Do you put on more weight when you eat late at night?

Most of us know strict dieters who eschew food in the hours just before bedtime, thinking that any calories they consume in the evening will count more. They usually cite the "fact" that a person's metabolism slows down significantly in the evening, or that no one burns calories in the middle of the night.

But in reality, a calorie at noon is really no different from a calorie at midnight.

The reason this myth is so widespread may have something to do with skewed perceptions. Many people who eat at night do so after skimping all day, leaving them with a ravenous nocturnal appetite. When they finally get around to eating, they are prone to grabbing the first thing in sight, which is likely to be something quick and easy, such as fast food. There are also those who eat full meals during the day and eat again at night anyway, packing in extra calories.

Does drinking milk make you phlegmy?

Most people believe that milk causes excess mucus and as a result should always be avoided during a cold, but this is plain wrong. What can seem like an increase in mucus after drinking milk is just a slight thickening of your saliva.

A team of Australian researchers weighed the nasal secretions of dozens of people who volunteered to have cold viruses squirted up their noses. For 10 days, the scientists followed the subjects, keeping track of how much milk they drank and how much mucus they produced. They found no connection between milk intake, nasal secretions and congestion.

Other studies that examined whether asthmatics or people suffering from a cold produce more mucus after a glass of milk also found no difference.

For the extremely small percentage of people with an allergy to the protein in cow's milk, there's a slight possibility that drinking milk could increase mucus.

Does cranberry juice prevent urinary tract infections?

In the past two decades, scientists have documented that cranberries contain a host of antibacterial properties, capable of preventing a variety of infections, chief among them UTIs. A study, published in the New England Journal of Medicine in 1998, showed that cranberries contain proanthocyanidin, a substance that prevents E. coli from adhering to cells that line the urinary tract. Three years later, another study, published in the BMJ found that women who drank cranberry juice every day for six months had a far lower risk of urinary tract infections than those in a control group. Six months after they ended their daily regimen, the women in the cranberry group still had a lower risk, suggesting there were long-term benefits.

But there are some limitations. It takes at least two glasses of cranberry juice a day to produce an effect and, while cranberry juice has preventive powers, there is no evidence that it can clear up infections once they begin.

Does alcohol really kill brain cells?

Alcohol is a powerful disinfectant and in high concentrations it can damage or kill off human cells. But the blood alcohol concentrations that make a person drunk - 0.1% or greater - are far below the extremely high concentrations that are lethal to cells (sterilisers, for example, are typically 100% alcohol). Even a person who drank non-stop would almost certainly stop breathing (alcohol causes respiratory depression) and hit the floor long before their blood alcohol level got anywhere near 1%.

In one study, the brains of heavy drinkers who died of non-alcohol-related causes were compared with those of non-alcoholics of similar age and background: both groups showed roughly the same number and density of neurons.

But while it may not kill brain cells per se, heavy alcohol use can cause long-term damage. Most of the damage is caused by the disruption of message-carrying dendrites attached to neurons in the cerebellum, a brain structure that's involved in learning and motor coordination. In the short term, it causes the hallmarks of intoxication that we're all familiar with: slow reflexes, loss of inhibition and slurred speech. In the long term, it permanently reduces communication between neurons, and alters their structure.

Does crossing your legs cause varicose veins?

More than 12 large studies that have looked at the risk factors for varicose veins have not found leg-crossing to be one of them.

About half of all women and 15% of men over 50 develop the swollen and unsightly blood vessels that characterise varicose veins, caused by blood pooling in the legs.

A study of 3,822 adults, published in 1988, found that for men, the strongest risk factors were smoking and low levels of physical activity. For women, a lack of exercise, high blood pressure and obesity were strongly linked to the condition.

Standing or engaging in sedentary activities for more than eight hours a day increased the risk as well - a finding that has been borne out in European studies of people whose jobs require them to be on their feet, such as nurses. Other studies have pointed to pregnancy and constantly wearing high heels as major contributors. Genes also play a part; more than 80% of people with varicose veins have at least one parent who also has them.

