Hormonal Nutrition

Discovery paper confirming leptin as the primary adiposity signal from white adipose tissue to the hypothalamus. Leptin levels are proportional to fat mass and suppress appetite while increasing energy expenditure. Critically, exogenous leptin restores body weight in leptin-deficient (ob/ob) mice but fails in high-fat-diet-induced obesity, demonstrating central leptin resistance — the hypothalamic inability to sense leptin signalling despite elevated circulating levels — as the key mechanism in diet-induced obesity.

Review of cellular mechanisms of leptin signalling and resistance. Leptin resistance develops via: (1) impaired blood-brain barrier leptin transport; (2) SOCS3-mediated intracellular signalling suppression triggered by chronic hyperleptinaemia from overfeeding; (3) endoplasmic reticulum stress in hypothalamic neurons (caused by saturated fat and fructose). Dietary strategies improving leptin sensitivity include caloric restriction, omega-3 supplementation, fibre-rich diets, and time-restricted eating.

Demonstrated that even modest fat loss (10%) during caloric restriction drops leptin disproportionately by ~50%, triggering powerful compensatory adaptations: increased hunger, reduced metabolic rate, and elevated NPY/AgRP (appetite-stimulating) neurons. This explains why dieters feel hungrier and their metabolism slows beyond what is explained by weight loss alone — a phenomenon known as adaptive thermogenesis that plateaus diet-induced weight loss and demands diet breaks or maintenance phases.

Comprehensive review of ghrelin as the only known hunger-stimulating (orexigenic) gut hormone. Ghrelin rises sharply before meals and falls after eating. Chronic dieting elevates fasting ghrelin 24–30% above baseline even after weight stabilisation — a hormonal drive to regain weight that persists for months to years post-diet. Exercise acutely suppresses ghrelin during exertion, providing an appetite-management benefit beyond energy expenditure.

Meta-analysis of 34 RCTs comparing subjective fullness across high vs standard protein diets. High-protein intakes (≥25% of energy) consistently produced greater postmeal satiety scores and reduced ghrelin compared to isocaloric high-carbohydrate or high-fat intakes. The satiety hierarchy across macronutrients is: protein > fibre-rich carbohydrates > fats > refined carbohydrates. This underpins the recommendation to anchor each meal with ≥30g protein to maximise both muscle anabolism and appetite control.

Reviewed gut hormone responses (GLP-1, PYY, ghrelin) to different meal compositions. High-protein, high-fibre meals produced the largest postprandial GLP-1 and PYY (satiety hormone) responses and the greatest ghrelin suppression. Fermentable fibres (FOS, inulin, resistant starch) additionally stimulate colonic GLP-1 via SCFA-mediated L-cell activation, providing a mechanistic basis for recommending fibre-rich whole foods over supplements for appetite management.

Iodine is the essential micronutrient for thyroid hormone (T3/T4) synthesis. Despite food fortification, iodine insufficiency remains common in athletes avoiding dairy and seafood. Low T3 reduces resting metabolic rate, impairs fat oxidation, and causes fatigue — symptoms often misattributed to overtraining. Recommended intake is 150 mcg/day (250 mcg in pregnancy). Key dietary sources: seaweed, dairy, seafood, iodised salt. Excessive uncooked cruciferous vegetables contain goitrogens that competitively inhibit iodine uptake.

Selenium-containing deiodinases are essential for converting inactive T4 to active T3. Selenium deficiency (common in areas with depleted soils) reduces deiodinase activity, raising T4 while lowering T3 — a pattern clinically identical to early hypothyroidism. RDA is 55 mcg/day; one Brazil nut provides ~70–90 mcg. Selenium supplementation (200 mcg/day) in autoimmune thyroiditis reduced thyroid peroxidase antibody titres by 40% in three independent RCTs.

RCT of 500–800 kcal/day very-low-calorie diets in obese subjects. After 17 weeks, total T3 dropped 27% and free T3 fell 24% with no change in TSH — the classic "low T3 syndrome" of dietary restriction. Resting metabolic rate fell 15%, of which 30–40% was attributable to reduced T3 beyond lean mass loss. Diet breaks (2-week maintenance phases) restored T3 to baseline, supporting the use of diet breaks in extended fat-loss protocols.

RCT (n=121) comparing calorie-restriction with/without carbohydrate restriction on 3-day 24-hour urinary cortisol. Calorie restriction alone elevated cortisol 18% at 3 weeks, with carbohydrate restriction adding a further increase. Elevated cortisol increases muscle protein catabolism, promotes visceral fat deposition, impairs immune function, and worsens sleep quality. Findings support maintaining adequate calorie intake, minimising unnecessary food restriction, and including higher-carbohydrate days to attenuate HPA axis activation during fat-loss phases.

Demonstrated that high-cortisol women consumed 62% more calories from a snack buffet after a laboratory stressor than low-cortisol women. Cortisol activates mu-opioid reward pathways in the brain, making hyper-palatable foods more rewarding under stress. For dieters, chronic cortisol elevation from aggressive restriction creates a neurobiological drive to binge on calorie-dense foods — mechanistically explaining why very-low-calorie diets have poor adherence and high relapse rates.