Image caption A baby's lifetime health is largely decided before birth

Why does one person die younger and another survive to old age? Lifestyle and genetic factors play a role, but argues Prof David Barker, a better predictor of future health is our birthweight and what it tells us about our development in the womb.

Heart disease, cancer, diabetes. These are some of the chronic diseases that determine lifespan.

We know some of the causes such as hardening of the arteries, rising blood pressure or insulin resistance, but why do some people suffer them more than others?

Obesity, cigarette smoking and psychosocial stress have all been implicated. Genes offer another possibility but they are unlikely to explain why coronary heart disease was rare 100 years ago but is now the commonest cause of death around the world.

FIND OUT MORE Horizon: The Nine Months That Made You is on BBC Two on Monday, 22 August at 21:00 BST

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Our search for ways to prevent today's chronic diseases has largely failed. Soon there will be 250 million people with diabetes worldwide, yet many of those are neither overweight nor inactive.

One of the most striking studies of the causes of diabetes was carried out in rural India among villagers living what might be considered a model lifestyle. They typically ate a vegetarian diet, were physically active and thin. Yet diabetes was prevalent.

Long-term studies there and elsewhere have shown that people who develop chronic disease often grew differently to other people in the womb and during childhood. Their birthweights tend to be towards the lower end of the normal range.

It seems people are like motorcars. They can break down if they are driven on rough roads but that is more likely if they were badly made in the first place.

Our early development in the womb sets up our constitution, how vulnerable we are to negative things that we encounter and how we will cope with them for the rest of our lives.

Genetic switches

Like other living creatures, our own development in the womb and during infancy is moulded by our environment, in particular the nutrition we receive.

The unique person that you are was not determined at conception, but by the interplay between the genes you acquired and the nutrition you did or did not receive as you developed.

If crucial genes are not switched on during foetal development as they should be, that can permanently change the structure and function of an unborn baby for life.

Image caption The Dutch wartime famine has provided lifelong case studies

Malnutrition and other adverse factors also slow growth, which is why chronic disease is associated with low birthweight.

It is a simple matter to alter the diet of a pregnant animal and show that its offspring have higher blood pressures, are obese or are unable to control how much sugar there is in their blood.

History has also provided us with real-life examples of the same effects on humans.

In the final stages of German occupation in World War II, the Netherlands suffered a five-month famine with a resulting fall in the average weight of babies born at the time. Lifetime studies of some of those people, now more than 60 years old, show they have suffered greater risk of high blood pressure, raised cholesterol, heart disease, diabetes and breast cancer than their brothers and sisters.

Around the world in China, America, Europe, the Middle East, records of low birthweight have also been linked to higher instance of poorer lifetime health.

Critical periods

Like many living things, the human baby is challenged and does not have sufficient resource to perfect every aspect of its body. So we have a hierarchy of priorities.

Brain growth is at the top and the development of organs such as the kidney and lungs, which do not function in the womb, are at the bottom. Their development can be traded off against other priorities.

Prevention of chronic disease... depends largely on the improvement in the nutrition of girls and young women Prof David Barker

For most organs there is a critical period during development when a system or organ has to mature. These periods are brief; they occur at different times for different systems; and for most systems they occur in the womb. Only the brain, liver and immune system continue to develop after birth.

If an organ fails to mature during that critical period, it has permanent consequences. For example, the capacity of our kidneys to do their job is determined at 36 weeks of gestation, and measured by the number of functioning units or nephrons in the organ.

More nephrons is good because it reduces the risk of high blood pressure. Some people have three times as many, and that was determined in the womb. For the unborn baby, the kidney is a low priority organ because the mother serves as the baby's kidney until birth.

The unborn baby is nourished by its mother's body. But that does not just mean what its mother eats each day. That would be far too dangerous a strategy. What she eats during pregnancy does matter.

But the baby's development depends more on the food stored in her body and on the way her body handles food, which is the product of her lifetime nutrition. This will decide her baby's health in later life.

Improving nutrition

The birthweight of a baby reflects how well it was nourished in the womb and the risk of chronic disease in later life.

It is better to be 7lb (3.2kg) at birth than 6lb - better to be 8lb than 7lb. This implies that variations in the supply of food from normal healthy mothers to normal healthy babies have huge implications for the long-term health of the baby.

So what are the lessons?

Prevention of chronic disease, allowing more of us to be healthier for longer, depends largely on the improvement in the nutrition of girls and young women.

Many babies in the womb in Britain and around the world today are receiving unbalanced and inadequate diets.

Coronary heart disease, type 2 diabetes, breast cancer and other chronic diseases are unnecessary. Their occurrence is not mandated by genes passed down to us through thousands of years of evolution.

Chronic diseases are not the inevitable lot of humankind. They are the result of the changing pattern of human development. We could readily prevent them, had we the will to do so.

The work of Prof David Barker MD FRS CBE and the Barker theory are explored in Horizon: Nine Months That Made You at 9pm on Monday 22 August on BBC Two.