Want to sip from the fountain of youth? Simply get moving — fitness is the key.

The pictures of toned celebrities from Cameron Diaz (42) to Sandra Bullock (50) may depress you beyond belief but they should be inspiring you. Sure the celebs have hugely expensive nutritionists, chefs, trainers and plastic surgeons on speed dial, but the fact remains that the slide into pudgy middle age isn’t inevitable.

The youthful vibrancy of older celebs isn’t just down to Botox, plastic surgery and liposuction. Stars in their mid forties like Jennifer Anniston have lean, toned, younger looking bodies. Celebrities are showing us the amazing potential of the human body, even at age 40 or 50.

The phenomenon isn’t confined to Hollywood. Socialites of all ages from New York to Karachi have taken the fitness fad to heart and are showing off toned, designer-clad bodies.

In Karachi, you’ll run into these women at Jeanette’s classes (such as her Insanity class which is extremely popular) at Studio X or Sheema’s classes at CORE. You’ll see them weight training under the eyes of the trainers at Shapes or Structure.

In Lahore, they can be found doing yoga with Zainab Abbas or Zumba at Knowledge Factory. They’ll be queuing up for the trainers at Suk Chan or even kick-boxing.

There’s a reason that the 42-day challenge has become so popular in both cities — people are realising that they can drastically improve their fitness levels and their appearance in a relatively short time.

Middle-aged spread isn’t a myth. As women age, their metabolism slows down. This is why women in their forties suddenly find themselves gaining weight, even when they haven’t started eating more. According to some experts, your metabolism can decline about 25 per cent in your forties — that means you would have to drop almost a meal a day just to stay the same weight.

Weight gain can be insidious. You only need to gain a couple of pounds a year to be nearly a stone heavier at the end of a decade. But keeping fit as you age isn’t just about looking good, it can also stave off osteoporosis and muscle loss.

The good news is you don’t have to go on crash diets or starve yourself to look good. Crash diets put your body into starvation mode and your metabolism actually slows down. This is why dieters often find themselves putting on more weight the minute they stop their diet.

The key to weight gain or loss is your Resting Metabolic Rate (RBM). This determines how many calories you burn when you are at rest. RBM depends on your age; weight and muscle mass, although it’s also affected by genetics. The good news is that you can boost your metabolism in the short term with exercise and in the long term by building muscle mass.

Your RBM changes throughout your lifetime. It’s very high in your teens as you go through puberty. This is why teens can pack away huge quantities of food without seeming to gain a pound. Your RBM levels off in your early twenties and then starts to fall. It spikes during pregnancy and breastfeeding, but only enough to allow you an extra 300 calories while pregnant and 500 calories when breastfeeding. Once you hit your forties, your RBM declines more rapidly — particularly once you hit menopause.

So what does RBM really mean?

Well, your body constantly burns calories — even when you are doing nothing. While each pound of fat burns two calories a day to sustain itself, each pound of muscle burns six calories a day — even when you aren’t exercising. This is why adding muscle boosts your RBM. Also a strength training session uses muscles all over your body, boosting the number of calories you burn that day.

Building muscle also supports weakening joints and adding Yoga or Pilates to your exercise regimen will help with balance and flexibility, both of which will help stave off the effects of aging. Exercising judiciously can also limit declines in bone mass, helping prevent osteoporosis as you age.

So what’s the ideal exercise regimen for a woman?

There’s no one-size-fits-all solution. Depending on your age and fitness levels, your exercise regime should include a mix of cardio and strength training. Cardio means anything that raises your heart rate — walking, running, aerobics or cycling all work.

The best way to improve your cardiac capacity and burn more calories is to aim for high-intensity-interval-training (HIIT). For example, run until you are tired, then walk until you get your breath back, then run again and so forth. HIIT delivers a longer and more sustained rise in RBM than low or moderate intensity workouts.

This should be supplemented by weight training. If you aren’t a fan of the bulky-muscled look (what woman is?), don’t panic. Lifting heavy weights doesn’t automatically result in bulk.

According to Sheema at CORE: “Many of my clients worry about lifting heavy weights, but you need to lift heavier weights to build muscle. You should be lifting enough weight that your muscles are hurting by the 10th repetition. You should not be doing more that 12 to15 repetitions for building muscle.”

Many people recommend using low weights for many repetitions rather than high weights for a smaller number of repetitions, but Sheema calls the former endurance training. You can increase endurance training at the expense of strength training if you are in maintenance mode, but building muscle needs genuine strength training.

Bear in mind that if you have a lot of fat on your arms and legs, you may end up looking bulkier initially anyway. This is because you are building muscles under the fat. As you continue with your exercise regime, you will eventually lose that fat and end up with a toned look.

Finally, do take some yoga or Pilates classes. Not only will they improve your strength and balance, the stretching exercises will help you achieve that long, lean look.

In terms of nutrition, it’s important to eat a balanced diet that includes protein, carbohydrates and good fat. Unfortunately, all that activity is not a license to eat whatever you want. While starving yourself is a no-no, you still have to eat smart. Aim for complex carbs like brown rice and brown bread and think about what you eat. Building muscle requires fuel and not eating properly will be counterproductive.

“Keep portion sizes small, include protein with every meal and remember to eat breakfast," advises Sheema. "Breakfast kick-starts your metabolism and helps control cravings. If you are aiming for weight loss, try to limit all processed carbs — even brown rice. The more natural you go, the leaner you will get — try to stay away from anything that comes in a box.”

There’s no need to jump straight into an intense program if you are really unfit. However, regular exercise can build your fitness levels rapidly. The couch-to-5k program, for example, is a proven way that can take just about anyone from taking no exercise to running 5k in nine weeks. There are several apps that help you implement the program, although any decent trainer will also be able to coach you through it.

You don’t have to head to the gym. You can walk in parks, take dance classes or join a cycling group. The most important thing is to get moving.

One of Hollywood’s top trainers, Harley Pasternak, told Vogue USA that to maintain weight, he advises his clients to move at least 10,000 steps a day (along with strength and core work); to lose, 14,000 steps a day.

“This is a lifestyle commitment, and these people understand the importance of exercise, nutrition, and especially rest," said Pasternak. "Lack of sleep makes you less apt to engage in physical activity and more prone to making poor eating choices.”

The great thing is that exercise releases all these lovely endorphins so you should come to really enjoy keeping fit. There’s no need to dwindle into a stereotypical dumpy aunty as you hit middle age — it’s possible to hit the pause button for a while if you get moving.

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The writer is a freelance journalist and the creative force behind style blog karachista.com. Follow her on Twitter @karachista