Hormones During a Cut

Fat loss isn't just a maths problem. Your hormones are in constant dialogue with your calorie intake, and during a cut they make themselves heard — through hunger, fatigue, mood, training performance, and the rate at which your body lets go of fat. Understanding the key hormones helps you manage the process intelligently.

Leptin: The Fat Loss Governor

Leptin is produced by fat cells and acts as a signal to the brain about energy availability. High leptin = adequate energy; low leptin = perceived starvation. When you cut calories, leptin drops — and it can drop substantially within days of starting a deficit, even before any significant fat is lost.

Low leptin triggers:

• Increased hunger
• Reduced metabolic rate
• Decreased energy expenditure through NEAT
• Reduced motivation for physical activity

This is the central mechanism of metabolic adaptation. Leptin levels correlate with fat mass, so leaner individuals experience more dramatic hormonal responses to cutting — another reason aggressive cuts from already-lean starting points are harder than they look.

What helps: refeed days and diet breaks at maintenance temporarily restore leptin, easing hunger and metabolic suppression. High carbohydrate intake drives the fastest leptin recovery.

Ghrelin: The Hunger Hormone

Ghrelin is produced primarily in the stomach and signals hunger to the brain. During a cut, ghrelin levels rise — and research shows they stay elevated even after reaching a lower body weight. This is a key reason weight maintenance after a cut is harder than losing the weight in the first place.

A 2011 study in The New England Journal of Medicine found that ghrelin remained elevated for at least a year following weight loss, suggesting the hormonal drive to regain weight is a long-term physiological challenge, not a short-term one.

What helps: Adequate protein and fibre blunt ghrelin's effects. Regular meal timing (eating at consistent times) can also regulate hunger signals more effectively than erratic eating patterns.

Cortisol: The Stress Response

cortisol is a catabolic stress hormone. In the short term it's useful — it helps mobilise fat stores. But chronically elevated cortisol (from large deficits, poor sleep, overtraining, or life stress) has significant downsides:

• Increased muscle catabolism
• Greater fat retention, particularly around the abdomen
• Impaired sleep
• Worsened insulin sensitivity
• Suppressed immune function

During a cut, some cortisol elevation is inevitable. The goal is to prevent it becoming chronic through excessive restriction, overtraining, or sleep neglect.

What helps: Adequate sleep (7–9 hours), controlled deficit size, deload week during training, and managing overall life stress where possible.

Testosterone (and Oestrogen)

In men, testosterone drops during a cut — often meaningfully. A 2021 review found testosterone levels fell by an average of 20–25% during aggressive cuts in resistance-trained men. Low testosterone impairs muscle protein synthesis, reduces training drive, and worsens mood.

For women, very aggressive or prolonged cuts can disrupt oestrogen and progesterone balance, potentially affecting menstrual cycle regularity — a clinical sign the body is under excessive stress.

What helps: Keeping dietary fat above 0.8g/kg bodyweight (fat is required for sex hormone production), avoiding excessively aggressive deficits, and including regular higher-calorie days or diet breaks.

Thyroid Hormones

The thyroid regulates metabolic rate. T3 (the active thyroid hormone) decreases during calorie restriction, contributing to reduced resting metabolic rate. A 2023 review found T3 levels dropped by approximately 20–30% during sustained aggressive cuts.

This is part of why metabolism slows beyond what weight loss alone explains. Fortunately, thyroid hormones tend to recover relatively quickly during maintenance phases.

Key Takeaways

• Leptin drops during a cut, driving hunger and metabolic slowdown — refeed days partially restore it
• Ghrelin (hunger hormone) rises during a cut and stays elevated long after weight loss, making maintenance hard
• Chronically elevated cortisol causes muscle catabolism and abdominal fat retention
• Testosterone falls during cutting, particularly with aggressive deficits — keep dietary fat adequate
• Women may experience menstrual disruption if cuts are too aggressive
• Thyroid hormone drops during restriction, contributing to metabolic adaptation