Role and structure of the Alimentary Canal

Alimentary canal diagram

The alimentary canal, also known as the gastrointestinal tract has two purposes

  1. break down food into its basic components, known as digestion and
  2. to absorb these basic components into the body

The process of digestion involves breaking down the food we eat in two main ways.

Mechanical breakdown. Food is broken up into smaller and smaller physical pieces. The main tool for doing so is our teeth, but the contraction of muscles along the alimentary tract also aids in mechanical breakdown.

Chemical breakdown sees the macronutrients of protein, carbohydrate and fat broken down from their original, complex molecular structures into their chemical basic components.

Chemical breakdown/digestion takes place in all major parts of the alimentary canal such as the mouth, small intestine and stomach, but not in the large intestine and colon.

However the only macronutrient that is significantly digested in the mouth is starch, which is a form of carbohydrate.

Protein is mainly digested in the stomach, which also has a minor role in fat digestion.

Most chemical digestion occurs in the small intestines, particularly the upper part. Here enzymes that are produced by the pancreas and intestinal cells go to work.

Any remaining undigested food will be broken down in the large intestine, especially dietary fibre. A large variety and quantity of bacteria reside in the lower intestine to assist with this.

A variety of cells along the alimentary canal produce digestive enzymes that assist with chemical breakdown. These digestive enzymes are known as catalysts.

What do catalysts do?

Simply put, for a chemical reaction to take place, it must reach a certain level of energy. Catalysts lower this energy barrier. Think of reducing the height of a wall to make it easier to jump over.

Reactions that take place using a catalyst comprise the process known as hydrolysis. In a hydrolysis reaction a chemical bond is broken via the addition of water to the reaction.

Proteins become amino acids, carbohydrates mainly become glucose and fats are broken down into fatty acids.

Once this process of digestion is complete, these components are absorbed into intestinal cells and then distributed throughout the body in a process called absorption.

The main role of the alimentary canal is to allow nutrients to be absorbed into the body, following the process of mechanical and chemical digestion, assisted by digestive enzymes.

Structure of the Alimentary Canal

Salivary glands in the mouth add moisture to the food. The primary role of this moisture is to lubricate the food and allow easy passage of it through the esophagus. However it also plays several other important roles. It starts the chemical digestion of food, helps us taste the food and even has anti bacterial/microbial properties.

The presence of food in the mouth causes saliva to be produced, but even just smelling or thinking about food can cause saliva production.

Salivary glands are located beneath the tongue, in the floor of the mouth and even up by the ears. The action of chewing with the mouth and teeth mixes saliva in with the food. Food is 99% water, and 1% enzymes, salts, mucus and anti bacterial compounds.

By swallowing, food is moved out of the mouth and into the esophagus. This long tube transports food from the mouth to the stomach. To ensure the esophagus stays closed when food is not being swallowed, a muscular ring called the upper sphincter contracts.

At the other end of the esophagus is the lower or esophageal sphincter, which prevents food from travelling back up from the stomach. The lower sphincter also relaxes and opens when we swallow to allow food to pass through it.

To ensure food only enters the esophagus and not the windpipe leading to the lungs, a leaf shaped flap called the epiglottis covers the entrance to the trachea, called the larynx.

Food does not just fall down the esophagus. The walls of the esophagus are muscular and they contract and relax to propel the food down, in a process called peristalsis.

As a result of this peristalsis, food arrives in the stomach. The stomach is essentially a hollow bag which in the average person has a volume of roughly two ounces when relaxed. However it can expand to over a litre after a large meal.

The main role of the stomach is protein digestion. A unique feature of the stomach is its high acidity. The stomach walls produce hydrochloric acid. Food remains in the stomach for a few minutes up to several hours. It is stopped from progressing any further by the pyloric sphincter, another muscular ring.

Once food leaves the stomach, it enters the small intestine. We can look at the small intestine in three parts.

The upper part of the small intestine is known as the duodenum. It’s a tube about 30 cm long and leads into the jejunum. The duodenum is where the digestive juices produced by the pancreas and bile from the liver enter the alimentary canal.

These digestive juices contain a lot of bicarbonate to neutralise the acidity in the food mass, which it picked up during its time in the stomach. This allows the process of digestion to continue.

The duodenum is in charge of regulating how quickly food exits the stomach and intro the small intestine. It does this by producing the hormones secretin and cholecystokinin. This hormone production is triggered by food entering the duodenum.

