You turned 35, then 40, and your clothes don't fit like they used to. The first explanation you hear from friends, colleagues, and even some websites is always the same: "it's your metabolism, it's slowed down."
I'll be honest with you: for years I believed that too. But when I looked at the data, the story was quite different. And more interesting, because what really counts for your energy expenditure is, for the most part, something you can influence.
In this article you'll see what science says about a slow metabolism, when it really can be a health issue, and what the real levers are for speeding up your metabolism sustainably. With numbers, no miracle promises. And there's a comparison further down that surprised me: the habit that counts the most is far less glamorous than the gym.
Does Metabolism Slow Down in Your 30s? The Data Says No
The idea that metabolism "drops" after 30 is so deeply ingrained that it seems like a fact. But in 2021, a study published in Science put this to the test with more than 6,600 people, from one week old to 95 years old, in 29 countries. They measured energy expenditure using the doubly labelled water method, the gold standard for measuring energy consumption in real life.
After adjusting for body size and muscle mass, energy expenditure remained stable from 20 to 60 years old. The decline only appears after 60, at about 0.7% per year.
At 90-something years old, daily expenditure is about 26% lower than in middle age. But what about 35? 45? According to this data, your engine is practically the same as it was at 25.
So why do you gain weight? Often for everyday reasons: less movement without us realising it, more hours sitting, changing routines. We've been there.
What Really Determines Your Resting Expenditure
Here comes a surprise. When you think of metabolism, you think of muscle. But, according to a review published in Frontiers in Physiology, fat-free mass (muscle, organs, bone and water) explains between 53% and 88% of the differences in resting expenditure between people, usually no more than 75%.
And within that fat-free mass, the brain, heart, liver and kidneys are only 5 to 6% of body weight, but account for over 80% of resting expenditure. It's like a car engine: small, but it's what burns the fuel.
Muscle also counts, of course. Only its effect on resting expenditure is more modest than people think. Let's look at the numbers.
How Much Does Each Lever Count? The Missing Table
I've brought together the numbers from the studies analysed in a single table. It's the clearest way to see how energy expenditure is worth spending:
| What influences expenditure | Observed dimension | Publication |
|---|---|---|
| Age (20–60 years) | Stable, after adjusting for body size | Science, 2021 (6,600+ people) |
| Age (over 60 years) | About −0.7% per year | Science, 2021 |
| Strength training | About +96 kcal/day at rest (95% CI: 45 to 147) | Journal of Sports Sciences, 2020 (18 studies) |
| Aerobic exercise | No significant effect on resting expenditure (+82 kcal, 95% CI: −58 to 221) | Journal of Sports Sciences, 2020 |
| Spending less time sitting (about 2.5 hours/day) | About 350 kcal/day difference (sample of 20 people) | Science, 2005 |
| Digesting what you eat | About 10% of total expenditure. Protein: 20–30% of your calories; carbohydrates: 5–10%; fat: 0–3% | Nutrition & Metabolism, 2004 |
Notice what this says. Strength training increases resting expenditure by about 100 kcal per day, the equivalent of a large apple. Not bad, but far from the "turbo engine" that is sold. The time you spend sitting made a difference of about 350 kcal daily. With the caveat that it's only 20 people.
The Silent Colossus: the Movement That Is Not Exercise
This is called NEAT (non-exercise activity thermogenesis). It's everything you do that isn't training: walking to the café, climbing stairs, standing up, tidying the house, moving around while talking on the phone.
In a study published in Science in 2005, 10 thin people and 10 with mild obesity wore sensors for 10 days. The latter spent, on average, 2.5 hours more sitting per day, and the difference in expenditure was around 350 kcal daily.
More importantly: this doesn't require heroic willpower. It requires small choices, repeated:
- Climbing the stairs at the metro station instead of using the escalator
- Getting off one stop earlier and walking the rest of the way
- Talking on the phone while walking, instead of sitting
- Getting up every hour if you work sitting down
Here is the most important thing: a healthy metabolism is not built in an hour at the gym, but in the other 23.
Muscle: the Guardian of Your Expenditure
If you want to really speed up your metabolism, it's best to start with muscle, but with realistic expectations. If strength only adds about 96 kcal per day, is it worth it? It is, and very much so, for another reason: it protects what you have. When you lose weight, your body can also lose muscle. And less muscle mass means less expenditure, and more difficulty maintaining your weight.
That's why the DGS, based on the WHO, recommends strength training at least 2 days a week. It doesn't have to be heavy or sophisticated: resistance bands at home, your own body weight, or a gym near you.
And protein helps maintain muscle mass. It also has a bonus: according to an article in Nutrition & Metabolism, the body spends 20 to 30% of the calories from protein just digesting it, compared to 5–10% for carbohydrates and up to 3% for fat. In plain English: eggs for breakfast, fish, chicken or chickpeas on your plate make sense for more than one reason.
