Have you ever wondered what happens to your food once you have digested it? What is the destination of the individual nutrients in your food? How does this influence your health and weight? What can you do to maximise your nutrition whilst preventing weight gain or lose excess weight? Read on to learn about the fascinating journey of nutrients in the body…
What happens to food after you’ve eaten a meal?
We all have some basic understanding of digestion: food we eat is chewed, mixed with enzymes that break down starches, swallowed, mixed with strong acid and mechanically churned to break it up in the stomach, it is then passed to the small intestine where enzymes and bile, secreted from the pancreas and gallbladder respectively, mix with the partially digested food (chyme). These enzymes break the bonds between food molecules to release the individual nutrients, whilst bile emulsifies and helps break down fats. Fibre is not digested and passes largely unaltered into the colon where it feeds the gut microbiome and bulks out the stool. Meanwhile, the individual nutrients in the intestine (glucose, fructose, triglycerides (fats), amino-acids, vitamins, minerals and various phytonutrients) are absorbed into the blood.
Nutrients have 1 of 3 fates in the body.
Nutrients are either used immediately to do their ‘work’ in the body, stored for later use, or they are excreted from the body if they are not needed and can’t be stored.
1. Using nutrients:
‘Energy’ nutrients (macronutrients).
These are glucose (from carbohydrates), triglycerides (from fats) and amino-acids (from protein). Amino-acids are only used to fuel the body as a ‘last resort’ when there is not enough glucose or fatty acids available – think starvation and the breakdown of muscle protein.
So, our main source of energy comes from the metabolism of glucose and triglycerides (further broken down into fatty acids and glycerol) to produce a chemical form of energy called ATP. This is a process that occurs in the mitochondria, which are tiny organelles present inside all our cells.
The hormone, insulin, largely determines whether our body metabolises glucose or fatty acids after a meal. If your insulin levels are high, due to eating carbohydrates, then we metabolise glucose, and if your insulin levels are low, due to eating minimal levels of carbohydrates, then we metabolise more fatty acids.
Fatty acids also have structural roles in the body – they are the main component of cells membranes and provide insulation to nerve cells, for example.
Amino-acids are needed to make enzymes, many hormones, transport molecules, immune molecules and of course, muscle, skin, hair and connective tissue.
Glucose has no other role in the body – just energy production. In fact, our liver can make all the glucose it needs from amino-acids and fatty acids, so in that respect it isn’t even an essential nutrient.
Vitamins and mineral (micronutrients).
These perform a range of roles in the body and many need to be constantly replenished on a daily basis. Many of them act as ‘assistants’ to enzymes and transport molecules, ensuring that all the thousands of different chemical reactions that go on inside cells 24/7 are supported and function properly.
By playing this supporting role, micronutrients enable us to fight infection, support energy production, make healthy blood cells, improve eyesight, support growth and repair, brain development, detoxification in the liver, maintain fertility and numerous other functions in the body.
Some, like calcium, magnesium and phosphorus, also have structural roles by providing strength and density to bones. Though we only need micronutrients in small quantities we do need a regular supply every day.
2. Storing nutrients.
If a particular nutrient is not needed after a meal, it may be stored for later use, but some nutrients can’t be stored or can only be stored in very small amounts.
Macronutrients
Glucose. Excess glucose will be stored as glycogen in the liver and muscles. However, once glycogen stores are full, any excess glucose still hanging around in the blood after a meal is converted by the liver into triglycerides (fat) and stored either in the liver or in the fat stores around the body.
How much glucose can be metabolised to ATP after a meal depends on how much energy the body needs at that particular time, and whether there is already a backlog of glucose cluttering up cells from a previous meal. If you are about to run a race, are healing an injury, growing rapidly (think puberty) or fighting an infection, you will need more energy; but if you are just sitting on the sofa after a meal or about to go to bed, you will use less energy and more glucose will be stored as fat.
Fatty acids. Fatty acids will be metabolised for energy when there is very little glucose available. However, when blood glucose levels are high, fatty acids will be stored in the fat stores (as triglycerides) for later use. This is why eating a meal that is high in both carbohydrates and fats will lead to weight gain. There is no upper limit to the amount of fat you can store on your body!
Eating less carbohydrate or less fat will ensure that you don’t have an excess of ‘energy nutrients’ after a meal and this will help prevent weight gain. However, if you already have excess fat on your body then you need to encourage your body to metabolise fats for energy i.e., ‘burn’ your body fat, by reducing the amount of carbohydrates (and thus glucose) available for energy. The body will always prefer to metabolise glucose for energy if it is available and will ignore your fat stores, so by depriving your body of glucose it will switch into ‘fat-burning’ mode to make energy.
Amino-acids. The body has a very small capacity to store amino-acids. Amino-acids play such an important role in the body that it needs a constant supply of protein in the diet, at regular intervals through the day, i.e., at each meal.
