Dalton Wong is one of the world’s leading personal trainers.

At the London gym Bodyism (www.bodyism.com) and the private Brompton Cross Clinic (020 7052 0070), both in South Kensington, he works with some of the most driven, wealthy and competitive people on earth.

They are often celebrities or big in the City; they head up hedge funds or run their own business.

Yet, despite their hectic lives, dedication to fitness and huge self-discipline, some of them consistently fail to lose weight. The reason? Too much exercise.

“I see it all the time,” Wong says. “These people push themselves relentlessly at the gym, but the numbers on the scales won’t budge. Yet, when I tell them to get off the treadmill, get some rest and eat well, they are often astonished to find they finally start losing weight.”

The reason for this paradox? Stress hormones. “These people run on stress all day, but what they don’t realise is that exercise is also a stress on the body, and causes it to produce the stress hormone cortisol. Normally, this is fine, provided there are also periods of rest. If, however, people have no downtime, levels of cortisol remain high, and that will affect their weight.”

Cortisol is a hormone secreted by the adrenal glands as a reaction to stress.

One of its main functions is to help the body produce blood sugar from proteins and pour it into your bloodstream, providing energy for the “fight or flight” response. Any excess sugar is then used for fat production.

Dr Henrietta Brain, an endocrinologist, says: “Hormones have a huge influence on our weight. Insulin and cortisol both contribute to fat accumulation, while human growth hormones and oestrogen affect fat mobilisation.”

Says Wong: “Elite athletes always build rest days into their schedule. Many of my clients never have a rest day. Not only do they work in highly pressured environments, they have family duties at the end of the working day, and they take strenuous exercise. Research shows that people with high lifestyle stress will release more cortisol during an intense bout of exercise than someone who has a lower stress level. At Bodyism, we carry out hormonal profiling on our clients – our Body Oracle system – to look at their stress levels. If we see high levels of cortisol in a client, and we think they are holding onto fat as a result, we build relaxation, rest and good nutrition into the plan, and that really helps them to lose weight.”

WHAT CORTISOL DOES TO YOUR BODY

As well as causing fat gain, especially around the stomach, chronically elevated cortisol levels also lead to muscle loss, a less effective immune system and slower recovery from injury; studies show that cortisol can even inhibit thyroid function. This lowers the metabolism, and can prevent weight loss and lead to weight gain.

Dr Marilyn Glenville, a nutrition and hormone specialist, and author of Fat Around the Middle (Kyle Cathie £9.99), agrees that stress is a potent saboteur of our weight-loss efforts. “The main reason some people gather more fat around their middle than others,” she says, “is specifically because of cortisol. Our bodies have evolved to produce short bursts of the hormones adrenaline and cortisol when we are in danger, to give our body energy in the form of high levels of blood sugar. However, our bodies can’t distinguish between late trains or missed appointments and being chased by wild animals. The stress hormones keep being produced and are never released, so the extra energy is deposited as fat around our middle. In addition, cortisol in the blood increases your appetite, particularly for carbohydrates, fat and sugar, which makes you lay down even more fat.”

That said, while everyone produces cortisol under stress, each person’s reaction to it is individual – hence the value of hormonal profiling, according to Wong. Researchers in the USA found that women who secreted higher levels of cortisol while under stress had a much greater tendency to snack on high-fat foods than women who produced less cortisol in reaction to the same stressful event. It seems that fish oils can reduce cortisol levels. In a Swiss study, men whose diet was fortified with omega3 fatty acids had unchanged levels of cortisol during stress tests, while the placebo group’s rose by more than 30%. Another study, published in the journal Diabetes & Metabolism, showed lower levels of cortisol in those taking fish-oil supplements.

THE SLEEP SOLUTION

Another reason to tame those cortisol peaks is that high levels lead to restless sleep, which is also disastrous for your weight. “Without sufficient relaxation and sleep,” Wong says, “the body cannot produce enough growth hormone, which makes you leaner and stronger.”

Lack of sleep is shown in study after study to be associated with obesity, at least partly due to the effects of the hormones leptin and ghrelin, which affect hunger and satiety. One study showed that people who slept only four hours a night for two nights saw an 18% decrease in levels of leptin, which seems to have a role in hunger control, and a 28% increase in ghrelin, a hormone that triggers hunger.

They also had a 24% increase in appetite, craved sweet, starchy and salty food, and lost interest in fruit, vegetables, protein and dairy products.

“Sleep is as important as exercise and diet,” Wong says. “I always tell clients that you lose weight while you are sleeping, not when you are working out in the gym.”

DO LESS, LOSE MORE

Just as slogging it out on the treadmill for hours after a hectic day at work isn’t a great idea, swapping the gym for the sofa won’t give you a supermodel body, either. It’s a balance. If you are highly stressed, the following tips will get you into shape faster while reducing your cortisol levels.

— Mix up your workouts. For every hard workout with strenuous cardio and weights, do two easy days, either resting or doing yoga, gentle cycling or walking, keeping your heart rate below 70 beats per minute.

— Build relaxing routines into your life – a warm bath before bed, simple stretching and relaxing exercises using a foam roller, which also massages the body (a roller with exercise chart starts at £15.95, from UK Sports and Fitness; 0845 463 3162).

— Keep a “gratitude diary”. Every night, write down three things that have gone well and that you are grateful for. Focusing on positive events and feelings has been clinically proved to reduce stress and elevate mood.

— Get enough sleep. You need more in winter because the darkness affects your sleep hormones.

— Eat plenty of healthy fats, such as fish oil, to keep cortisol in check.

— Do diaphragmatic breathing. Inhale through your nose, filling up your abdomen. Hold for four or five seconds, then exhale slowly through your mouth. Repeat 10-20 times before bed.

— For every three to four weeks of exercise, have an “unloading week”. This will include massage, gentle yoga-type exercises, walking, gentle cycling and stretching. When you head back to the gym, your body will be fully rested and you will be able to train harder, which will mean greater weight loss and an improvement in strength.

— Want to check your hormonal stress profile? Ki Performance offers a saliva test, which can be done at home, then posted for analysis by a British laboratory. It examines your key stress hormones, including cortisol, DHEA and testosterone (£130; 020 7307 8280, www.kiperformance.co.uk).

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