18%
minimum body fat for regular ovulation
<80 cm
waist circumference target for metabolic health
BMI
cannot distinguish fat from muscle — use metabolic markers
500 kcal
maximum safe daily deficit during fertility treatment
The Mechanisms
How body composition affects reproductive hormones — in both directions.
Body Fat, Oestrogen & Ovulation
- →Adipose (fat) tissue is an endocrine organ — it produces and metabolises oestrogen via the enzyme aromatase. Both insufficient and excess body fat disrupt oestrogen balance, but through opposite mechanisms.
- →Underfuelling and low body fat (<18% in women) impairs the hypothalamic-pituitary-ovarian axis through reduced leptin signalling. Leptin, produced by fat cells, is a permissive signal for gonadotropin-releasing hormone (GnRH) pulsatility. When leptin falls below threshold (as in low body fat or caloric restriction), GnRH pulsatility slows, follicle-stimulating hormone (FSH) and luteinising hormone (LH) decline, and ovulation ceases — a condition called hypothalamic amenorrhoea.
- →Excess body fat (particularly visceral fat) drives aromatase overactivity, converting androgens to oestrogen in peripheral tissue. This creates oestrogen excess and progesterone relative deficiency, disrupting the follicular-luteal cycle and impairing implantation.
Why BMI Is a Poor Fertility Predictor
- →Body mass index (BMI) measures weight relative to height — it cannot distinguish between muscle mass, fat mass, bone density, or fat distribution. Two women with identical BMI can have vastly different body compositions and fertility profiles.
- →A woman with BMI 22 but low muscle mass and high visceral fat (a pattern called 'normal weight obesity' or 'TOFI — thin outside, fat inside') may have significant insulin resistance and inflammatory burden, impairing fertility despite a 'normal' BMI.
- →Waist circumference and waist-to-hip ratio are more clinically meaningful than BMI for fertility assessment. A waist circumference >80 cm in women is associated with insulin resistance and elevated inflammatory markers regardless of BMI.
- →The most fertility-relevant metric is not weight — it is metabolic health: fasting insulin, HbA1c, sex hormone-binding globulin (SHBG), and inflammatory markers tell a more complete story than the scale.
Underfuelling, Restriction & Fertility
- →Chronic caloric restriction — even without reaching clinical underweight — suppresses the reproductive axis. The body interprets energy deficit as an unsafe environment for reproduction and downregulates fertility as a survival mechanism.
- →Relative Energy Deficiency in Sport (RED-S) describes a state of insufficient energy availability relative to exercise load. It is common in athletes and active women and is characterised by menstrual irregularity, low bone density, and impaired fertility — even at normal or above-normal BMI.
- →Disordered eating patterns (restriction, skipping meals, fear of specific foods) activate the HPA axis and elevate cortisol, which directly suppresses gonadotropin-releasing hormone (GnRH) and progesterone. Recovery requires not just caloric adequacy but psychological safety around food.
- →If you have a history of restriction or disordered eating, work with a practitioner experienced in both fertility and eating disorders — rapid weight gain protocols are not appropriate and can worsen the underlying pattern.
Practical Approach
1
Assess metabolic health, not just weight
Request fasting insulin, HbA1c, SHBG, and waist circumference measurement. These markers reveal metabolic status that BMI misses.
2
Prioritise protein adequacy
1.2–1.6 g/kg body weight daily supports muscle mass, stabilises blood glucose, and reduces appetite dysregulation. Protein adequacy is more important than caloric restriction.
3
Resistance training over cardio for body composition
Resistance training builds metabolically active muscle, improves insulin sensitivity, and supports progesterone production. Excessive cardio without adequate fuelling can worsen hypothalamic suppression.
4
Avoid extreme caloric restriction during fertility treatment
A modest deficit of 250–500 kcal/day is the maximum safe range during fertility treatment. Aggressive restriction impairs oocyte quality and endometrial receptivity.
5
Address visceral fat through insulin sensitivity
Visceral fat responds best to reduced refined carbohydrates, increased fibre, resistance training, and improved sleep — not caloric restriction alone.
Your Action Plan
Body Composition Checklist
0/14 completed
Assessment
Request fasting insulin and HbA1c — not just weight or BMI
Measure waist circumference — target <80 cm
Check sex hormone-binding globulin (SHBG) — low SHBG indicates insulin resistance
Track menstrual cycle regularity as a marker of energy availability
Discuss body composition concerns with your practitioner before making changes
Nutrition Approach
Aim for 1.2–1.6 g protein per kg body weight daily
Do not skip meals — three balanced meals daily stabilises hormones
Prioritise whole food carbohydrates: legumes, sweet potato, oats, fruit
Avoid ultra-processed foods and refined sugars — these drive visceral fat regardless of total calories
If restricting, limit to a maximum 250–500 kcal/day deficit during fertility treatment
Movement
Include 2–3 resistance training sessions per week
Walk 8,000–10,000 steps daily
Avoid excessive cardio if menstrual cycle is irregular — this may indicate underfuelling
Post-meal walking (10–15 min) improves insulin sensitivity without adding stress load