If you are heading for Israel this summer, do not clog up the case with extra sunblock. In fact, consider stepping out for half an hour after breakfast or tea wearing only the merest smidge of low-factor protection; it could prove a life-saver.
It is true that UVB rays can burn and age the skin, but they also bestow a gift whose benefits are only now becoming fully appreciated - they enable our body to manufacture and store vitamin D.
"We must create a new relationship with the sun," says Dr Soram Khalsa, a professor of medicine and the author of The Vitamin D Revolution. He believes D has the power to prevent cancer and other serious diseases.
It is certainly vital for bone health. Doctors began to see rickets in children 300 years ago when the Industrial Revolution moved populations off the farm and into factories. Exposure to sunlight was identified as a cure a century before vitamin D was finally given a name in 1920.
But there is much more to this powerful agent that we can store in our bodies against a rainy day- a whole winter of rainy days in fact. And we need to; a deficiency of D has been linked to heart disease and diabetes, as well as 17 different forms of cancer. Finnish researchers have just published a 30-year study showing a deficiency can also predispose a sufferer to Parkinson's in later life.
Surprisingly, given that low-cost flights have made year-round sun a possibility for almost everyone, huge numbers of us are suffering from vitamin D deficiency. We have, it seems, taken too literally warnings to cover up against what can be harmful rays .
That perfect pale blonde Gwyneth Paltrow revealed recently that she was suffering from a deficiency of D and heading for osteoporosis as a result. And she is not alone - recent research shows half all British adults are in the same boat and may not know it.
"We are seeing an increasing incidence of vitamin D deficiency in patients whose blood we test when they complain of vague symptoms including tiredness, muscle aches and bone pains," says Dr Garry Savin, medical director of the London clinic Preventicum.
Jews could be at extra risk, he surmises, first because we have predominantly olive skin which requires twice as much sun exposure to produce vitamin D as those with fairer complexions. But we also have an increased tendency to spend long hours working inside. Plus the assiduousness of the strictly-Orthodox in covering up limbs predisposes to a deficiency.
Workaholics and Charedim alike are two groups who need to be taking supplements, since according to Dr Savin, the fair-skinned need 20 to 30 minutes of sunlight on their faces and arms two to three times a week, and a dark-skinned person twice as much.
And women at least have been programmed out of getting even that small amount of sunshine, as Dr Khalsa found out when he wondered why he was discovering vitamin D deficiencies even in the heart of southern California:
"I realised most of my patients see dermatologists, who for years have been warning about the dangers of sun exposure and its role in skin cancer," he reflects. "They have trained us to shield ourselves before we even go out for the day."
So given the justifiable concerns about sunburn and skin cancer and the premature ageing soaking up sun can bring to faces, what is to be done? In Britain we have the additional problem that we do not get strong enough rays between October and April to facilitate Vitamin D production.
It comes down to supplements, experts agree, particularly since there is pressing evidence that maintaining bone health involves taking Vitamin D with calcium to help the body absorb the vital mineral.
But what supplements? When, like Gwyneth, I was diagnosed as being on the road to osteoporosis far earlier than normal, I was warned that most calcium on the market is the carbonate variety, less well absorbed by the body than the more expensive calcium citrate. The latter is hard to find here, considerably more expensive and not to be confused with lemon-flavoured calcium carbonate tablets.
Of these, Adcal may be prescribed free to women diagnosed with osteoporosis - but, packed with a whopping 1500mg of calcium carbonate per tablet, they taste like chalk, a shame considering each one also contains a healthy dose of Vitamin D3.
Calci-Plus, which does not require a prescription, is smoother and more palatable, and also balanced with magnesium, another important consideration when choosing a Vitamin D and calcium supplement.
Osteo Plus, a blend of D3, calcium citrate and magnesium citrate developed by nutritionist Dr Marilyn Glenville for the National Health Practice, is even certified kosher. Plus the capsules do not need to be sucked or chewed, a bonus for those who have no sweet tooth. However, at £19.06 for six weeks' supply they are not cheap - certainly not compared to the zero cost of a life-giving stroll in the morning sunshine.
'The Vitamin D Revolution' is published by Hay House