Dietary Fat Intake for Hormone Health and Body Recomposition

To maintain endocrine function and support body recomposition, evidence recommends establishing dietary fat intake between 20% and 35% of total daily energy intake, which corresponds to approximately 0.5 to 1.0 g/kg of body weight per day for active individuals [14, 17, 19, 22]. The primary caveat is that during severe, short-term hypocaloric phases—such as competitive physique preparation—fat intake is occasionally compressed to 10% to 25% of total calories to prioritize high protein targets and carbohydrate availability for training, though this low intake increases the risk of temporary androgen and reproductive hormone suppression [18, 22].

Dietary Fat and Endocrine Function

Dietary fat plays a structural and metabolic role in steroidogenesis, providing the cholesterol substrate required for the synthesis of sex hormones [17]. Dropping dietary fat intake below 20% of total daily energy intake or below 0.5 g/kg/day consistently compromises the hypothalamic-pituitary-gonadal (HPG) axis [17, 22].

A systematic review and meta-analysis of intervention trials in men demonstrated that low-fat diets significantly decrease total testosterone compared to higher-fat diets (standardised mean difference [SMD] = -0.38, P = 0.04) [4, P1]. Reductions were also observed across free testosterone (SMD = -0.37, P = 0.005), urinary testosterone (SMD = -0.38, P = 0.009), and dihydrotestosterone (DHT; SMD = -0.30, P = 0.03), while luteinizing hormone (LH) and sex hormone-binding globulin (SHBG) remained unchanged [4, P1]. In subgroup analyses of European and North American men, the reduction in total testosterone on low-fat diets was more pronounced (SMD = -0.52, P < 0.001) [4, P1]. In these trials, the weighted mean difference in fat intake between low-fat and high-fat protocols was 20.1% of total energy intake (19.5% vs. 39.6%), with an overall weighted body weight change of only -0.8 kg, indicating that the hormonal drop was primarily driven by dietary macronutrient composition rather than extreme weight loss [4, 5].

For female athletes, dropping below the 0.5 to 1.0 g/kg/day threshold similarly disrupts circulating oestrogen and progesterone concentrations and elevates the risk of menstrual dysfunction [22].

Fat Quality: Saturated vs. Polyunsaturated Fatty Acids

Beyond total fat volume, research has explored whether fat quality (the proportion of saturated, monounsaturated, and polyunsaturated fatty acids) influences circulating androgens. Over the last 50 years, population-level total testosterone concentrations in Western countries have declined by up to 1% annually, with up to two in five men aged 45 and older meeting the threshold for testosterone deficiency [3]. Epidemiological trends over this period show relatively stable saturated fatty acid (SFA) intake alongside increased monounsaturated (MUFA) and polyunsaturated fatty acid (PUFA) consumption [3].

In animal models, SFA intake correlates positively with testicular function and steroidogenic enzyme levels, whereas high PUFA intake—particularly omega-6 PUFAs—demonstrates suppressive effects [3]. However, human observational data present a more nuanced picture. In the Kuopio Ischaemic Heart Disease Risk Factor Study (KIHD), which evaluated 2,546 middle-aged men using 4-day food records and standardized fasting blood samples [3, 6, 8], initial models adjusted for age, examination year, and total energy intake indicated that SFA was positively associated with total and free testosterone, while PUFA was negatively associated [6]. Yet, after multivariable adjustment for confounding factors such as body mass index (BMI), smoking status, physical activity, and disease history, dietary fat quality metrics (SFA, MUFA, PUFA, and trans fatty acids) were no longer independently associated with serum total testosterone, free testosterone, or SHBG [6]. When testing isocaloric nutrient replacement, substituting dietary protein with SFA was associated with higher total testosterone and SHBG, but this relationship lost statistical significance after excluding participants with a history of cardiovascular disease or diabetes [6].

Fat Targets for Body Recomposition

Body recomposition—the simultaneous reduction of fat mass and accretion or maintenance of fat-free mass (FFM)—requires managing energy balance and macronutrient distribution [11, 12]. When assessing body composition, the four-compartment (4C) molecular model (tracking fat mass, total body water, total body protein, and bone mineral content) remains the gold standard, as 3-compartment methods like dual-energy X-ray absorptiometry (DXA) are sensitive to shifts in hydration and glycogen [12].

