Nutrient Absorption: What It Is and How to Boost Yours

Congratulations! You’ve made changes to your diet and are eating more fresh fruits and vegetables, less processed foods and sugars, and supplementing with a multivitamin. You’ve begun the journey ofoptimizing your nutrition. According to your food log, you’re consuming almost all of the 60 essential nutrients that your body needs every day. When it comes to maintaining optimal health, consuming proper nutrition is just the first step. The next step is making sure your body is absorbing the nutrients you’re consuming. 

Nutrient absorption is often overlooked. If your body can’t absorb and use the nutrients you’re feeding it, those dietary changes won’t make much of an impact. So it’s important to understand how the body absorbs nutrients and the ways in which you can support it in doing so.

The digestive process

The digestive process consists ofmechanical digestion, or the physical breakdown of food into smaller parts, andchemical digestion that breaks food down into its chemical building blocks. There are four essential parts to this process.

Chewing

When you start chewing your food, your teeth grind and crush the food, breaking large chunks into smaller parts. Food is softened, allowing more surface area to become exposed. This allows the saliva to break down the carbohydrates more quickly. 

Mucus then lubricates the food and the chewed substance is calledbolus. Bolus is swallowed and passes through the esophagus into the stomach. As you will see later, chewing provides the first opportunity to improve nutrient absorption.

Mechanical and chemical digestion in the stomach 

Once the bolus travels down the esophagus and enters the stomach, muscles contract causing powerful gastric enzymes like hydrochloric acid and pepsin to mix with the bolus. This marks the beginning of the advanced work of digesting the proteins. After about two hours, the bolus becomes a thick liquid calledchyme. 

Chemical digestion and absorption in the small intestine

The chyme exits the stomach and enters the upper part of the small intestine called the duodenum. Here, more enzymes from the pancreas and bile juice from the liver further digest the chyme into subunits that pass into the small intestine. These units are small enough to be absorbed by the lining of the alimentary canal. About 95 percent of nutrient absorption into the blood occurs when the fully digested chyme enters the part of the small intestine called thejejunum(1).

Absorption and elimination in the colon 

Material that hasn’t been absorbed into the blood in the small intestine passes to the large intestine or colon. From there, water and minerals are absorbed into the blood and the vitamins biotin and vitamin K are produced by bacteria in the colon. All other waste material is eliminated from the rectum during defecation. 

Absorption

Every day in the small intestine, 10 liters of food, liquids and gastro-intestinal secretions are processed into molecules small enough to enter the bloodstream. Ninety percent of the foods and liquids are not absorbed and instead enter the large intestine(2). Thus, most of the nutrient absorption takes place in the small intestine. This absorption occurs through the following mechanisms:

• Active transport: movement of a substance across a cell membrane from an area of lower concentration to an area of higher concentration requiring energy in the form of adenosine triphosphate (ATP). It’s like pushing a boulder uphill and, as a result, requires a lot of energy. 

• Passive diffusion: movement of a substance across a cell membrane from an area of higher concentration to an area of lower concentration. It’s like rolling a boulder downhill which is fairly easy—push it and it will roll by itself. It doesn’t require sustained exertion of energy.

• Facilitated diffusion: movement of a substance across a cell membrane from an area of higher concentration to an area of lower concentration using a carrier protein. Imagine you’re the boulder sitting atop a snow-covered hill. On your own, you probably won’t slide down. However, sit on a sled and you’ll quickly slide down the mountain. The sled here represents the carrier protein.

• Co-transport uses the movement of a molecule through the membrane from higher to lower concentration to power the movement of another molecule from lower to higher concentration. It’s like playing pool: When you strike the cue ball, the transfer of energy to the cue ball is used to move the next ball that the cue ball hits.

• Endocytosis: the cell membrane engulfs material requiring energy in the form of ATP. Think of this like lava swallowing whatever is in its path. It needed the energy of a volcanic eruption to propel it. 

Water soluble versus lipid soluble

Another factor that affects nutrient absorption is the structural component of the cell membrane. It’s made from a substance called alipid. A class of lipids that are a major component of cell membranes are called phospholipids. Some of these phospholipids are hydrophobic, meaning they avoid interaction with water. 

