Your body absorbs two types of nutrients: macronutrients and micronutrients. Macronutrients (carbohydrates, proteins and fats) are your body’s direct fuel or energy sources, whereas micronutrients (vitamins, minerals, etc.) indirectly influence available energy, by serving as catalysts to release the macronutrients. But they require efficient absorption to fulfil their duties optimally.
The journey of absorption begins with digestion. After food is chewed and broken down by enzymes in your saliva, it travels down the oesophagus and into your stomach. There, digestion intensifies through the action of hydrochloric acid and enzymes like protease (breaks down protein), lipase (breaks down lipids or fats) and amylase (breaks down carbohydrates). The food is then broken down into what is called chyme.
Some types of nutrients take longer to digest than others (e.g. fat and protein take longer than carbohydrates because enzymes that begin to break down carbohydrates are released earlier in the digestive process, in the mouth). According to nutrition experts, carbohydrates, proteins and fats leave the stomach in approximately three to five hours.
Apart from alcohol, no nutrients are absorbed through the stomach. The chyme leaves the stomach and is pushed into the small intestines by a process called peristalsis. The small intestines is where virtually all nutrient absorption takes place, including proteins, carbohydrates, fats and water, as well as vitamins and minerals.
There are two primary ways that nutrients cross through the walls of the small intestine and enter the bloodstream: passive diffusion and active transport absorption. Passive diffusion is when nutrients move from an area of high concentration (from inside the intestine) to an area of low concentration (the bloodstream). Active transport absorption is when nutrients require a helper or carrier molecule to get through the intestinal wall, into general circulation.
Some nutrients are more prone to one type of absorption than the other. All vitamins, both fat-soluble ones (like A, D, E, and K), and water-soluble ones (like B’s and C), are absorbed through passive diffusion. Minerals like calcium and magnesium are transported actively. Many nutrients can be absorbed more easily than others, depending on what food they are being consumed with, and the body’s current nutritional status. The less your body has of a specific nutrient, the more readily it will be absorbed.
You generally don’t have to worry about absorbing more nutrients than you need. That’s because your body constantly strives for homeostasis, a physiological term for balance. It will up-regulate its absorption for nutrients that it is deficient in, and down-regulate absorption for the nutrients is has an abundance of.
Absorption of protein
Proteins (amino acids) are absorbed through the intestines via active transportation. There are different transporters for different amino acids. The most common transporters are sodium (Na) dependent transporters, which take up neutral or charged amino acids. There are chloride (Cl) transporters as well. There are transporters for both two amino acids (di-peptides) and three amino acids (tri-peptides). The rate-limiting factor for amino acid transport is the type of transporters in the intestines.
It is in the small intestines that 95% of dietary protein is absorbed; the remainder passes through to the colon to be fermented by bacteria. Protein is responsible for the building and repair of tissue. The small intestines not only absorb protein for the body but also take what it needs for its own survival.
By measuring the amino acids in the stool, it is possible to tell which amino acids are not being absorbed.
There is so much contraindicating information about how much protein can be absorbed at once. The amount is thought to be 5-10g per hour depending on the source of protein consumed. After a workout, when your body needs protein, a protein shake may allow for greater absorption than a steak eaten at a different time of the day. There is no real evidence showing how much protein can be absorbed at one time. The current suggestion is 30g, based on assuming 10g/hour can be absorbed, with a typical meal taking three hours to digest.
The digestive hormone CCK is released when dietary protein is present in the intestines. CCK has the ability to slow down intestinal contractions (as well as regulating appetite and satiety), allowing for more protein to be absorbed before passing through and excreted.
The intestines use the amino acid glutamine for fuel, and can turn certain amino acids into glutamine for its own use.
It's actually been shown in a few studies that the body can absorb more protein than 30g, although most evidence suggests that consuming more than 30g of protein after exercise has no additional benefits. YBP have given you a recommended amount of protein to consume around your workout, the remainder would be best spread as evenly as possible between your other meals that day.
Absorption of carbohydrates
Carbohydrates are found in a wide variety of foods including, fruits, vegetables, grains, potatoes, pastries and sweets. They are the body’s preferred source of energy. Carbohydrates are made up of carbon, hydrogen and oxygen (CHO).
Carbohydrates are broken down by the body into sugar (glucose). There are three types of carbohydrate, and the number of sugar molecules they are made up of defines them.
Monosaccharide’s are a one sugar molecule, examples include glucose, galactose (in milk), and fructose (in fruit).
Disaccharide’s are two sugar molecules, examples include sucrose (glucose+fructose: table sugar), lactose (glucose+galactose: in milk) and maltose (glucose+glucose: in beer).
Polysaccharide’s are several sugar molecules, examples include starchy foods like pasta, or potatoes, and fibre, which is the indigestible part of a plant that aids digestion.
Mono and disaccharides refer to simple sugars, which are easily and quickly absorbed due to their simple molecular structure (milk, fruit, table sugar). The term complex carbs refers to polysaccharides, which due to their complex structure are slower to absorb (vegetables, potatoes, grains).
