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Nutrition & Public Health

In this series of articles, medical students from across the country will share their knowledge of medical physiology, anatomy and biochemistry to give you a taster of medical school. This will be a fantastic opportunity to build upon your A-Level Knowledge.

Remember it's more important to understand the principles than to mindlessly memorise facts.


Nutrition and Public Health - if there were ever 2 topics that would make medical students’ eyes glaze over and have us questioning some life decisions, it would be these. Thanks to COVID-19, the latter has become remarkably more popular and epidemiology lecturers everywhere are experiencing a wave of appreciation from their students unlike anything they’ve seen before. Nutrition, on the other hand, is very much in the same position it was circa January but I’m hoping I can open your eyes to its importance.

Why Nutrition:

Is it a bit dull when you start? Oh yes. Is it very important and applicable across most specialties? You bet. Granted, on the surface there are lots of things that are more interesting than how much protein compared to carbohydrates somebody’s eating, or how Mrs Jones in bed 3 is going to eat after her surgery. Stay with me though - I’d like you to take a minute to consider all the conditions that are affected by diet. On top of that, have a think about all of the invasive procedures we do in hospital (I’m looking at you budding surgeons 👀). Do they help with the condition the patient came in with? Sure - but what comes next? The patient’s body needs to rebuild around all of the damage we did, and all of the building blocks come from...(it’s in the title)...nutrition!


As a general overview there are 2 aspects we look at:

1) Fluid

2) Macronutrients and Micronutrients


It always looks very exciting when doctors on Grey’s Anatomy have to give IV fluid to a patient that’s just presented to A&E encased in concrete (what an episode that was), but I’m afraid the reality is somewhat less dramatic. Maintaining a patient’s hydration is extremely important for many reasons including to allow their organs to function correctly and ensure the volume of blood circulating around their body is sufficient.

Before we start pumping fluids into people, we need to know how much they’re using to keep a balance between the fluid entering their body and fluid leaving their body (having too much fluid can be just as bad as not having enough). We look at this in terms of:

- Sensible losses - Water lost in urine

- Insensible losses - Water lost in faeces and exhaled from the lungs

- Additional losses - Loss from things like nasogastric tubes (more on these later), stomas etc.

There are some equations that help with the calculations using body weight, but the TL;DR is that a healthy adult requires about 2L per day, so we can assume that most ill patients in hospital will need somewhere around 3L per day. Within these fluids we also give electrolytes such as sodium and potassium, again based off calculations for individual patients. These calculations are something that still baffles me sometimes so it’s good for you guys to be aware of, but don’t worry too much about the ins and outs of it!

Macro and Micronutrients:

Micronutrients refer to vitamins and minerals. If you are deficient in any of these it can cause conditions such as anaemia (iron deficiency), osteomalacia (vitamin D deficiency in adults), rickets (vitamin D deficiency in children). Macronutrients are just the fancy way of referring to carbohydrates, proteins and lipids. Thanks to A-level Biology we know that carbohydrates provide us with glucose for respiration, proteins are important structural molecules and lipids provide us with an energy store. There’s a bit more detail surrounding each of them to say the least, but in the interests of keeping things clinically relevant lets look at how we can give these to people.

The first way is fairly obvious - we let people eat! Great in theory, but doesn’t alway work particularly well. For example, if patients can’t eat enough to cover what they should be taking in to recover or they can eat but their gastrointestinal tract isn’t functioning properly. These are two very different situations needing different (albeit very logical) approaches.

Enteral Nutrition

Patients that can’t take in enough food via their mouth or aren’t allowed/able to, can receive enteral nutrition. There a few variations on this including:

1. Nasogastric tube - Feeding tube is run through the nose into the stomach

2. Nasojejunal tube - Feeding tube is run through the nose into the jejunum (a section of the intestine)

3. Percutaneous endoscopic gastrostomy (PEG) - A feeding tube is inserted into the stomach through the skin overlying the stomach

4. Percutaneous endoscopic jejunostomy (PEJ) - A feeding tube is inserted into the jejunum through the overlying skin

Patients are given formulations with the correct proportions of nutrients that they require. There are also different types of feed that are dependent on how well their gastrointestinal tract is working and whether they have the ability to break up the macronutrients given into smaller molecules for absorption. For example polymeric feeds include whole proteins, carbohydrates and liquid fat that the patient’s body must then break down to absorb. In contrast, elemental feeds provide proteins as amino acids and carbohydrates as mono or disaccharides with very little fat included.

Parenteral Nutrition

If the patient is unable to absorb the nutrients from the gastrointestinal tract, even if they are given as an elemental feed, parenteral nutrition is used. This is where the entire gastrointestinal tract is bypassed completely, and the patient is given the nutrients directly into their blood. It’s helpful if there are no other means of ensuring patients are getting adequate nutrition, but it comes with risks such as infection meaning it’s only used if enteral nutrition will not work.

Back to Mrs Jones

Let's revisit Mrs Jones from earlier. I asked where all of this information fits in, so lets put a bit more of a history in. Mrs jones initially presented with severe abdominal pain on a background of atrial fibrillation. This condition predisposes her to blood clot formation in her heart (more on this in another article) and one of these clots lodged on her abdominal vessels and blocked blood flow to her intestines. As a result the intestine tissue began to die and had to be removed. She is currently recovering from that surgery and the only way that she can receive the nutrients allowing her to heal is via parenteral nutrition.


In summary, nutrition is really important in many aspects of medicine. Whether it’s to do with drug reactions (and there are some weird ones), symptom and disease control or healing. Ideally patients can manage to eat enough and the correct quantities but with patients who have been admitted, or those with chronic conditions, we sometimes have to step in. We can do this by delivering food directly to their stomach/small intestine (enteral nutrition) or bypassing the gastrointestinal system entirely and delivering nutrients directly into the blood (parenteral nutrition). It’s something that won’t necessarily be pointed out as the most important thing, but is worth looking out for and asking about in your work experience!

I’m hoping that after reading this (well done if you’ve got all the way to here!) that:

1. You aren’t bored senseless

2. You have an appreciation for the role nutrition plays in a clinical setting

Key words:

Sensible fluid losses - water lost in urine

Insensible fluid losses - Water lost in faeces and exhaled from the lungs

Macronutrients - Proteins, carbohydrates and lipids

Micronutrients - Vitamins and minerals

Enteral nutrition - Feeding a patient through a tube passed directly into their stomach or first part of the small intestine

Parenteral nutrition - Feeding a patient in a way that bypasses the digestive system entirely (ie nutrients delivered directly into bloodstream)


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