Water-soluble nutrients require a “friend” or transport molecule embedded in the membrane to invite them into cells. Lipid-soluble nutrients, on the other hand, can enter the cell membrane without the use of active transport since they’re already friendly with the cell membrane. 

It is this very complexity that makes digestion and nutrient absorption so important and misunderstood. Understanding the details of each step in the system can inform the habits to improve the process of your own digestion, so let’s look at some of those details.

Carbohydrate absorption

Carbohydrates provide the body with the ingredients it needs to produce energy. The average American diet consists of 42 percent low-quality carbohydrates and 9 percent high-quality carbohydrates(3). Digestion and absorption of carbohydrates, therefore, is a critical function of the body and involves complicated processes and terminology. While reading the next section, don’t get caught up on the words—they’re just scientific names for the building blocks of energy that make up the carbohydrates in food. They could just as easily be called the red thing, the blue thing and the yellow thing!

Carbohydrates are classified assimple sugars (monosaccharides and disaccharides) andcomplex sugars (polysaccharides). 

Monosaccharides such as glucose, galactose and fructose found in fruits are readily digested and absorbed in the body. During this process, the chemical bonds are broken, releasing energy that cells use for all of their functions. 

The digestive system is also capable of breaking down disaccharides, which are combinations of the monosaccharides. Sucrose, or regular table sugar, is a combination of glucose and fructose. Lactose, or milk sugar, is a combination of glucose and galactose. Maltose, or grain sugar, is a combination of two or more glucose molecules. A chain of monosaccharides also forms the polysaccharides glycogen and starch, which the body can also digest. Some polysaccharides such as cellulose, however, are indigestible and provide no nutritional value. What these molecules do provide is fiber that helps aid digestion by propelling food through the small intestine.

Digestion of complex carbohydrates begins in the mouth with the salivary amylase which breaks down starches into their constituent disaccharides: maltose, sucrose and lactose. The gastric enzymes maltase, sucrase and lactase then break down the maltose, sucrose and lactose into the monosaccharides glucose, fructose and galactose at a rate of 120 grams per hour(4). 

Glucose and galactose then use sodium ions and proteins as co-transport carriers into epithelial cells—the cells that line surfaces of the skin, blood vessels and organs. Nutrients must pass through these epithelial cells that serve as barriers between the inside and outside of the body. The monosaccharides then exit these cells through facilitated diffusion (like riding on a sled) and enter capillaries through small openings between cells. 

Protein absorption

While carbohydrates are primarily used to produce energy, proteins are used as the building blocks of all tissues in the body and help regulate the body’s tissues and organs. Proteins are long chains composed of amino acids linked by peptide bonds. Digestion reduces the protein into its component parts. This process is more complex than digesting carbohydrates and takes more time.

Whereas digestion of carbohydrates begins in the mouth, digestion of protein starts in the stomach. The hydrochloric acid and pepsin break the chain of amino acids and peptides into smaller pieces which are then chemically digested in the small intestine by the enzymes chymotrypsin and trypsin. These enzymes are necessary for acting on specific peptide bonds. Aminopeptidase and dipeptidase enzymes further break down the peptide chains so that the resulting molecules are small enough to enter the bloodstream. Like the monosaccharides, the amino acids require active transport and the carrier is usually linked to sodium.

Fat absorption

Fat digestion and absorption gets even more complicated. Americans get about 33 percent of their daily calorie intake from fats, also called lipids, in the form of triglycerides, cholesterol, and phospholipids(5). Again, these are just more building blocks needed to facilitate the absorption of fat-soluble vitamins (A,D,E, andK) and carotenoids(6). 

Linoleic acid (anomega-6 fatty acid) found in safflower, sunflower and corn oils, andalpha-linolenic acid (an omega-3 fatty acid) found in green leaves, flax, rapeseed, walnut and soy, are also important. To digest these fats, the pancreas must secrete more enzymes called lingual lipase, gastric lipase and pancreatic lipase into the small intestine. There, the most common dietary lipid triglyceride is broken down into two free fatty acids and a monoglyceride. 

