As a mother to three children under 12, I expect to feel totally exhausted at the end of the day. What I didn't realise, though, was that a lack of an essential vitamin could be to blame for my paltry physical stamina.
A while ago, during a series of blood tests for some imaginary ailment (no doubt tiredness-related), my levels of vitamin D - essential for the absorption of calcium - were found to be very low.
The doctor didn't seem to think this was significant but, as my mother, aged 78, has osteoporosis, I thought perhaps I should take a supplement. The condition, where bones become brittle, has been linked to a lack of vitamin D, although there are strong genetic factors as well.
I'd probably have thought little more of it, but last month I read about intriguing research which found boosting vitamin D intake can raise energy levels and lower blood pressure.
In the study, by a medical team in Edinburgh, volunteers were asked to cycle for 20 minutes. They were then given either a placebo or vitamin D and, two weeks later, were asked to cycle for 20 minutes again.
Those who'd taken the vitamin D were able to "cycle longer with less effort" and showed diminished levels of the stress hormone cortisol, which can also cause raised blood pressure as it narrows blood vessels.
Vitamin D deficiency is a peculiarly contemporary phenomenon. The most efficient way of absorbing what is an essential vitamin is through sunlight directly on the skin.
But, ironically, in our bid to avoid sun damage - a contributory cause of both skin cancer and premature ageing - many of us just don't get enough.
Though the Scottish study only had 13 participants, its lead author, Dr Emad Al-Dujaili, of Queen Margaret University, is convinced of its importance.
"Vitamin D deficiency is a silent syndrome linked to insulin resistance, diabetes, rheumatoid arthritis, and a higher risk for certain cancers," he says.
It all sounds so convincing I decide to conduct my own experiment to see if boosting my vitamin D levels would have a similar effect. First, I arrange for blood tests through online service Medichecks. My baseline reading for vitamin D comes back as 53 nmol/L, which is just within the "normal" range this time.
The truth is, experts don't agree on where the optimal range, or even "normal", is, though 25 is considered severely deficient. Nutritionist Dr Marilyn Glenville says: "Some laboratories define more than 75 as normal and 80 to 100 as optimal."
She, too, believes Vitamin D deficiency is one of the health stories of our age. "Research shows vitamin D can be anti-inflammatory and that, aside from osteoporosis, its levels affect cardio health, type 2 diabetes, breast cancer, joint pains, allergies, fertility and immune function," she says.
"The trouble is, we have so taken on the skin cancer message - plus sunscreen is in women's skin creams, so we tend to be worse off than men - that we are even seeing the return of rickets." Rickets is childhood bone malformation caused by a lack of vitamin D and cereals were fortified to try to see it off.
My blood tests also record a mid-range level of cortisol, the fight or flight "stress" hormone, again implicated in inflammation. My blood pressure, meanwhile, is fine.
The next step in my experiment is the bike test, which I do in my gym. I manage 10.1 km in 20 minutes and feel pretty tired when I come off. Then, for the next two weeks, I take a daily vitamin D supplement with food. Vitamin D3 is the best sort to raise vitamin D levels. It's important to get the right amount so do consult a doctor or nutritionist. Too much vitamin D can be toxic so, as I am under 50 kilograms, I go for 1000 iu which is the maximum Britain's NHS advises people to take without specific medical advice; that will still give me twice the recommended daily amount.
A few days in, I begin to think - imagine? - I feel perkier in the mornings and less manic at bedtime. Ten days in, and I even have enough energy to go for a short run, much to the children's amazement. By the end of the two weeks, I'm not exactly bouncing out of bed but I do feel more focused and somehow, stron-ger, like the voltage has been turned up.
The results of new blood tests show my vitamin D levels have risen and although my resting heart rate is the same, my blood pressure has gone down slightly and so have my cortisol levels.
The moment of truth, though, is on the bike. I cycle again for 20 minutes and this time I cycle 11km - a boost of 0.9km - and I could easily have carried on.
Grand claims are made for Vitamin D these days, but I will say this: I am going to carry on taking these supplements whole-heartedly because they really do make me feel better. And that is pure, unadulterated sunshine.
- Daily Mail