Sleeping Little Also Costs
A study published in Annals of Internal Medicine in 2010 tested this: ten adults did two 14-day phases with the same moderate calorie restriction, one sleeping 8.5 hours and the other 5.5. With less sleep, they lost less fat (0.6 kg versus 1.4 kg) and more lean mass (2.4 kg versus 1.5 kg). Small sample, clear message: short sleep pushes the loss in the wrong direction.
If you want to protect your metabolism, protect your nights.
What If It Really Is the Thyroid?
There are cases where "slow metabolism" has a medical cause. The best known is hypothyroidism, in which the thyroid produces fewer hormones. According to CUF, the signs include fatigue, weight gain, sensitivity to cold, dry skin and hair, and muscle pain. They appear slowly and go unnoticed.
The diagnosis is made with blood tests (TSH, T3 and T4) and the treatment, when necessary, is done with tablets that replace the hormone, always accompanied by a doctor. CUF also warns to avoid supplements rich in iodine without medical indication.
If you see yourself in these symptoms, don't guess: book an appointment with your family doctor. No supplement, not even this one, replaces a diagnosis.
"Did I Ruin My Metabolism with Diets?"
This is a question many people ask in silence, after years of quick diets. The most cited evidence comes from a study published in Obesity in 2016 with 14 contestants from The Biggest Loser, who lost an average of 58 kg in 30 weeks. Six years later, their resting expenditure was about 500 kcal/day below expected, and 13 of the 14 had regained weight.
But beware: it's an extreme case, with very rapid weight loss and only 14 people. The practical lesson is another: gradual losses and sustainable habits are the most sensible approach. Losing weight moderately, while maintaining strength training, protein, and sleep, is what best supports your pace.
Where a Supplement Fits In
To maintain a healthy metabolism, habits come first. If you already have them, a supplement can provide extra support. But that's all: support. What research shows, for example in a 2009 meta-analysis on green tea and caffeine, is an effect in the order of −1.31 kg on average, over several months of the study.
Kyoslim, from PrimeCell Nutrition, brings together five ingredients (EGCG from green tea, guarana, L-carnitine, raspberry extract and niacin) in one capsule per day, formulated to support metabolism and healthy weight management when combined with balanced nutrition and exercise. It is manufactured in the European Union, with DGAV authorization, is vegan and has a 180-day warranty, so you have nothing to lose by trying it. You can learn about the formulation here: Kyoslim.
If you are interested in the habits side, also check out our article on how to lose fat with a weekly plan.
Frequently Asked Questions
How can I speed up my metabolism?
There is no single trick. The levers with the most evidence are moving more throughout the day, strength training 2 times a week, protein in meals, sleeping well and avoiding very drastic diets.
Does metabolism slow down with age?
According to the study published in Science, only after 60 years old, at about 0.7% per year. From 20 to 60 it remained stable.
How do I know if I have a slow metabolism?
You can't guess: only a doctor, with tests and evaluation, can say if there is a cause like the thyroid. Persistent fatigue and unexplained weight gain are reasons to book an appointment.
Do supplements speed up metabolism?
Some ingredients, like green tea with caffeine, have a modest effect in studies. They work as support for good habits, never as a substitute. Results may vary.
The Essentials To Take With You
A healthy metabolism is not built with tricks. Yours probably isn't "broken". The data shows that energy expenditure is stable for decades, and that the real levers are daily movement, muscle, diet and sleep. Choose one this week, maybe get off one stop earlier every day, and give it time. The rest comes by addition.
Sources
- Pontzer H. et al. (2021). Daily energy expenditure through the human life course. Science. Summary at Duke
- Bosy-Westphal A. et al. (2013). Issues in characterizing resting energy expenditure in obesity and after weight loss. Frontiers in Physiology. View article
- MacKenzie K. et al. (2020). The effect of exercise interventions on resting metabolic rate: a systematic review and meta-analysis. Journal of Sports Sciences. View article
- Levine J. et al. (2005). Interindividual variation in posture allocation: possible role in human obesity. Science. View article
- Westerterp K. (2004). Diet induced thermogenesis. Nutrition & Metabolism
- Nedeltcheva A. et al. (2010). Insufficient sleep undermines dietary efforts to reduce adiposity. Annals of Internal Medicine
- Fothergill E. et al. (2016). Persistent metabolic adaptation 6 years after "The Biggest Loser" competition. Obesity. View article
- Hursel R. et al. (2009). Meta-analysis on green tea, catechins and caffeine and body weight. International Journal of Obesity
- CUF. Thyroid and weight loss
Important Health Information
Disclaimer: This article is for informational purposes only and does not constitute medical advice. Kyoslim is a dietary supplement designed to support healthy weight management when combined with balanced nutrition and regular exercise. Results may vary by individual. Consult your healthcare professional before starting any new supplementation regimen, especially if you have existing health conditions or take medications.
The statements in this article have not been evaluated by regulatory authorities. This product is not intended to diagnose, treat, cure or prevent any disease.