Short-chain fatty acids are water soluble and can be absorbed via simple diffusion (like a boulder rolling down hill). From there, they take the same path as monosaccharides and amino acids into the blood capillary. Large, hydrophobic long-chain fatty acids, however, require bile salts and lecithin to enclose them in a special sphere known as a micelle. Without micelles, these lipids would sit on the surface of chyme and never come into contact with the absorptive surfaces of the epithelial cells. The micelle gets near the cell surface and the lipid substances are secreted out of the micelle and absorbed via simple diffusion and form a new complex called achylomicronthat is then transported to the lymphatic vessels and eventually into the bloodstream. 

Nucleic acid absorption

Nucleic acids are the large biomolecules composed of sugar, a phosphate group, and a nitrogenous base that is essential to all forms of life. Nucleic acids refer specifically to deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). These nucleic acids create, encode and store all the information of every cell and transmit that information inside and outside the cell nucleus. 

DNA and RNA are found in most foods and digestion reduces them into their component sugar, phosphate group and a nitrogenous bases that are then absorbed through the alimentary canal wall. 

Mineral absorption

With the exception of iron and calcium, all minerals that enter the intestines are absorbed. During the process, sodium concentrates inside the cell while potassium concentrations are reduced. To maintain the proper balance, it’s necessary for the cell to pump out sodium and pump in potassium. This requires energy in the form of ATP.

Iron and calcium are absorbed in the upper part of the small intestine. Iron is absorbed into mucosal cells via active transport (rolling the boulder uphill) where it combines with ferritin and is stored until it’s needed by the body. 

Calcium absorption is regulated by the amount of calcium in the blood. Low levels signal parathyroid hormone release of calcium from bone matrices to be reabsorbed by the kidneys.

Vitamin absorption

Most water-soluble vitamins are easily absorbed by simple diffusion (rolling the boulder downhill) in the small intestine. Vitamins A, D, E and K are fat-soluble and require co-transport (like playing pool) with a dietary lipid for absorption. 

Vitamin B12 is a large molecule that binds with other substances in the stomach, preventing its digestion. It’s absorbed through endocytosis (swallowed by lava) through mucosal receptors in the terminal ileum at the end of the digestive tract. 

Improving nutrient absorption

Now that you understand the process of digestion and absorption, here are five ways to improve nutrient absorption.

Chew, and keep chewing

The process of mechanical digestion begins when you chew your food. Chewing stimulates the production of stomach acid and pancreatic juices that will be needed for digestion in the stomach. The more you chew, the more the food is broken down into smaller parts that increase the surface area of the resulting bolus that is exposed to contact with the digestive enzymes in both the mouth and the stomach. This will increase the speed and efficiency of the chemical digestion. 

One study showed that chewing longer can increase the amount of protein your body is able to absorb(7). Another study showed that chewing longer can make some vitamins, minerals and fats more readily available for absorption(8,9). In addition, better chewing means less undigested food in the stomach, which can help to reduce gas. Finally, some studies have found that taking smaller bites and chewing them more can decrease your food intake at mealtime by as much as one-third, assisting with weight management(10).

How many times should you chew each bite? There’s no specific number. Just take your time and be aware of how the food feels in your mouth. When it feels warm, soft and gooey, then swallow.

Maintain the proper gut microbiota

There are over 100 trillion microbes in the human body, most of which live in the intestines(11). This includes bacteria, fungi, protozoa and viruses. The bacteria in the digestive system come from 300 to 1000 species, 90 percent of which belong to Firmicutes and Bacteroidetes(12, 13).  These bacteria help digest food, protect against disease, and even produce vitamins B12, thiamine, riboflavin and Vitamin K. 

These bacteria cover themselves with sugar molecules taken from the cells of the digestive system, disguising them so that the immune system won’t attack them(14). When it comes to digestion and absorption of sugar polymers to simple glucose—especially from fruits and vegetables—there are about a dozen bacteria that play an important role in digestion(15). 

