We’ve all heard the saying about people having a fast or a slow metabolism, but is this true and how does it come about?

Clare Wall, Auckland university’s medical faculty professor, joined Jesse Mulligan to discuss metabolism and how it impacts weight and general health.

The metabolism is simply how we convert the food that we eat into energy to run our bodies, how we store that energy and how we get rid of waste products from this process, Prof Wall says.

And the human body never turns off, so that energy need is constant, she says.

“When you’re asleep still need enough energy to breathe and for your heart to beat that’s called the basal metabolic rate.

“It is quite individual, we’ve all got a different basal metabolic rates depending on the size we are, how tall we are, whether we’re male or female and how old we are.”

There is also some research to suggest there is a genetic component to our basal metabolic rate, she says.

So, are some people ‘lucky’ in that they can seemingly eat what they want but not gain weight?

“As well as the total amount of energy that you need, you’ve got your basal metabolic rate which is the minimum amount of energy you need just for breathing and for your heart to keep beating and then anything that you do over and above that you need obviously more energy so your metabolic rate increases.

“If you want to stay the same weight the amount of food you eat has to be what you need to stay alive plus the energy you need for activity.”

People with a faster metabolic rate are likely to be more active, she says.

“What we do know is that slimmer people are often more generally active, like fidgeters, they’re busy people.”

The science is unclear, she says, whether this is genetically determined or learnt behaviour.

As we age we start to lose lean muscle mass, she says, which has an effect on our metabolism.

“As you get older and your muscle mass decreases, particularly if you’re female, then your basal metabolic rate or the amount of energy that you require starts to go down.

“The key is to keep your lean body mass up and to keep active.”

This is because muscle cells require more energy for them to function, by virtue of having more lean body mass you will increase your metabolism, she says.

She recommends a mixture of exercise to achieve this – high intensity work outs, aerobic work outs, resistance training and weights.

Hunger and the human metabolism are intrinsically linked, she says.

“The thing about our bodies is that they are really amazing in terms of survival. Our bodies metabolically are set up to store energy and to keep us alive.

“We are very quick to react to starvation, so hunger is a really good cue to drive us to eat.

“When we are hungry it decreases our metabolic rate too, so our body is really set up to conserve energy and prevent us from starving to death.”

This is why reducing our food intake is so difficult, she says.

“When we eat less so our bodies can use up our stored energy the immediate thing our body does is to reduce the metabolic rate and also make us hungry so we want to eat.”

Diets, such as keto, can have a short term benefit, but are not a long term solution to a healthy weight, she says.

“Research shows that trying to stay on those diets long term is really quite unsustainable, and that people tend to drift back to their previous eating behaviours and put the weight back on that they’ve lost.”

Some more radical diets may also deprive us of vital nutrients, she says.

“They look good on paper, and make sense scientifically, but weighed against the consequences long term they don’t always look that great.”

Her general diet advice is a healthy proportion of key food groups.

“When we think about our plate at dinner time, half our plate being full of veggies and a quarter wholegrain carbohydrates and a quarter protein.”