Do you struggle to run for a bus because you're unfit? It may be because you weighed less than your peers at birth, scientists find

• Researchers tracked 280,000 Swedish men from birth until aged between 17-24
• On being called up for military conscription in Sweden, their fitness was tested 
• Per 450 grams of birth weight, bicycle ergometer output increased by 7.9 watts

Many of us get out of breath just running for a bus and feel guilty about not going to the gym.

But fitness levels could actually be down to how much you weighed at birth, according to scientists.

Researchers looked at more than 280,000 Swedish men aged between 17 and 24 - the years when they could be called up for military conscription in the country.

On being called up, the men underwent tests on stationary bicycles to test their fitness at Sweden's Department for Global Public Health.

The findings show that for every 1lb (450g) of weight at birth, the participants' capacity on the bikes increased by an average of 7.9 watts.

This translates to a 34 per cent increase in fitness levels and a 13 per cent difference in risk of premature death, the researchers said. 

Co-author Dr Daniel Berglind said the difference he and his team observed was 'alarming'.

The average weight for a baby born in the UK is 7lbs 3oz and the NHS classified a low weight as anything below 5lbs 8oz.

Low birthweight babies struggle to feed, gain weight, fight off infections and can develop breathing difficulties. 

As they grow older, they are also known to face a greater risk of suffering problems such as diabetes and cardiovascular disease. 

Dr Berglind and his colleagues wanted to see if low birth weights played a role for cardiorespiratory fitness in individuals born after pregnancy of 37-41 weeks.

Cardiorespiratory fitness is the ability of the body to supply oxygen to the muscles during sustained physical activity.

People who were born before 37 weeks – when they would be regarded as premature – were not included in the study.

When the men in the study were called up to serve in the military, their maximum aerobic performance was tested on a bicycle ergometer.

The machine measures someone's fitness by getting them to pedal as hard as they can for as long as possible.

A person's V02 max - how much oxygen they can use during intense exercise- is recorded in the test.

The researchers found that those born with higher birth weights performed significantly better on the test.

Dr Berglind said the 7.9 watt increase in output for every 1lb (450g) of birth weight translated into a 1.34 (34 per cent) increase in metabolic equivalent (MET). 

WHAT ARE THE RISKS UNDERWEIGHT BABIES FACE? 

The average newborn weighs about 8lbs and anything under 5.8lbs is considered underweight.  

Low birthweight babies struggle to feed, gain weight, fight off infections and can develop breathing difficulties.

As they grow older, they are also known to face a greater risk of suffering problems such as diabetes and cardiovascular disease. 

In developing countries, poor growth in the womb is one of the major causes of low birthweight. 

In Western nations, low birthweight is often associated with prematurity - a baby born earlier than 37 weeks gestation. 

Low birthweight is often due to prematurity as a result of high maternal age.

Smoking, medically unnecessary caesarean sections and fertility treatments can also all increase the risk of a baby being born with a low birthweight. 

MET is a measure of how much energy someone expends during exercise relative to their weight. The higher a MET score, the more physically fit they are thought to be.

Scores range from one to 12 - where one is considered the equivalent of sitting on the sofa, three is associated with walking, seven with jogging, 10 with skipping rope and 12 with sprinting.

He said the observed differences were, 'associated with a 13 per cent difference in the risk of premature death and a 15 per cent difference in the risk of developing cardiovascular disease.'

The results remained true even when researchers took body mass index - a measure of body fat based on your weight in relation to your height – into account.

The findings stayed the same when the researchers compared the participants to their siblings, suggesting shared genetics alone do not explain fitness levels. 

The researchers said the findings could impact public health policy, because around 15 per cent of babies born globally weigh less than 5lbs 8oz (2.5kg).

Viktor Ahlqvist, who co-authored the study, said: 'Providing adequate prenatal care may be an effective means of improving adult health not only through prevention of established harms associated with low birth weight but also via improved cardiorespiratory fitness.'

Cardiorespiratory fitness is declining globally, both for youths and adults, according to a study published in the British Journal of Sports Medicine. 

Scientists at the University of Essex found that from 1998 until 2008, there was an annual decrease in fitness among both boys and girls in middle and high income countries of around 0.8 per cent.

There are also spiralling rates of obesity in children, with the numbers of children who are morbidly obese doubling since 2008, according to NHS figures.

HOW MUCH EXERCISE DO YOU NEED TO DO?

To stay healthy, adults aged 19 to 64 should try to be active daily and should do:

• at least 150 minutes of moderate aerobic activity such as cycling or brisk walking every week and

• strength exercises on 2 or more days a week that work all the major muscles (legs, hips, back, abdomen, chest, shoulders and arms)

Or:

• 75 minutes of vigorous aerobic activity such as running or a game of singles tennis every week and

• strength exercises on 2 or more days a week that work all the major muscles (legs, hips, back, abdomen, chest, shoulders and arms)

Or:

• a mix of moderate and vigorous aerobic activity every week – for example, 2 x 30-minute runs plus 30 minutes of brisk walking equates to 150 minutes of moderate aerobic activity and

• strength exercises on 2 or more days a week that work all the major muscles (legs, hips, back, abdomen, chest, shoulders and arms)

A good rule is that 1 minute of vigorous activity provides the same health benefits as 2 minutes of moderate activity.

One way to do your recommended 150 minutes of weekly physical activity is to do 30 minutes on 5 days every week.

All adults should also break up long periods of sitting with light activity.

Source: NHS