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Digestion of foods

The process of digestion starts once we have placed the food in our mouth and start to chew. A couple of things occur here. As we chew, our teeth start to break the food up and this is mixed with saliva (produced by our salivary glands) by the tongue. The saliva contains a chemical called salivary amylase, which starts breaking down complex carbohydrates, and another called mucin, which helps to soften the food. The chewed and softened ball of food is called a bolus.

The bolus of food is then propelled down our oesophagus  by the process of peristalsis – a wave-like contraction of the muscles of our digestive system. It then reaches the stomach, where the next stage of digestion occurs. Receptors in our stomach sense that food has (or is about to) enter and a hormone called gastrin stimulates the release of hydrochloric acid and digestive enzymes. The stomach is then turned into a highly acidic environment (acidic enough to burn your skin) and this helps to further break the food down.
 

After being processed in the stomach, the food is then passed into the first part of the small intestine, known as the duodenum, which is where most of the digestive process takes place. The partially digested food is mixed with three other liquids here:
1.    Bile; a substance that is released by the gallbladder to help in the digestion of fat.
2.    Pancreatic juice and enzymes (released into the intestine by the pancreas)
3.    Other alkaline intestinal enzymes, such as maltase, lactase and sucrose, which help to further break down different types of sugars (this is why some people have reactions to dairy products – they lack, or have very low levels of lactase, so can’t break down lactose, the main sugar in dairy products).

Absorption

Now that the food has been processed, it needs to be absorbed by the body, so it can provide us with the energy and nutrients that we need. This occurs mainly in the small intestine, which has small, finger-like structures called villi protruding from the walls.

The process of digestion breaks the food up into tiny particles, which are collected by the villi and passes into the bloodstream going to the Liver, where the blood is filtered to remove any toxins and the nutrients are processed. Protein, most carbohydrate and some fats use this pathway to be absorbed. From the liver, the nutrients are delivered to the organs and cells of the body via the bloodstream, where they are used to assist in the millions of chemical processes and reactions that occur every second within our cells.

There are a few important points of note here:
1.    We have around 50 to 70 trillion cells in our bodies and every one of them needs a steady supply of nutrients to stay alive, reproduce and communicate with each other.
2.    The processes of digestion and absorption require energy, vitamins and minerals to take place. Thus, the body will lose nutrients through these processes. If a food contains lots of nutrients, the body ends up with a net gain of energy, vitamins and minerals.
3.    The body does not have reserves of all nutrients. For example, carbohydrate stores are limited and protein stores are almost non-existent. When we look at vitamins, the body stores fat-soluble vitamins (A, E, D & K), but has no storage facility for water-soluble vitamins (B vitamins, vitamin C & folate) – therefore it is very important that we get a steady supply of water-soluble vitamins.
4.    Vitamins and minerals from natural foods are easily digested and absorbed by the body, but this is not always the case with synthetic forms that we get from tablets. Also, we need to realise that a multi-vitamin/mineral pill contains around 30-40 nutrients, whereas the average natural food can contain between 10,000 and 15,000 nutrients.

What Happens to our Apple and our Doughnut?

Now that we have an understanding of the bodily processes involved, let’s have a look at the fate of our two different foods as they are digested and absorbed by the body.
The apple, which contains water (about 80%), carbohydrate (15 %, mainly fructose with some dietary fibre) and a small amount of protein (5% or less), has been mostly digested by the time it gets through the small intestine. The body has used up energy and nutrients to complete this process, but it is rewarded with an abundance of nutrients from the apple. Firstly, there are amino acids from the protein, which are sent to a pool from which individual cells can access in order to repair themselves and communicate with other cells. Next we have energy released from the carbohydrate, which goes to fuel our muscles, nervous system and brain. Because most of the sugar is fructose, it is stored in the liver until it is needed, helping blood sugar to remain relatively steady.
The humble apple also contains an abundance of nutrients for the body. Just under the skin lies half of the Vitamin C content of the apple. It also contains calcium, phosphorus, iron, Vitamin A and a healthy dose of potassium. In addition to those, the skin contains a compound called quercetin, a powerful antioxidant that reduces cardiovascular risk. If that wasn’t enough, the flavonoids and phytochemicals that it contains seem to help fight against cancer. Finally, the skin contains lots of fibre, which helps to improve bowel function and reduced cholesterol absorption.
The doughnut, however, is a different proposition altogether. It is loaded with saturated fats, trans fats and refined sugar and is largely devoid of any nutritional value, other than energy, which it has in abundance. Digestion of the carbohydrate component will start in the mouth and the stomach will assist in further breaking the doughnut down. When it reaches the small intestine, the simple sugars will be readily digested and then absorbed by the villi. The bile that is released by the gallbladder will help to break down the different fats, but the picture here gets a little complicated. Saturated and trans fats take a different pathway than the more healthy fats, such as monsaturated or polyunsaturated, and the pathway dictates whether the fats are healthy or unhealthy. The healthier fats generally are absorbed via the liver, whereas saturated and trans fats pass through the villi and are converted into triglycerides – the main form of fat storage in the body. They are also coated in cholesterol (from the liver) and hence the fats in a doughnut will raise the bad (LDL) cholesterol and reduce the good (HDL) component.
Trans fats, however, surpass saturated fats in the damage that they do. They have been shown to wreak havoc with the body’s ability to regulate cholesterol and massively increase your risk of heart disease. They also get into the membrane (outer lining or skin) of our cells, causing them to harden. This has a negative effect on the functioning of our cells and disturbs the delicate ecosystem.

The Take-home Message

Hopefully this article has demonstrated why Nutritionists and Dieticians are always harping on about eating natural foods. It is not just the energy content that is important, but the nutrients within the foods play a large role in our long-term health. Although there is room for everything in moderation, the vast majority of your food should be natural and healthy. As a general rule, I tell my clients that if a food looks like it has never been alive at any point, then it’s best avoided or at least put in the category of ‘occasional food’.

Trans Fats

Also known as trans fatty acids, trans fats are the most damaging of all fats to our health – even more so than saturated fats. Although a small amount of trans fats are produced in nature (through the digestive processes of cows and sheep; hence they are present in small amounts in beef, dairy products and lamb), the majority of trans fats in our diets come from the food processing industry. They are produced by the chemical process of hydrogenation of oils, which turns oils into solids and increases the shelf life of products, as well as improving the texture of a food. The greatest source is deep fried foods, such as french fries, but trans fats are also present in high amounts in cakes, cookies, biscuits, some breads (especially croissants and pastries), processed foods (especially pies, sausage rolls etc), snack foods (potato chips, some museli bars) and margarines (in small amounts). All US and most European foods must declare how much trans fats are in the product (Denmark has banned all foods with more than 2% trans fats), but there is no requirement to declare it in Australia and NZ. Look out for trans fat on the label or the words hydrogenated or partially hydrogenated.


Article Contributed by
Paul Taylor
Personal training courses and Certification
www.ptacademy.com.au