Thus, it is important to maintain the health of good bacteria in your gut microbiota. This is where diet andprobiotics can help increase digestion and nutrient absorption. Generally speaking, a diverse microbiota is a healthy microbiota. Since certain bacteria thrive on specific nutrients, microbiota are dependent on an individual’s food consumption. Change the diet and you change the microbiota. 

Eating fermented foods such as yogurt, sauerkraut and kimchi supports the growth of the “good” bacteria. Feeding the good bacteria with foods high in dietary fiber or “prebiotics” such as vegetables, fruits, nuts, seeds and legumes while consuming foods rich in polyphenols like cocoa, dark chocolate, red wine, blueberries help probiotics survive and do their job. On the other hand, avoiding artificial sweeteners can prevent negative effects on the gut microbiota. 

Combine the right foods and prepare them properly 

As previously discussed, in order to facilitate the absorption of fat-soluble vitamins (A,D,E, andK) and carotenoids, some dietary fat is needed.Large, hydrophobic long-chain fatty acids however, require bile salts and lecithin to enclose them in a special sphere known as a micelle.Vitamin B12 is a large molecule that binds with other substances in the stomach, preventing its digestion. 

These are examples of how various nutrients can either aid or prevent digestion and absorption. The practice of food combining to aid digestion has been practiced throughout human history for this exact reason. Here are some of the best food combinations for nutrient absorption:

• Ginger and various foods
• Turmeric and black pepper 
• Leafy greens with peppers and berries
• Beans and rice
• Eggs and leafy greens
• Nuts, seeds and berries
• Kale, broccoli and avocado
• Olive oil, vegetables, tomatoes and herbs
• Vitamin C and plant based (non-heme) iron

How you prepare food can also make vitamins, minerals and other compounds more bioavailable. Here are some tips to get you started:

• Cut fruits and vegetables to break down rigid plant cell walls
• Soak grains and beans to reduce phytic acid that can block the absorption of iron, zinc, calcium and magnesium
• Eat sources of vitamin B1, B5, folate and vitamin C raw since heat degrades them (sunflower seeds, peas, beet greens, brussels sprouts, broccoli, cauliflower, kale, spinach, and bell peppers)
• Opt to blanch, steam, saute, roast or microwave food with water-soluble nutrients like vitamin C rather than boiling them
• Cooking increases the nutrient absorption of tomatoes, carrots, sweet potatoes, spinach, eggs and meat

Avoid caffeine and alcohol

According to nutritionist Jayta Szpitalak, “Alcohol and diuretics [like coffee] not only lesson [sic] the number of digestive enzymes in your system, it also damages the cell linings of the stomach and intestines, making it harder for the nutrients from digestion to enter the bloodstream”(16). 

Alcohol is devoid of proteins, minerals and vitamins and inhibits the absorption and usage of vitamins B1, B12, folic acid and zinc(17). This is especially important considering vitamin B1 is involved in the metabolism of proteins, fats and carbohydrates. 

Alcohol further impairs absorption by decreasing secretion of digestive enzymes from the pancreas(18). Caffeine, meanwhile, can cause the depletion of vitamin B6 and interfere with the absorption of calcium, iron, magnesium and other B vitamins(19).

Manage stress 

Yes, stress can affect digestion. This is because stress can lead toadrenal fatigue depleting the eight B vitamins, vitamin C and magnesium. Stress also causes the release of cortisol, the hormone responsible for the “fight-or-flight” response. Blood is diverted from the digestive system to the muscles in the arms and legs. As a result, less hydrochloric acid is produced in the stomach and this delays digestion. One study showed that mineral absorption is decreased because of stress(20).

In summary

Consuming proper nutrition is essential for maintaining good health, but are you getting the most out of the nutritious foods you eat? If your body fails to absorb the nutrients from food, you can developnutritional deficiencies. Properly chewing food, maintaining a proper gut microbiota, combining and preparing the right foods the right way, avoiding alcohol and caffeine, and managing stress are five of the most important ways to improve nutrient absorption so that you can optimize your health.

Siphiwe Baleka - Contributing Writer, Physician’s Choice