These two hormones have other functions too, including causing the pancreas to release bicarbonate and digestive enzymes. The pancreas is located behind the stomach and produces several digestive enzymes called lipases, amylases and proteases.

The duodenum leads into the second part of the small intestine, called the jejunum. It is the major site of nutrient digestion. Here food is broken down into the basic chemical components which can be absorbed by the body. The cells which make up the intestinal tract, known as enterocytes produce several different enzymes, such as disaccharidases and peptidases to aid in the break down of food into its components chemicals.

Most the nutrient absorption takes place in the third and final part of the small intestine, the ileum.

To maximise the effectiveness of the intestine, it has several key features to maximise its surface area. Firstly it has many folds and creases. Additionally, tiny finger like projections called villi stick out into the intestinal path. The fingers have their own blood supply and work to absorb desired nutrients. Microvilli and even smaller and stick out from the enterocyte cells lining the small intestine.

Food moves out of the ileum and small intestine, into the large intestine or colon. The colon exists to remove water from the remaining indigestible food mass. It also provides a way for the body to excrete unwanted faecal waste.

Large numbers of bacteria, known as gut microbiota are present in the colon. These ferment and further break down dietary fibre.

The composition of this microbiota, both in terms of variety and effectiveness, is highly dependant on the type and quality of a persons diet.

By now the body has extracted and absorbed any nutrients from the food. What’s left in the colon is waste faecal matter. This collects in the colon and rectum, staying here for one to two days, and is released during a bowel movement.

Observation versus Intervention in nutrition research

Diagram of research methods

In our previous post discussing methods of recording food intake for nutrition research we discovered several ways that researchers can find out what people are eating. From this they can further calculate a persons consumption of macro nutrients, micro nutrients, calories and much more.

This data by itself, whilst interesting, is not very useful. Only when data is collected and analysed as part of a study can we really begin to make discoveries and reach solid, evidence based conclusions.

There are many methods available when conducting research. These generally fall into two categories, observation and intervention. Let’s start with the observational methods.

Ecological study

Ecological studies are focused not on individuals, but groups or national populations. For example, consumption of olive oil per capita vs heart related illness in that country.

However the data can be misleading. For example, countries that consume the most alcohol per capita have lower rates of traffic related deaths. Clearly it would be wrong to assume that alcohol consumption is somehow a preventative factor for traffic deaths.

Cross sectional study

In a cross sectional study researchers take a group of people and look at the exposure and outcome at the same time. For example, the exposure might be the persons body weight, and the outcome their blood pressure. Both sets of data can be collected at the same time and examined. In this case, people with higher body weight tend to have higher blood pressure.

As with ecological studies, cross sectional studies can be misinterpreted. For example, people who drink large amounts of diet sodas rather than the non-diet versions tend to have higher BMI and body fat levels.

It would be incorrect to assume that the diet soda intake is causing the higher BMI. In this case, participants are drinking the diet soda rather than the non-diet version in an attempt to help them lose weight. The high BMI and body fat levels came first, followed by their choice to drink diet sodas.

With cross sectional studies it is difficult, if not impossible to know for sure which factors are the causes and which are the results. Cross sectional studies can show us interesting correlations, but cannot provide proof of what factors cause what results.

Case control study

Say we are interested in the potential causes of a certain type of disease. In a case control study, we would take a group of people who have the disease, the “cases”. We would ask about their dietary history, for example frequency and quantity of meat consumption.

We would then do the same for a “control” group; people who do not have the disease. By comparing the results we may be able to see a correlation between the amount of meat eaten, and the chance of developing the disease.

Selection bias can play a factor in the quality of case control studies. Both the cases and control groups should be from the same geographical area, be of similar economic status and so on. It is important to normalise as many variables as possible in order to have a fair comparison between the groups. Otherwise external factors such as quality of healthcare and housing, environmental pollution etc can skew the results.

Confounding is another possible limitation to case control studies. We know that people who eat large amounts of meat tend to be heavier. We also know that being heavier can be a risk factor for the disease we are researching in this study. So perhaps any correlation between meat consumption and occurrences of the disease may be due to the cases group being heavier, and not the amount of meat they are eating.

We can attempt to minimise the amount of confounding in a study. In the example above, we may add the BMI of the participants in the data as a confounder.

