By THÉRÈSE MARGOLIS    

It’s one of those sad realities that come into play after age 40: the thinning of hair.

Blame it on your genes, hormonal changes, pollution or a poor diet, but by the time you hit the Big Four-Oh, chances are your tresses are thinner, less voluminous and less resilient than they were when you were in your 20s.

But fear not, even though you are never again going to have that shiny, thick, full head of hair you had in college, there are things you can do to keep thinning hair at bay, starting with using the right shampoo and conditioner.

“After age 40, 81 percent of women notice a loss of hair volume and density,” explained dermatologist Adriana Aguilar Donis from Mexico City’s Médica Sur Hospital.

“And hair doesn’t just lose volume and density with age, but also becomes stiffer, duller and more fragile.”

Speaking at a conference to introduce Vichy Laboratoires’ new Dercos Technique’s Densi-Solutions line of hair thickening and protecting products, Aguilar Donis went on to say that at the root of most hair thinning is the hair growth cycle.

“All hair follicles go through three stages, known as the anagen, catagen and telogen phases,” she said.

During the anagen phase, which is also known as the growth phase and which can last between three and seven years, healthy hair grows from the root or follicle at a rate of about a centimeter a month.

This is followed by the catagen or transition phase, during which the hair follicle stage detaches from the dermal papilla in the scalp. This can last about 10 days.

The third and final stage is the telogen or resting phase, which lasts about three months. This is when new hair follicles begin the growth phase.

Every hair follicle has its own distinct cycle, so one follicle can be in the anagen phase and the one next to it might be in the catagen or telogen phase.

On average, we all lose about 50 to 150 hairs every day, which Aguilar Donis said is a normal hair shedding process.

But as we age, she said that the hair growth cycle slows and we produce less hair than before.

Also, the length of the anagen stage decreases, leaving follicles weaker and thinner with each cycle.

The secret to ensuring healthier, thicker and more voluminous hair is to make sure that the follicle is nourished and protected when it is in the anagen and telogen phases, Agular Donis said.

Products like the new Vichy Dercos’ Densi-Solutions line – which includes as deep-cleaning Thickening Shampoo, enriched with rhamnose (to help in scalp health and follicle strength), filloxane (to make hair follicles thicker and more resilient from the inside out) and vitamin E (to protect hair from environmental aggressions and free radicals); a Restoring Thickening Balm conditioner, with ceramides (to protect hair’s fiber and help restructure) and candelilla (to seal follicles and protect against frizz); and a leave-in Hair Mass Recreating Concentrate, with the antioxidant resveratrol and stemoxydine (to fight against hair loss) – can help make your tresses stronger and more resistant to breakage and fall out.

Agular Donis said that, used in tandem, the three Dercos products can produce visually thicker, stronger hair in just six weeks.

One more tip Agular Donis said: When you wash your hair, be gentle with it.

Hair is at its most fragile state when it is wet, and thinning hair may not be well anchored at the root.

Don’t rub your hair dry. Instead, gently pat it dry with a towel.

Also, extreme heat can cause hair to weaken and fall out, so washing your hair with scalding water is a big no-no.

Blow driers and chemical processing, such as perms and coloring, can also thin hair.

When you do use a blow drier, be sure to use a heat protecting product, and remember to avoid exposing your hair to too much sun. UV rays can wreak havoc on your tresses.

Try to avoid tight hairbands or ponytails that can put tension on delicate hair and cause breakage.

There are other factors that can weaken and thin hair, starting with your diet.

A diet rich in biotins and other B vitamins, along with folic acid and vitamin E will help nourish your hair from within, and there are special hair supplements available on the market.

Lack of sleep and smoking can also thin and dull hair.

Finally, if you have a sudden change in the amount of hair loss you are experiencing, it is best to consult your doctor because this could be an early sign of a hormonal problem, infection or autoimmune disease.