We live in a time of peak wellness and an obsession with optimisation. We slavishly adopt the latest fad, whether it be peptides, collagen or extreme longevity diets, and we track every aspect of our lives. An ageless body and a quest for immortality are our goal. But among all the noise we miss the one thing that trumps all others when it comes to our health and wellbeing, and it is the one thing we are in danger of letting slip in a world of social media: our friendships. 

There is now a robust body of scientific evidence that our relationships are the biggest factor in our mental and physical health, our longevity and our wellbeing. Maintaining strong and healthy friendships is more impactful than quitting smoking, maintaining a healthy body weight or following government exhortations to get your five-a-day. The newest health trend is one you have never heard of. Friendship. 

As an evolutionary anthropologist who studies love, I find the power of friendship unsurprising. After all, love evolved to help us survive. To make sure we invested in what we refer to as our survival critical relationships. To ensure we did this, evolution gifted us a set of neurochemicals which motivate and reward us for sticking with each other. Some of these chemicals you will have heard of. Oxytocin is the one with the best PR, but others, such as the hormone of long-term love, beta-endorphin, are less well known. And it is this set of neurochemicals, which also includes dopamine and serotonin, which bestow the health and wellbeing benefits of friendship on us. 

Because as well as making us feel the euphoria of love, they also play a key role in maintaining the health of both your body and mind. Take beta-endorphin. As well as being your body's natural painkiller, this morphine-like opiate underpins your immune system. This means that when you spend time with your friends and receive a hit of beta-endorphin, you not only feel relaxed, safe and bathed in love, but your immune system is given a boost, meaning you are less likely to succumb to the latest bug doing the rounds. 

Research has shown that having a strong network of friends reduces your risk of suffering from chronic illness, developing heart disease, diabetes, kidney disease and some cancers. Recovering from an operation? The best medicine is to spend time with your friends. In part this is because friends bring practical help, the homemade soup or the latest prescription from the doctor, but it is also because oxytocin speeds up wound healing, beta endorphin lowers pain and reduces inflammation, and serotonin improves our mood. 

Beyond the impact of friendships on our physical health, perhaps their greatest health-giving property comes when we consider their ability to counteract stress. When I first arrived at Oxford in 2008, I carried out a study exploring the benefits of women's best female friendships as compared to their romantic partnerships. What I found surprised me. It was overwhelmingly the case that women were more emotionally intimate with their female best friend than their romantic partner. 

Think of the no holds barred conversations of the women of Sex and the City, or the deeply empathetic and transformative relationship between Alexis and Twyla in Schitt's Creek. These are not just satisfying fictions. They reflect something true about how women use friendship to process the world. Dealing with our emotions in a healthy way is absolutely fundamental to our health. Undisclosed or unrecognised emotions ramp up our cortisol levels, increasing inflammation, decreasing the efficacy of our immune system and lowering our mood. Our female friends are our most powerful source of emotional good health. 

And they do this, in part, through the power of co-regulation. Because while talking through our feelings is vital, perhaps the most consequential work is being done within our bodies and brains without us even noticing it. When we are with our closest friend, every mechanism in our body comes into synchrony with theirs: our behaviour, our physiology, our heart rate, body temperature and blood pressure, our hormone levels and our brain activity. We can tell how close two best friends are simply by viewing their brain scans. The more similar the activation pattern, the closer they are. This is known as biobehavioural synchrony, and when two people are in this state they become as close as is possible to being one person. Acting as one, feeling as one, seeing the world as one. Your friend's warmth, understanding and calm become your warmth, understanding and calm. Your body recovers its equilibrium while being bathed in the health-giving benefits of the bonding hormones. 

There is nothing on the market that can produce such an all-encompassing health benefit, or that has such far-reaching consequences as your friendships. And it is free. So you could take your collagen, adopt that longevity diet or invest in the latest health tracker, but arguably you would be doing more for your health by grabbing a bottle of cold rosé from the fridge and heading out to meet your friends, ready to talk, laugh and commiserate while your body and mind receive the most powerful health-giving boost they can experience. 

I know which I would choose. 

Dr Anna Machin is one of the voices shaping theALLBRIGHT's Live Well campaign this September. On 25 September at Old Sessions House, London, the Community and Connection pillar puts the science of female friendship, and what it actually does to our health, at the centre of the conversation. Tickets available now.