eating disorders as well as poor transmission of nerve impulses.
Decreased levels of HDL: Intake of inadequate levels of healthy fats could result in decreased levels of HDL (High Density Cholesterol) and increase the risks of stroke and heart disease. This is due to the fact that HDL draws away 'bad' cholesterol from the heart and transports it to the liver for excretory purposes.
Another important point to be aware of is that low fat foods are often laden with sugars to make up the taste and the calories. It is a better idea to eat the right kind of fats instead of consuming a low fat diet over an extended period of time.
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Are all fats unhealthy?
5.5 Introduction to Micronutrients
Micronutrients are nutrients that the body needs in small quantities in order to survive. There are chemical reactions in every cell of the body, and catalysts involved in helping the reactions proceed in the right manner. While most of the catalysts are enzymes (which are made up of proteins), there also certain organic cofactors that help these reactions take place. These factors include vitamins and minerals. Since the body is unable to synthesise vitamins on its own, it is necessary to consume sufficient amounts in the diet. Although vitamins and minerals are required in very small amounts, they are essential for the normal functioning of the human body.
Metabolic requirements and the role of micronutrients in biological functions determine the body's needs for vitamins and minerals. Vitamins were discovered in 1912 by Casmir Funk, a Polish scientist who called them 'vital amines'. In ancient cultures, vitamins were often consumed in different forms; such as soaking maize in lime water in order to release niacin or making drinks made from pine-needles. These were all forms of vitamin consumption.
5.6 Vitamins
Vitamins can be essentially categorised into fat-soluble and water- soluble vitamins. They possess the following characteristic features.
They are organic in nature and can be easily destroyed Vitamins are essential nutrients (implying that we need to consume them from food and that the body is unable to synthesise them on its own) and inadequate consumption or malabsorption often results in cessation of vital functions.
Vitamins may work individually or together with other vitamins to perform specific metabolic functions. Sometimes different vitamins can be involved in different parts of the metabolic process. Vitamins are present in foods from plants as well as animals in small amounts. Vitamins are chemically different and can be synthesised in the laboratory. This is why they are available in the form of supplements and they function in a way that can be compared to natural vitamins and provide similar benefits.
Vitamins and minerals are needed by the body in small amounts and are measured in milligrams or micrograms. The ability to absorb vitamins varies from person to person.
Fat Soluble Vitamins
Fat-soluble vitamins are found in oily or fatty parts of food and are usually absent from fat-free foods. The digestion and absorption of vitamins requires normal fat digesting mechanisms to be in place. They are digested and absorbed along with other fatty foods and released into the lymphatic system.
Fat-soluble vitamins require a carrier that is lipid-soluble in nature and they are not excreted in urine; they are stored in adipose tissue as well as in the liver (like other fats). Vitamins should be taken in small amounts and excessive intake can have toxic side-effects.
Fact
According to the National Diet and Nutrition Survey, 46% of Britons consume vitamin supplements and two-thirds of the total population fail to consume βive portions of fruits and vegetables on a daily basis.
Source: www.express.co.uk
5.7 Vitamin A
Vitamin A was the first fat-soluble vitamin that was identified and there are three types of vitamin A that are found in the body. These are called retinal, retinol and retinoic acid. Together, they are known as retinoids. While there is inter-conversion between the βirst two forms, once the vitamins are converted to the acid form, they cannot be converted back to the original state. However, a class of compounds known as 'carotenoids' can be converted slowly into retinol and the most important carotenoid is beta- carotene.
In the UK, the mean total consumption for retinol for men and women has been pegged at 940 μg/day. Normal recommended intake for retinol is 600-700 μg per day. However, research indicates that intake of vitamin A has fallen in recent years due to reduced consumption of fat. Vitamin A that is derived from animal foods is easily hydrolysed in the intestine. Eggs, butter, dairy products, cod liver or βish oils are all good sources of vitamin A. Plant foods contain vitamin A mainly in the form of carotenoids which can be found in the form of red or yellow pigments in vegetables and fruits. Rich plant sources of carotenoids include carrots, dark, green, leafy vegetables, tomatoes, red peppers and broccoli. They can also be found in mango, peaches and apricots. Normal cooking processes do not have any adverse impact on the carotenoids or retinol.
For example
Cooking carrots make them soft and enhances their ability to be easily digested. Vitamin A is carried to its target sites with the help of RBP or Retinol Binding Protein as well as pre-albumin in the blood plasma. Actually, this double carrier molecule (consisting of RBP and albumin) is too large to be excreted through the kidneys. This protects the body from vitamin loss through urine. The body usually
controls the level of plasma retinol levels but this may vary during inflammatory disease states when the RBP produced in the blood may be lower than normal.
Some functions and beneβits of vitamin A include the following.
Important role in improving vision
Retinyl esters are transported to the retina where they get converted to retinal form, from where they are transported to photoreceptor cells. The retina is located at the back of the eyes and is a light-sensitive layer. The retina is made up of two types of cells: The rods (that are sensitive to dim light) and the cones (that are sensitive to colour and daylight).
In the photoreceptor cell, the retinal binds itself to a protein known as opsin in order to form a visual pigment known as rhodopsin. Cells that contain rhodopsin are extremely sensitive to light and can detect even small amounts of light. People who lack vitamin A have inadequate rhodopsin cells and consequently cannot see well at night when there is less light. This condition is commonly known as 'night blindness'. Most of us find it easy to 'adjust' to dim light.
Important role in gene expression
Retinol is oxidised to retinoic acid in the cell and in the acid form, it plays an important role in cellular differentiation and gene expression. Retinol as well as its metabolites play a crucial role in forming a first line of defence against infection. Their important role in cell differentiation helps produce white blood cells called 'lymphocytes' that help improve immunity function. Children with inadequate levels of Vitamin A are found to be more vulnerable to respiratory infection.
Similarly, retinol plays an important part in embryonic development including the formation of the heart, eyes and ears through the process of cell differentiation. Vitamin A also plays a crucial role in the differentiation of epithelial cells (epithelial cells are found on the surfaces of the body including urinary tract, skin and blood vessels). In fact, vitamin A is responsible for the structure and integrity
of lungs, trachea and conjunctiva of the eye. This is why a lack of vitamin A results in a condition called 'dry eye'. In addition to epithelial cells, vitamin A also helps in the remodelling, resorption and deposition of bone.
Vitamin A for red blood cells
Vitamin A also helps promote development of precursor (or stem cells) into red blood cells. Vitamin A also appears to help in the mobilisation of iron into red blood cells from storage sites for the purpose of incorporation into haemoglobin.
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MODULE SUMMARY
Contrary to popular belief, a certain amount of fat intake is necessary and important for health and wellbeing. The underlying issue is consuming sources of healthy fats (monounsaturated or polyunsaturated fats). Since fats provide a high amount of energy per gram, they need to be consumed in smaller quantities.
Fat digestion begins with fat emulsification in the small intestine, Bile and pancreatic lipase help break down fats into its constituents (fatty acids and glycerol) and these are then released into the bloodstream through the lymphatic system. Micronutrients are required in small quantities but are crucial for multiple biological functions.
Vitamins and minerals act as organic cofactors in catalysing chemical reactions in body cells. Vitamins can be classified as fat-soluble and water-soluble and they may work alone or together other vitamins to perform certain metabolic functions. High levels of vitamins may result in toxicity and this may result in adverse side-effects. There has been concern over the growing number of over- the-counter vitamin supplements that actually provide very high levels of vitamins. It is important to be aware of daily recommended allowances.
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