A common thought process is to label simple sugars as bad and complex as good, this is not the case, all carbohydrates have their uses.
Carbohydrates are broken down and enter the bloodstream, increasing the amount of sugar (glucose) in the blood. The level of sugar in the bloodstream is called your blood sugar level. After consuming carbs, the blood sugar level rises, which activates the hormone insulin. Insulin’s job is to help the transportation of the blood sugar, out of the blood stream (large amounts are toxic) and into the muscle cells (which can absorb about 300-400 grams) and liver cells (which can absorb 100 grams). Here they are converted to the stored form of glucose called glycogen.
The amounts of carbohydrate you require depends on several factors, physical activity levels, gender, your training and of course body type. When the muscle and liver stores are full, any excess glucose will be converted to fat (De novo lipogenesis). After training, your muscles become extra sensitive to insulin, meaning that more glucose will be drawn into the muscle, rather than converted to fat.
The Glycaemic Index was created to measure the speed in which carbohydrates are converted to glucose, measured from 1 to 100. Foods that digest quickly are measured high on the index, while foods that digest slowly are lower. The higher up the index the faster and higher the subsequent insulin release will be. This is important because large spikes in insulin affect your hunger (can make you even more hungry, as they crash down), negatively impacting fat loss, and can even lead to diabetes if levels are chronically elevated from over consumption of fast digesting carbohydrates.
What is fibre?
Dietary fibre is the indigestible portion of carbohydrate derived from plants. It can be divided into two components:
Soluble fibre, which dissolves in water, is fermented in the colon, producing gases and physiological by products. Soluble fibre is viscous and can be a prebiotic (feeds probiotics).
Insoluble fibre, which is not dissolved in water, but absorbs water, provides bulking and is metabolically inert. It can be a prebiotic, and metabolically ferments in the large intestines. The absorption and bulking of the insoluble fibre helps ease defecation.
Fibre changes the nature of the contents that pass through the gastrointestinal tract and by changing how other nutrients and chemicals are absorbed.
Chemically, dietary fibre consists of non-starch polysaccharides, such as arabinoxylans, cellulose, resistant dextrins, inulin, lignin, waxes, chitins, pectins, beta-glucans, and oligosaccharides.
Food sources of dietary fibre are often divided according to whether they are predominantly soluble or insoluble fibre. Plant foods contain both types of fibre in varying degrees, according to the plant's characteristics.
Soluble fibre helps to remove hardened faecal matter from the lining of the colon wall. Soluble fibre binds to bile acids in the small intestine, making them less likely to enter the body, this in turn lowers cholesterol levels in the blood. Soluble fibre also attenuates the absorption of sugar, reduces sugar response after eating, normalises blood lipid levels, and once fermented in the colon, produces short-chain fatty acids as by products with wide-ranging physiological activities. Short chain fatty acids are a major energy source for colonocytes (cells of the colon) and they produce an acid environment preventing growth of harmful bacteria and yeasts. It may also be beneficial in lowering overall blood cholesterol and bad LDL cholesterol. It also slows the rate at which glucose enters the blood so helps diabetes and blood sugar control.
Insoluble fibre helps keep the bowels regular. Consuming insoluble fibre also produces healthy compounds during its fermentation and has the ability via its passive hygroscopic properties (the ability of a substance to attract and hold water molecules from the surrounding environment through either absorption or adsorption), increasing in volume, stickiness and bulk, softening the stool, to shorten transit time through the intestinal tract.
There are disadvantages of too much fibre in the diet, high intestinal gas production, bloating and elimination of important nutrients. Constipation can occur if insufficient fluid is consumed with a high-fibre diet. The re-absorption of toxins can also result in constipation. Soluble fibre is mainly found in legumes, vegetables, fruits, oats, barley, rye and seeds. Insoluble fibre is mainly found in wheat bran, other whole grains and some vegetable and fruit skins.
Functions of fibre in the body
It improves transit time and assists the movement of waste products through the gut. Fibre acts like a broom and sweeps waste material along the digestive tract, thus preventing the build-up of stale faeces in the colon, which can lead to toxins being released which irritate the digestive tract.
It helps balance the levels of glucose by binding to the sugar molecules and slowing down their absorption into the blood.
It helps stabilise blood cholesterol and therefore reduces the risk of heart disease.
It helps to protect against haemorrhoids, varicose veins, diverticular disease, gallstones, kidney disease and constipation.
It binds to and deactivates carcinogens therefore helping reduce the risk of colon cancer.
It helps reduce the build-up of toxins, yeasts and unfriendly bacteria.
Conditions fibre can help improve
Irritable bowel syndrome
High blood pressure
Sources of fibre
Nuts and seeds, especially almonds
Whole grains, such as barley, brown rice, rye and oats
Pulses, lentils and beans
Fruits, including apples, apricots, bananas, berries, figs, peaches, pears, prunes, raisins,
Vegetables, such as asparagus, beetroot, broccoli, brussels, cabbage, carrots, celery, courgettes, peas, potatoes, spinach, sprouts and sweet potatoes
Always drink plenty of water and fluids as this helps your body to use fibre safely.
