Why Your Metabolism Slows Down at 40, and the Supplements That Help

You turn 40 and something feels different. The jeans that fitted fine at 38 feel tight by 43, even though your eating habits have barely changed. The energy that used to carry you through to bedtime now dips somewhere around the school run or the mid-afternoon meeting. It is easy to blame your metabolism and assume it has quietly downed tools. That is the story most of us grew up with. The science, it turns out, tells a more useful one.

Does your metabolism actually slow down with age?

Your metabolism does not slow down nearly as early or as much as people think. One of the largest studies ever done on human energy use tracked more than 6,400 people aged from eight days to 95 years across 29 countries, measuring the calories they burned each day using the gold-standard doubly labelled water method. After adjusting for body size and muscle, the researchers found that daily energy expenditure holds steady from age 20 right through to 60. No slump in your thirties. No dip at 40. No measurable drop at menopause. So if your resting metabolism is not the culprit, what is actually going on?

When does your metabolism really slow down?

The same research found that the genuine decline does not begin until around age 60, after which daily energy expenditure falls by a little under one percent a year. That means the changes you notice through your forties and fifties are real, but they are not your cells suddenly burning fuel more slowly. The cause sits elsewhere, and it is something you can do a lot about.

What actually changes in your body at 40?

Three shifts tend to arrive around this age, and together they explain most of what gets blamed on a slow metabolism.

The first is muscle loss. Muscle is metabolically active tissue, so the more of it you carry, the more energy you burn even while sitting still. From around the fourth decade of life, you gradually start to lose it. Studies commonly report a loss of somewhere between 3 and 8 percent of muscle mass per decade, and the rate picks up speed after 60. Less muscle means slightly fewer calories burned at rest, and noticeably less strength, which is why hauling shopping bags up the stairs starts to feel like more of an event than it used to.

The second is that you tend to move less. Work gets busier, knees get a bit creakier, and the constant low-level movement of younger years quietly fades. This is the part people rarely notice, because it happens so gradually, but it adds up to a real drop in the calories you burn each day.

The third is hormonal. For many women, perimenopause and menopause arrive during the forties and fifties, and falling oestrogen affects sleep, mood, and where the body tends to store fat. Your basal metabolism may be steady, but how you feel, and how your body looks, can change all the same.

Here is how those pieces fit together:

What changes with age	What the evidence shows	Why you feel it
Resting metabolism	Stays stable from 20 to 60, then declines after 60	Not the real reason for midlife weight gain
Muscle mass	Falls by around 3 to 8 percent per decade from your forties	Less strength and a slightly lower calorie burn at rest
Daily movement	Tends to drop as routines get busier and more sedentary	Fewer calories burned through everyday activity
Hormones	Oestrogen falls during perimenopause and menopause	Affects sleep, mood and where fat is stored

Can supplements speed up your metabolism?

Here is the honest answer. No supplement meaningfully speeds up your resting metabolism, and you should be cautious about any product that promises it will. What the right nutrients can do is support the systems behind your energy and your muscle, particularly when your diet falls short. Several of the nutrients your body uses to turn food into usable energy, and to protect muscle, get harder to obtain as you age. That is where a well-chosen supplement earns its place. Not as a metabolism booster, but as a way to close the gaps that hold you back.

Which supplements actually help after 40?

Magnesium for energy-yielding metabolism

Magnesium does a great deal of quiet work in the background. It acts as a cofactor for more than 300 enzyme reactions, including the ones that convert ATP, your cells’ energy currency, into usable power. When you are running low, those reactions become less efficient, which can show up as the kind of flat, foggy tiredness that no amount of coffee quite fixes. Magnesium is officially recognised as contributing to normal energy-yielding metabolism and to the reduction of tiredness and fatigue, which is one reason our Pro Magnesium 4 Complex is a favourite with readers in their forties and fifties. It also supports normal psychological function and the nervous system, which matters more during perimenopause, when falling oestrogen leaves the body more reactive to stress.

Omega-3 to help protect ageing muscle

Since muscle loss is one of the genuine drivers of how your body changes at 40, anything that helps you hold on to muscle is worth your attention. In a randomised controlled trial published in the American Journal of Clinical Nutrition, older adults who took omega-3 fish oil for eight weeks showed a stronger muscle-building response than those given a placebo oil. A follow-up trial by the same team found that six months of fish oil increased muscle mass and strength in healthy older adults. Omega-3 will not build muscle on its own, but paired with regular movement it gives your body better raw materials to work with. Our High Strength Omega-3 delivers 2000mg of fish oil per serving, and it pairs neatly with magnesium if you are tackling both energy and muscle.

A daily multivitamin for turning food into energy

The B vitamins, especially B12, B6 and folate, are the spark plugs of energy metabolism. They help your body release energy from the food on your plate. The snag is that B12 becomes harder to absorb as you get older, because the stomach produces less of the acid needed to free it from food. B12 deficiency is estimated to affect 10 to 15 percent of people over 60. Worth being clear here, though: topping up B vitamins only lifts your energy if you were genuinely short to begin with, so this is about covering a real gap rather than chasing a buzz. A good formula uses the active, ready-to-use forms of these nutrients, which is why our Daily Multivitamin is built with methylated B12 and folate, plus CoQ10 and vitamin D, rather than the cheaper standard forms.

Vitamin D for muscle strength

Vitamin D is hard to get from food, and in the UK it is close to impossible to make from sunlight between October and March. Its link to muscle is striking. In an analysis of 4,157 UK adults aged 60 and over from the English Longitudinal Study of Ageing, muscle weakness was twice as common among those who were vitamin D deficient compared with those who had healthy levels. Topping up your vitamin D will not transform your strength on its own, but a shortfall clearly works against you, which is why it appears in both our magnesium and multivitamin formulas.

What works better than any supplement?

If you change only one thing after reading this, make it resistance training. Lifting weights, using resistance bands, or even bodyweight moves like squats, sit-to-stands and press-ups directly counter the muscle loss that drives so much of what feels like a slowing metabolism. Add enough protein, aiming for a decent portion at each meal, get reasonable sleep, and keep moving through the day, and you are dealing with the real causes rather than the imagined one. Supplements support that foundation. They were never meant to replace it.

This matters even more if you are using a GLP-1 weight loss medication. The newer injectable treatments, semaglutide (sold as Ozempic and Wegovy) and tirzepatide (sold as Mounjaro), are very effective at reducing weight, but clinical trials that scan body composition show the weight that comes off includes lean tissue, not only fat. In other words, you can lose muscle along with the fat. The same levers covered above, regular resistance training and enough protein, are what protect that muscle, and omega-3 gives your body better material to work with. If you are on one of these medications, it is sensible to treat strength work, protein and a quality multivitamin as part of the plan rather than an optional extra.

The bottom line on your metabolism after 40

Your metabolism is not the villain it has been made out to be. The evidence shows your resting energy burn holds steady through your forties and fifties, and what you are really feeling is the slow loss of muscle, a drop in daily movement, and shifting hormones. The encouraging part is that all three respond to action. Stay strong, keep moving, eat well, and use targeted support like magnesium, omega-3 and a quality multivitamin to back up your energy and protect your muscle as the years go on.