Food and Energy: It's a Balancing Act

Summary

Food provides the energy every cell in our body needs to work and grow. We use our teeth and digestive system to break food down into its building blocks: sugars from carbohydrates, amino acids from proteins, and fatty acids from fats.

Our blood carries these substance to our cells. Chemical reactions inside the cells make different amounts of energy from them. We store anything extra as glycogen (a carbohydrate) in our liver, as body fat, or as protein in our muscles. We also keep small amounts of glucose (a sugar) in our blood ready for immediate use.

If our blood glucose level falls, one set of chemical messengers (called hormones) from our pancreas tell the liver to change its glycogen into glucose. If levels rise, other hormones like insulin tell the liver to store glucose as glycogen. If we’ve used up all our glycogen and glucose supplies, we’ll start using our body’s stored proteins and fats for energy.

How do we get energy from our food?

Foods are broken down mechanically by our teeth (and churning in our stomachs) and chemically (by enzymes) during digestion. Carbohydrates, our main source of energy, are generally broken down to simple sugars like glucose and fructose. Food proteins are broken down to amino acids, and complex fats to simple fatty acids and glycerol. These breakdown products are absorbed into our circulatory system and transported into cells.

How do we store energy from our foods?

Glucose is the main quick source of fuel for our body’s cells and the preferred one for our brain, but we can have only a small amount circulating in our body fluids before it starts to cause physiological problems. Instead, we store energy in three main ways:

Glycogen is a carbohydrate made up of many linked glucose molecules. It’s made in our liver and mostly stored there, with some in our muscles to fuel them quickly when needed like when we run. Because it can be easily broken down to release glucose, it’s the main way we store it. An average man (70 kg) contains about half a kilo (480 g) of glycogen. That’s just under a day’s supply of energy for someone using 2000 to 2100 Calories (8400 to 8700 kJ).

Fats (triglycerides) are made up of fatty acids and glycerol. Fat is our body’s main energy store, but it takes longer to break down than glucose or glycogen. We store fat as fatty (“adipose”) tissue in our bodies, as free fatty acids in our liver, and even have some circulating in our blood. If we need energy, our body’s cells absorb and break down fats to release large amounts of it. An average man contains about 12,000 g (12 kg) of stored fat: about 50 days’ supply of energy.

Proteins are made of amino acids. Most of our proteins are stored in our muscles, which also provide structure for our bodies. We also use proteins to make enzymes, hormones and DNA. Proteins are our body’s least favourite substance to use for energy. Their amino acids must be broken down and their nitrogen removed before they can be made into new sugars we can burn. If we have extra protein in our diet it can be broken down and stored as fat. An average man contains about 6000 g (6 kg) of protein: about 12 days’ supply of energy.

Breaking down fats releases twice as much energy as from carbohydrates and protein.

 Why is fat our main energy store?

• It stores more energy in its bonds than the other substances.
• Unlike proteins, it’s not needed in large amounts for other major roles in the body – so can be used without risk of compromising other systems and processes.
• It’s pretty unreactive (inert), non-polar (no electric charge), and hydrophobic (meaning it doesn’t attract water) – so it can be stored easily and won’t weigh much.

If a 70-kg man used only hydrated glycogen (glycogen plus all the water it attracts) to store the same amount of energy that his 12-kg store of fat provides, he would need more than his bodyweight of it. That’s another reason why we use fat!

How do we balance storage and use?

Hormones respond to changes in our blood glucose level. Many affect the liver, which works like the body’s glucose reservoir. If we need energy (have low blood glucose levels) the pancreas releases the hormone glucagon. This triggers the breakdown of glycogen to glucose (“glycogenolysis”) in the liver and releases glucose to the bloodstream. If we’ve been exercising hard and used up all our muscles’ glycogen stores, it can take hours or days to restock them.

But if we have high blood glucose levels (representing an excess of energy, like after a meal), the pancreas releases the hormone insulin. That triggers all our body’s cells to take up glucose; it also tells liver cells to convert any extra glucose to glycogen (“glycogenesis”). This removes glucose from the bloodstream to safe storage. The contrasting effects of insulin and glucagon normally control our blood sugar, but people with diabetes either can’t make insulin (type 1) or don’t respond effectively to it (type 2) and may need help balancing their blood glucose level and sugar intake.

 If our body still needs energy but has used up all its glycogen/glucose (perhaps after a lot of exercise, or eating less than we need) the liver and kidneys may start breaking down stored body fats and proteins to make new sugars to burn for energy (“gluconeogenesis”). Many hormones are involved in these complex processes including insulin, glucagon, adrenaline, noradrenaline, thyroxine, growth hormone, and cortisol.

When we have enough to eat and adequate stores of glycogen in our liver and protein in our muscles, any extra carbohydrate or protein is converted to fatty acids for safe storage in our fatty tissues. If we don’t use it, it stays there!

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References

Antranik.org, March 18 2012. “The Catabolism of Fats and Proteins for Energy”. Retrieved from: http://antranik.org/the-catabolism-of-fats-and-proteins-for-energy/ 10 October 2017.
Colorado State University, July 2015. “Nutrition for the Athlete”. Fact Sheet 9.362. Retrieved from: http://extension.colostate.edu/topic-areas/nutrition-food-safety-health/nutrition-for-the-athlete-9-362/ 9 October 2017.
Diabetes.co.uk, n..d.”The Liver and Blood Glucose Levels”. Retrieved from: http://www.diabetes.co.uk/body/liver-and-blood-glucose-levels.html 10 October 2017.
Khan Academy, 2017. “Introduction to Energy Storage”: created by Jasmine Rana. Retrieved from: https://www.khanacademy.org/test-prep/mcat/biomolecules/fat-and-protein-metabolism/v/introduction-to-energy-storage 9 October 2017.
NOVA, 15 December 2015. “The Ins and Outs of Our Digestive System”. Retrieved from: http://www.nova.org.au/people-medicine/digestive-system 9 October 2017.
TeensHealth.org, June 2015. “Metabolism”. Retrieved from: http://kidshealth.org/en/teens/metabolism.html# 9 October 2017.

Useful links

Bioman, n.d. “Endocrine Ed – Blood Sugar Hormones”. Retrieved from: https://biomanbio.com/HTML5GamesandLabs/Physiogames/endocrine_edhtml5page.html  9 October 2017.
MedicineNet.com, n.d. “Diabetes Quiz”. https://www.medicinenet.com/diabetes_quiz/quiz.htm 11 October 2017..
Seavuria Projects, n.d. “Glucose in the Balance”. http://seavuriaprojects.pbworks.com/w/page/100929613/Lesson%202-4%3A%20Glucose%20in%20balance 11 October 2017.