Increase your fibre intake gradually over several weeks. If you increase your fibre intake too quickly you may experience gas and bloating.
Avoid coarse wheat bran as it can be irritating to the digestive system. Instead opt for more soluble fibre as found in oats, beans, brown rice, pulses, fruit and vegetables.
Eat at least two pieces of fresh fruit a day.
Eat at least five different vegetables with your meals during the day.
Snack on nuts, seeds and dried fruits as these all contain fibre.
Flaxseeds/linseeds are an excellent source of fibre and if soaked in water before eating are easily digested and gentle on the digestive system.
Oat bran is a finer ground form of oats and is gentler on the digestive system. It can be made into a smooth porridge to which seeds and/or fresh fruit can be added.
Ideally you should be eating at least 35 g of fibre a day. If you make sure that 2/3 of your meals contain fruit, vegetables or grains then you will be getting sufficient fibre.
Nutrient absorption dynamics
In addition to your body’s current nutritional status, there are many other factors that influence how well your body absorbs nutrients.
Stress: Digestive problems like indigestion and heartburn, can be related to stress. These conditions are common byproducts of the body’s biochemical response to being under constant, low-grade “fight or flight” status. Stress causes blood flow away from non-emergency body functions, like digestion, and negatively affects absorption. Many people take antacids to reduce their symptoms but antacids can also decrease absorption, by increasing the PH (lowering acidity) in the stomach reducing nutrient absorption.
Drugs: Nutrient-drug interactions can work both ways. For example, corticosteroids, often prescribed to reduce inflammation from sports injuries, decrease absorption of calcium and vitamin D. On the other hand, grapefruit and grapefruit juice can enhance the absorption of some pharmaceuticals like Tegretol (an anticonvulsant) and Zocor (used to treat high cholesterol), which may result in toxic levels.
Alcohol: Alcohol consumption can cause nutrient deficiencies. Alcohol damages the lining of the stomach and small intestines, altering or reducing absorption of vitamins and minerals, as well as inhibiting the breakdown of nutrients by decreasing secretion of digestive enzymes.
Caffeine: It’s a good idea to wait at least an hour between consuming caffeine and meals or supplements. Iron is one nutrient particularly affected by caffeine, absorption can be decreased by up to 80 percent.
Exercise: Intense exercise (above 75 percent of your maximal oxygen uptake [VO2max]) can affect how efficiently you absorb nutrients. Exercise is good for intestinal health in the long term, as it increases intestinal mobility but during exercise blood is needed for working muscles and directed away from the digestive system, decreasing nutrient absorption. That’s why it’s important to wait a couple of hours between consuming a big meal and working out. You should also eat something small and easy to digest prior to training.
Intense exercise increases your need for fluid (after about 20 minutes) and electrolytes (after about an hour) to replace the vitamins and minerals lost by sweating. Sports drinks (commercial and homemade) are an exception to the ‘do not eat and exercise’ rule and carbohydrate drinks may also be needed during endurance events to prevent fatigue. It’s important to eat the right foods at the right time to optimise energy. Macronutrients (carbohydrates, fats and proteins) are used for energy. Micronutrients, such as the B vitamins and other vitamins and minerals, although not used for energy directly, are needed for energy production.
Nutrients also compete for absorption. Calcium interferes with iron, copper and zinc absorption. Zinc and iron compete also. Mother Nature packaged nutrients together, so separating them isn’t necessary, unless you have a need for a single nutrient e.g. if it has been recommended that you take iron supplements to correct anaemia, extra calcium for female athletes or older people or women of childbearing age, trying for children, are recommended to take extra folic acid to prevent some types of birth defects. It is generally beneficial to combine nutrients in foods, or in a multivitamin/mineral supplement e.g. vitamin C greatly enhances absorption of iron and vitamin D improves calcium uptake.
Vitamins and mineral supplements are mostly absorbed the same way as whole foods. There are a few steps that may help you maximise their digestion and absorption.
Bioavailability: This refers to how well a supplement tablet or capsule disintegrates during digestion. Regardless of how potent a product is, if it doesn’t break down easily, your body can’t use it. Most supplements break down properly, but for extra assurance look for the United States Pharmacopeia (USP), NSF International seal, or a manufacturer's statement regarding disintegration time. A supplement passing a USP or NSF review has been tested and deemed satisfactory for disintegration ease.
Chelation: Chelation (wrapping a substance in amino acids) is a process that enhances bioavailability of minerals, but experts opinions differ on their actual effectiveness. It is thought that chelation greatly improves mineral absorption and that amino-acid-bound chelated minerals are absorbed three to ten times better than non-chelated ones. It is essential that the chelation is performed correctly, otherwise it can actually decrease or even block absorption. The NSF label on supplements indicates that the product has been reviewed for effective chelation.
Dosage Size: Generally, absorption efficiency decreases as vitamin or mineral quantity increases. If the recommended dosage is three tablets daily, spread them over the day.
Always Take With Food: Combining supplements with food slows down the transit time of the nutrients, the longer nutrients remain in the system, the higher the rate of absorption.