Factors That Affect Your TDEE: Age, Hormones, Body Composition

Once calculated, many people treat their TDEE as a permanent value. That is a mistake: total daily energy expenditure is a dynamic figure that changes with age, hormonal status, body composition and environment. Understanding what modifies it lets you anticipate changes and adjust your nutritional strategy before the body "stops responding."

1. Age: the decline is not inevitable

BMR decreases approximately 2–3% per decade after age 30.^[1] However, the main cause is not ageing itself, but the progressive loss of muscle mass (sarcopenia) that accompanies the years without training. A 60-year-old who maintains the same muscle mass as at 35 will retain a very similar BMR.

This means strength training is not optional in adulthood: it is the most effective tool to counteract the age-related metabolic decline.^[2]

2. Sex: beyond hormones

Men have, on average, a BMR 5–10% higher than women of the same weight and height.^[1] The difference is not directly caused by testosterone, but by the greater muscle mass proportion that androgens favour. At equal lean mass, the metabolic difference between sexes shrinks significantly.

In women, the menstrual cycle introduces variations of ±100–300 kcal/day: BMR is slightly higher in the luteal phase (days 15–28) due to the thermogenic effect of progesterone.^[3]

3. Body composition: fat is not inert

Fat-Free Mass (FFM) is the main predictor of BMR.^[4] When body weight is not a reliable indicator (athletes, highly muscular individuals), the Katch-McArdle formula — which uses FFM directly — is more accurate than Mifflin-St Jeor:

• BMR = 370 + (21.6 × FFM in kg)

4. Metabolic adaptation: the body resists the deficit

Under prolonged calorie restriction, the body activates energy-saving mechanisms. This phenomenon — metabolic adaptation or adaptive thermogenesis — can reduce BMR by 10–20% beyond what muscle loss alone explains.^[5]

Practical implication: if after 12 weeks at 1,600 kcal/day you have stopped losing weight, your calculations may not be wrong — your BMR may have adapted. Counter-strategies include maintenance weeks (refeeds), moderate deficit cycles, and prioritising protein to preserve muscle.

5. Hormonal status

Hormone	Effect on metabolism
Thyroid (T3/T4)	Hypothyroidism can reduce BMR 10–30%; hyperthyroidism raises it
Cortisol (chronic)	Promotes muscle catabolism and insulin resistance
Insulin	Insulin resistance reduces metabolic efficiency
Oestrogen/progesterone	Menopause can reduce BMR 50–200 kcal/day
Testosterone	Low levels linked to less muscle mass and reduced BMR

Conclusion

TDEE is a dynamic estimate. Recalculating it every 3–4 months, or whenever weight, activity level or hormonal status changes significantly, keeps your nutritional planning accurate. Formulas are starting points: tracking your own weight trend over 2–4 weeks remains the best fine-tuning tool.

If you'd like to skip the manual calculation, our free TDEE calculator gives you your personalised number in seconds.

Scientific references

1. Speakman JR, Selman C. "Physical activity and resting metabolic rate." Proc Nutr Soc. 2003;62(3):621-634.
2. Peterson MD et al. "Resistance exercise for muscular strength in older adults." Ageing Res Rev. 2011;10(3):333-340.
3. Solomon SJ et al. "Menstrual cycle and basal metabolic rate in women." Am J Clin Nutr. 1982;36(4):611-616.
4. Cunningham JJ. "Body composition as a determinant of energy expenditure." Am J Clin Nutr. 1991;54(6):963-969.
5. Müller MJ et al. "Metabolic adaptation to caloric restriction." Am J Clin Nutr. 2015;102(4):807-819.