We’ve all had those conversations, that we used to be able to eat whatever we wanted and not seem to gain any weight, in addition, we seem to accept that as we age our metabolism changes. That is true, but perhaps not at the time you would think.
But, before we establish how and when our metabolism changes, let’s get to grips with what metabolism actually is.
What is Metabolism?
Metabolism describes the processes that occur to provide energy to biological systems throughout the body. In short, it is concerned with converting what we eat and drink into energy.
You may have heard people asking how to speed up their metabolism in the hope that they can increase the rate at which their body uses calories for energy, and therefore prevent the storage as fat. But the body isn’t like a car with an accelerator – it’s doesn’t have speed settings. Metabolism is just something your body does in line with the demands placed upon it.
Metabolic States
There are three metabolic states in the human body.
Fed
The fed state occurs after a meal, and it is also known as the absorptive state. This state is characterised by a high insulin to glucagon ratio – largely to the rise in circulating glucose from the intake of food.
This state is largely anabolic, meaning things are grown here! Fuel stores are replenished, and it’s where recovery can occur for those muscles after exercise.
When food is eaten, carbohydrates are converted to glucose to use as energy, if the glucose isn’t used, it is stored in the liver and muscles for later. It can also be converted into fatty deposits known as triglycerides. When glucose levels drop between meals, those stores are called upon. Triglycerides are split into fatty acids and glycerol, and it’s glycerol that can be converted into glucose. This is what happens in the fasted state.
Fasting
The fasting state occurs between meals and its purpose is to maintain a sufficient blood glucose level. This state is largely catabolic, meaning that things are broken down here. This is important because glucose can be made from triglyceride storage, but also protein catabolism (hence why if you’re not eating enough you can struggle to build muscle). This is also where the popularity of fasting comes from – the catabolic state is thought to remove damaged cells (true autophagy can take anywhere between 18 hours and 4 days to achieve however).
Starvation
If the fast isn’t broken, starvation sets in and after the remaining glucose has been used, ketone bodies are created from fats and serve as the preferential fuel source. Once these stores are depleted, proteins will be catabolised first from the organs with fast turnover, like the intestinal lining. Muscle may be spared to prevent muscle wasting, but if there are no other alternative sources, muscles will be broken down.
The main purpose of metabolism is to fuel physiological function – the brain and red blood cells have an absolute need for glucose, which is why the metabolic states are so concerned with maintaining sufficient levels.
So Does Metabolism Change As We Age?
Yes, but most of the change occurs when we are 1 year old!
A team of international scientists analysed the average calories burned by more than 6,600 people from one week old to 95 years old as they went about their daily lives in 29 countries worldwide.
What they found was that energy needs soar in the first 12 months of life, unsurprisingly when we think of how much developing the new-born does. The average 1 year old burns calories 50% faster for their body size than an adult. After this initial surge in infancy, metabolic processes slow down around 3% each year until we reach our 20s. It then seems to level off to a new normal. What’s really interesting is that when we experience those growth spurts in adolescence, there isn’t a significant uptick in caloric needs.
Midlife also surprised researchers. During our 30s, 40s, and 50s, energy expenditure is relatively stable. Data suggests that our metabolisms don’t really change until after age 60, and they tend to slow around 0.7% a year. A person in their 90s needs 26% fewer calories than someone in midlife.
The researchers concluded that the thickening waistlines experienced throughout midlife is relatively unrelated to metabolism, but that the slight decline after 60 could be related to muscle loss, as muscle has a high energy demand.
Muscle loss could also highlight fatty deposits, making us feel “fatter!’
So, the jury is in, the data so far suggests that our metabolism does change, but perhaps not like we thought.
To this end, it’s important to support our metabolism, whatever it may be doing, and consider our lifestyle choices if we don’t feel a healthy weight.
Supporting a Healthy Metabolism
- Eat regular meals with protein, fat and complex carbohydrates.
- Ensure you are eating sufficient calories to suit your lifestyle,
- Move more
- Hydrate – the body needs water to carry out all of its physiological functions,
- Reduce and manage stress – the stress response requires energy and so glucose is liberated leading to high glucose levels in the blood.
