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If you tell someone you’re feeling a bit low, one bit of advice you might receive is to do some exercise, like go for a walk.

Similarly, if you’re feeling a bit under the weather, you might also feel less bubbly.

What this suggests is that our physical health and our mental health are intertwined, and when one takes a hit, it’s likely the other will too.

What’s less certain though, is why or how the two interact. We’ve spoken to two researchers about their theories.

Like 'a jigsaw puzzle'

There’s a million and one different things that can affect your physical and mental health. Some of these will be obvious - if you don't eat enough during the day you might feel moody, and exercising might make you feel less sluggish.

Studies have shown that there are other, perhaps less straightforward, interactions going on. For example, stress and depression can lead to a weaker immune system, which in turn can increase the likelihood of coming down with something. Mood disorders have also been linked with a higher chance of developing heart disease. So it’s a two-way thing: if your physical health is affected, your mental health is too, and vice versa.

But it gets more complex. Factors like your social interactions and even things that happened when you were a child can play a big part too.

In fact, Dr Julius Ohrnberger, who works in the department of Infectious Disease Epidemiology at Imperial College London, explains these are part of a group of factors known as “determinants of health”. They include genetics, social and support networks, and environmental factors like where we live – and they influence our overall health.

However, what he’s found through his research is that it works the other way around too: both our mental and physical health can also affect all these other aspects of our lives.

We are more likely to exercise, for example, if we are in good mental health, but crucially this exercise will also improve our mental health. If you have depression, you may find it hard to motivate yourself and to carry on, which may influence your ability to exercise. You could become less physically fit and so therefore less likely to exercise and so on so forth.

In other words, it’s like “a jigsaw puzzle” according to Dr Ohrnberger - each piece not only fits together to form the bigger picture of our health as a whole, but this picture also dictates what the separate pieces will look like. This he says can help explain why our physical and mental health often go hand in hand.

Here are just a few of many things you can do to take care of yourself and keep on top of your mental and physical health:

• exercise regularly
• eat a balanced diet
• get sufficient sleep
• balance school/work and other commitments
• make time for relaxation and hobbies

It’s all in your head - sort of

There are some who think, in some cases, we may need to rethink our traditional understanding of how physical and mental health interact. Professor Edward Bullmore is a neurologist and professor of psychiatry at the University of Cambridge. He's written a book called ‘The Inflamed Mind’, in which he suggests some mental health issues are caused by inflammation of the brain.

Inflammation is the body’s reaction to injury or infection. Your immune system will trigger it when it thinks it’s under attack, and is part of the healing process. Problems arise, though, when there’s a false alarm, and the body gets inflamed in parts that aren’t injured. What Prof. Bullmore argues is that this inflammation, if it reaches the brain, could help explain why some people suffer from depression, anxiety and other mental health issues.

This was observed in animals in a lab setting and, while it is much harder to measure in humans, there has been some research that provides evidence for such a link. Prof. Bullmore says one such study observed a potential link between childhood inflammation and a risk of developing depression later on in life, an example of “inflammation in the body preceding depression.”

Understanding that depression can have more than one root, says Prof. Bullmore, can be helpful in many ways, for example when offering treatment for it. He thinks that historically, medicine has been “organised as if everybody falls into one of two boxes”; you either have a physical condition, for which you’d see a physician, or a mental condition, for which you’d see a psychologist. This he thinks is the wrong approach - people should be treated as a whole, not two separate halves.

Mystery - solved?

There are other examples of ways in which our physical and mental health interact but, as Prof. Bullmore says, “generations of research has gone in to the mind, and generations of research has gone into the body, but relatively little has gone into the relationship between the mind and body.”

But it is being researched, and while we wait for more definitive answers, we can keep doing the things that we know make us feel good – like spending time in nature, eating lots of delicious healthy food, and keeping in touch with our friends and family, to name just a few.