Micronutrients.
Fat soluble vitamins. Many micronutrients can also be stored in the body, these include all the fat soluble vitamins (A,D,E and K) which are stored in the liver and fat cells. Vitamin B12, though a water-soluble vitamin, can also be stored in the liver. In fact, the liver can store several years’ worth of vitamin B12. These stored vitamins will be released into the blood as they are needed.
Water-soluble vitamins. These are all the B vitamins (apart from B12) and vitamin C. Since they are only soluble in water, they cannot be stored anywhere in the body. This means we need to consume these vitamins every day for good health.
Minerals. Some minerals can also be stored. Calcium, phosphorus and magnesium can be stored in bones and iron is stored in red blood cells and as ferritin in liver, spleen, muscles and bone marrow. Minerals are often stored for structural or metabolic reasons rather than as nutrient stores. Most minerals are only needed in very small amounts, and mineral levels in the body are carefully controlled by homeostasis, but excess consumption of minerals can lead to toxicity.
3. Excreting nutrients.
Glucose. In health, glucose is never excreted in urine. All of the glucose that gets filtered by the kidneys is reabsorbed back into the blood. Glucose will only be excreted in the urine when there is a failure of proper glucose metabolism within cells and excess glucose has built up in the blood to the point where the kidneys can no longer keep up with reabsorbing it back into the blood and it spills into the urine, a condition called diabetes.
Fatty acids. Excess fatty acids are not excreted via the kidney as they are stored in fat stores instead. The only way of removing fatty acids from the body is to metabolise them for energy and then excrete the waste products of CO2 and water via the lungs and kidneys respectively.
Amino acids. Excess amino-acids are excreted by the kidneys. This is why it is important to spread your protein intake throughout the day and not load it all into one meal at the end of the day. You will deprive your body of amino-acids throughout the most active part of your day and then ingest excess amino-acids in the evening, of which much will end up being excreted. This pattern risks causing protein depletion and muscle wasting over time.
Vitamins. With the exception of vitamin B12 all the other water-soluble vitamins (rest of the Bs and vitamin C) cannot be stored in the body and any excess will be excreted in the urine. This means that water-soluble vitamins need to be replenished every day. Fat soluble vitamins are not excreted so there is the potential for toxicity if excess amounts are consumed in food or supplements over a period of time.
Minerals. The body needs to maintain a constant amount of most minerals and so excess minerals will be excreted by the kidneys or through bile back into the gut. However, iron cannot be excreted from the body and so excess intake can lead to toxicity. Matching your energy requirements to your activity levels
Since your main sources of energy (glucose and fatty acids) cannot be excreted from the body when consumed in excess, it is important to not overeat these nutrients but to match intake to your activity levels to minimise the amount that will get stored. This means thinking about your day ahead and working out what times you are going to be sedentary and when you are going to be active.
Ideally you want to eat a bigger meal before activity to provide fuel, amino-acids and vitamins and minerals that will be used in metabolism during the post-prandial period (the 3-4 hours after a meal is eaten), and smaller meals when you know you are going to be sedentary in the following hours. By doing this you can maximise nutrient delivery to cells and tissues when it is needed and minimise energy storage as fat. Also increasing the time between meals and fasting for at least 12 hours overnight will increase the amount of time your body burns fat and this will aid with weight loss.
Conclusion
Every meal matters!
Your body needs a regular supply of nutrients through the day, particularly nutrients that can’t be stored (B and C vitamins).
Nutrients that cannot be excreted via the kidneys will be stored in various parts of the body e.g. fat stores, liver, bone, spleen, muscle and gall bladder. Vitamins and minerals that cannot be excreted (vitamins A, D, E and K, iron and vitamin B12) and are consumed in excess, can potentially lead to toxicity.
Amino-acids from protein need to be consumed at regular intervals through the day as they cannot be stored and are needed for a wide variety of functions.
Glucose and fatty acids are the main source of energy nutrients, and their intake needs to be matched carefully to activity levels through the day, to minimise the amount of excess energy stored as fat.
Your plate:
Plan your meals to:
Include a source of protein at every meal (meat, fish, eggs, cheese, legumes, tofu, soy beans);
Reduce the amount of sugar and starchy carbohydrates you eat (less: bread, breakfast cereals, cakes, pies, potatoes, rice and pasta, sugary drinks)
Add more fibre and micronutrients by eating a variety of above ground vegetables, fruits and salad (tomatoes, cucumber, lettuce, greens, broccoli, cauliflower, mushrooms, aubergines, courgettes, bell peppers, green beans, berries, apples etc) plus nuts and seeds.
Restrict yourself to 2 or 3 meals per day,
Don't overload your plate (food should be in a single layer and you should see a good 1-inch of plate around the rim),
Fast for at least 12 hours overnight (up to 16 hours if you can).
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