Fat loss is primarily dictated by a sustained energy deficit, where higher baseline body fat allows for more aggressive deficits, whereas leaner individuals require slower rates of weight loss (such as ≤0.5% body mass per week) to prevent lean tissue loss [12, 18]. When calories are restricted, basal muscle protein synthesis (MPS) decreases rapidly (by ~27% after 5 days and 19% after 10 days), while muscle protein breakdown (MPB) rises by day 10 [18]. To counteract this catabolic environment, high daily protein targets are required:

• Baseline hypertrophy / Maintenance: 1.6 to 2.2 g/kg/day [11].
• Hypocaloric lean mass retention: 1.8 to 2.7 g/kg/day of total body mass, or 2.3 to 3.1 g/kg of fat-free mass [11, 12, 18]. Meta-regressions in resistance-trained individuals suggest optimal ranges between 1.9 and 3.2 g/kg/day of body mass (2.5 to 4.2 g/kg FFM) during energy deficits [11].
• Empirical Recomposition Protocol: Consuming 2.5 g/kg/day of protein alongside 4-day-per-week resistance training elicited significant recomposition over 10 weeks under both an energy deficit (Δ FFM = +1.04 kg, Δ FM = -2.94 kg) and an isocaloric state (Δ FFM = +0.97 kg, Δ FM = -1.41 kg) [11].

Because high protein targets occupy a substantial share of a hypocaloric energy budget, dietary fat must be budgeted carefully. While moderate deficits of 500–600 kcal/day carry minimal risk of severe endocrine suppression in overweight individuals (and may even improve testosterone by reducing adipose aromatase activity) [17], active individuals who restrict fat below 20% of energy intake frequently face impaired satiety, fat-soluble nutrient malabsorption, and suppressed androgens [17, 19]. Even short-term 25% caloric deficits combined with resistance training have been shown to reduce total testosterone and resting energy expenditure alongside fat loss [15].

Energy Availability and the Risk of REDs

Dietary fat cannot be evaluated in isolation from total energy availability (EA). Under the Relative Energy Deficiency in Sport (REDs) framework, low energy availability (typically <30 kcal/kg FFM/day) downregulates the HPG and hypothalamic-pituitary-thyroid axes [17, 21, 23]. In national-level athletes, high prevalence of REDs is characterized by elevated cortisol, elevated cortisol-to-insulin ratios, and depressed resting metabolic rates [21].

When low energy availability is compounded by low carbohydrate intake (<3 g/kg/day or <30% of total calories), cortisol output increases, which suppresses pituitary thyroid-stimulating hormone (TSH) and inhibits peripheral conversion of T4 to active T3, shifting production toward reverse T3 (rT3) [25]. This hormonal milieu further suppresses gonadotropin-releasing hormone (GnRH) and LH pulsatility, blunting testosterone, oestradiol, and progesterone production [25]. Early mammalian ecology models established that concurrent restrictions in both carbohydrate and fat availability are particularly potent triggers for reproductive axis suppression [23].

Practical Recommendations

Based on the collective evidence across sports nutrition and endocrinology guidelines, the following ranges apply:

1. Optimal Baseline Range (Health & Recomposition): Consume 20% to 35% of total daily energy intake from dietary fat, or 0.5 to 1.0 g/kg/day [14, 19, 22]. This supports steroidogenesis, satiety, and fat-soluble vitamin absorption without crowding out required protein or performance carbohydrates [14, 19].
2. Low-Carbohydrate / High-Fat Variations: In low-carbohydrate or ketogenic contexts, dietary fat intake typically scales up to 1.5 to 2.0 g/kg/day to meet energetic requirements while maintaining adequate protein [22, P2, P4].
3. Temporary Floor (Physique Peaking / Aggressive Cuts): In contest preparation settings where protein is elevated to 1.8–2.7 g/kg/day and carbohydrates are prioritized to maintain training volume, fat may be set between 10% and 25% of total calories on a temporary basis, with the understanding that androgenic and metabolic markers are likely to decline during this phase [18].
4. General Minimum: Diets should not drop below 20% of total energy or 0.5 g/kg/day outside of short, highly monitored phases, as prolonged intake below these levels offers no performance benefit and risks systemic endocrine suppression [14, 17, 22].