Are those white spots on your fingernails a sign that you need more calcium?

More often than not, the white spots - or leukonychia - are caused by mild trauma to the base of the nail plate, usually from a hard knock or blow to the fingers. A rough manicure that puts a lot of pressure on the nails, an allergic reaction to nail enamels, and bacterial and fungal infections can also be the culprit.

The white colour that you see is usually one of two things: trapped air or a structural defect in the nail itself. It takes more than eight months for a fingernail to grow out, so if you see a white spot halfway up your nail it means the damage occurred within the last four months.

Do wounds heal better when they are allowed to breathe and scab up?

Researchers have discovered that exposing a wound to the air is a terrible mistake because it creates a dry environment that promotes cell death. A handful of studies have found that when wounds are kept moist and covered, blood vessels regenerate faster and the number of cells that cause inflammation drop more rapidly than they do in wounds allowed to air out. Today's best medical advice: keep a wound moist and covered for at least five days.

A plaster wards off infection, keeps the wound moist and protects the area from sunlight, which stimulates the production of pigment and can cause discolouration.

And yes, you can go ahead and pick away at that unsightly scab when no one is watching: a small initial scab will help stop the bleeding, but leaving it to mature for too long will increase scarring. The best way to remove the scab is to soak and slowly remove it, then dry the area and slather on some Vaseline.

Does cracking your knuckles cause arthritis?

Surveys show a large percentage of arthritis sufferers attribute their condition, at least in part, to a lifetime of popping their joints. But here's what's really going on: the "pop" of a cracked knuckle is caused by synovial fluid, the thick lubricant that surrounds every joint. When you stretch or pull your fingers backward, the bones of the joint pull apart, creating a low-pressure environment that creates a bubble which implodes, or collapses in on itself, sending synovial fluid crashing inward.

That noise you hear when the bubble implodes is not as bad as it sounds, and there are studies that confirm this. One of the largest was published in the Annals of the Rheumatic Diseases and looked at 300 healthy people over 45-years-old, about a quarter of them habitual knuckle crackers.

The rates of arthritis of the hand were similar in both groups, though the knuckle crackers, on average, had reduced grip strength along with higher rates of swelling of the hand.

Do you need less sleep as you get older?

It is well known that older people are typically early to bed, early to rise, and studies show that the average person spends about two fewer hours in bed each night aged 70 than they did aged 30.

Conventional wisdom is that older people just don't need as much sleep, but research suggests that is not the case.

What happens instead is that the composition of sleep changes, as people gradually spend less and less time in the deep, restorative stages of sleep. The duration of the REM phase, the stage in which we dream and our muscles relax most completely, diminishes markedly with age, as do the phases of sleep that are the deepest and most refreshing, stages three and four. In people who are older than 90, in fact, stages three and four may even disappear for good.

Meanwhile, stage one of the sleep cycle, the phase that constitutes light sleep, increases. That means there's a greater likelihood of being easily awakened by noise or something as minor as the person lying next to you rolling over. The elderly are also more likely to have their sleep disturbed by pain, chronic illness, drug side effects, the need to use the bathroom or some other physiological discomfort.

As a result, they end up with fewer hours of sleep each night - and subsequently a need to make up for that loss during the day

Is breakfast really the most important meal of the day?

People who eat a normal breakfast each morning - usually consisting of fibre and a protein source such as eggs, meat or soya - have been shown in repeated studies to be healthier than those who do not. The most immediate benefits from breakfast are increased energy levels and a better ability to concentrate during the day.

It is also clear that eating breakfast can help ward off disease. In a study at Harvard Medical School, researchers tracked thousands of people and found that those who ate breakfast every day were far less likely to be obese than those who skipped it. Again, part of the reason is that people who skimp on meals in the morning - whether for lack of time or in the hope of losing weight - tend to compensate by gorging later in the day, often on junk food.

For many people, breakfast is also the greatest source in their diet of wholegrain foods, which are associated with better health and a longer life span.

· Don't Go To Work on Mondays by Anahad O'Connor is published on September 6 by Michael O'Mara Books at £9.99