Cohort study

This is the most powerful observational study. Participants are assessed and continually studied over long periods of time.

Cohort studies can be extremely large, such as the Nurses Health studies in the US and the EPIC study in Europe involving over 100,000 people. They provide info on their dietary intake and food frequency every couple of years.

This data is analysed and linked to the rate that the participants develop diseases or other illnesses.

The main limitation in these studies are the food frequency questionnaires which are used to gather the data. See the previous post for more about the issues with FFQs.

Commonly known and followed nutrition guidelines such as to eat more fruit and vegetables, or to eat less saturated fat all come from cohort studies.

Intervention study

In an intervention study we no longer just watch, as with the observational studies above, but actively intervene in the lifestyle of our subjects. A group of subjects will be randomised and split into two or more groups. One will get the intervention, which for this example is a fish oil pill. The other group will get a pill that they think is fish oil but is actually inert and will not contain any active ingredients. However they will not be told this.

After a period of time the subjects will be reexamined. In this example study, the subjects may have their cholesterol levels taken before and after the fish oil pill intervention. We can compare the results between the groups that did and did not have fish oil pill.

Ideally these studies would run for many years and include many people, however this is very expensive and difficult to do. However even short term intervention studies can yield valuable results.

The advantage of experimental, or intervention studies is that it is possible to establish causality.

In other words, does doing/eating/having X cause Y (weight gain, an illness etc). This is not possible with observational studies, which can only establish if there is a correlation or association between the factors being studied. Observation studies can suggest, whereas intervention studies can prove.

It is very important to find out and analyse what study design was used in any research that we look at. We cannot rely on other people such as press officers, authors or journalists to do this for us. Often press coverage is given to poorly designed or significantly flawed studies simply because the conclusion can provide a catchy headline. Eating chocolate causes you to lose weight! for example.

Methods of recording food intake for nutrition research

An example of a food diary

When conduction nutrition research,  it is frequently necessary to find out exactly what the experiment participants are eating. Three of the most common methods for doing so are as follows.

Food Frequency Questionnaires (FFQ)

The participant in the study is asked what types of food they eat and how often. For example, do they eat bread? If so, what type (white, wholemeal etc),?How many slices do they have per serving, and how many times per day and per week do they eat this. Using this data, researchers can estimate how much food and what types a person consumes.

24 Hour Dietary Recall Questionnaire

Similar to the FFQ, this method involves administering a questionnaire to the research participant. In this case the participant is only asked to recall what they have eaten during the last 24 hours. This can be a more accurate method, as the participant only has 24 hours worth of eating to remember, and does not have to factor in foods that they eat occasionally. The flip side to this is that the 24 hours being studied may not be indicative of their usual diet.

Food Diary

The participant is given a diary to fill out with the details of what they eat on an ongoing basis. The food diary may be used for a few days or several months. Diaries tend to be more accurate than FFQ or other questionnaires, however they are not perfect. Users of food diaries often change their eating habits when they know they are being monitored. The motivation to accurately complete the diary can fade over time, leading to less accurate reporting. Diaries also take more time and money to issue, and then analyse.

Each of these methods provide “usable accuracy”, but are far from perfect. Participants may fill out the questionnaires or diaries incorrectly. Or they may deliberately under report certain eating habits. We know that people tend to under report alcohol and unhealthy food intake. Overweight or obese people have been shown to under report total food/calorie intake.

Some researchers have asked participants to take photos of the food they eat. This can provide additional accuracy, but still does not show all ingredients and their quantities in each meal.

With the rise of nutrition apps such as MyFitnessPal, it may become easier for researchers and participants to obtain accurate data at a reasonable cost. Nutrition researchers are actively exploring the role that technology and apps can play in accurate and efficient food intake reporting.

Data from these reporting methods is then turned into nutrient intake data using a food composition table. The macro and micro nutrients, along with other data of each food are listed. These tables are tailored to certain regions or countries.  For example, the UK food composition table will list different foods and data than the USA table. This is to take into account which foods grow where in the world, different food treatment, storage processes etc, all of which affect the nutrient content of the food.

The International Network of Food Data Systems website keeps an updated list of many of the food composition tables used around the world.

Once this nutrient intake data is collected and analysed it can be used in a variety of ways. In the next post we will look at different techniques used in nutrition research.