Beyond the basics
Lesson 1 introduced the calorie deficit and the recommended 15 to 20% range below TDEE. This lesson goes a level deeper: the actual maths behind fat loss, why bigger deficits are not always better, and a practical tool, the diet break, used on longer cuts.
The rough maths of fat loss
Body fat stores roughly 7,700 kcal of energy per kilogram, based on adipose tissue being mostly fat with a small amount of water and other tissue. This is a widely used approximation, not a precise, unchanging constant, but it gives a genuinely useful way to think about the numbers.
A sustained deficit of 500 kcal a day works out to:
500 × 7 = 3,500 kcal a week ≈ 0.45 kg a week
This is exactly why Lesson 1’s recommended range (0.25 to 0.5 kg a week) lines up with a moderate 15 to 20% deficit for most people, rather than being an arbitrary number.
Why a bigger deficit is not automatically a better one
The maths above makes an aggressive deficit look tempting: cut 1,000 kcal a day and, in theory, lose roughly double the fat per week. In practice this rarely holds up, for reasons already touched on in Lesson 14: severe, prolonged restriction triggers genuine metabolic adaptation, and the risk of losing muscle alongside fat rises sharply without very deliberate protein intake and training.
There is also a practical ceiling: hunger and food-focused thinking intensify as the deficit deepens, which is exactly the kind of pressure that tends to end in a binge, followed by a longer break from the plan than a smaller, steadier deficit would ever have required.
Why it gets harder the leaner you get
The same 15 to 20% deficit feels increasingly difficult as body fat drops, for two compounding reasons. First, TDEE itself falls as body weight falls, so the same percentage deficit becomes a smaller and smaller number of actual calories to play with. Second, hunger-regulating hormones respond more strongly to a deficit at lower body fat levels, a mechanism covered in detail in the next lesson.
This is a normal, expected part of the process, not a sign that something has gone wrong. It is also the reason many structured fat-loss plans build in periodic breaks rather than one long, continuous cut.
Diet breaks: a planned pause, not giving up
A diet break is a short, deliberate period, typically one to two weeks, of eating at maintenance calories in the middle of a longer cut, before returning to the deficit. It is scheduled in advance, not triggered by a bad day.
A controlled study of obese men (Byrne et al., 2018, published in the International Journal of Obesity, known as the MATADOR study) compared continuous dieting against a schedule alternating two weeks of deficit with two weeks at maintenance. The intermittent approach produced greater fat loss over the full study period and better preserved resting energy expenditure.
Further reading
- Byrne NM, Sainsbury A, King NA, et al. (2018). Intermittent energy restriction improves weight loss efficiency in obese men: the MATADOR study. International Journal of Obesity, 42(2), 129-138.
Today’s task
Using the 7,700 kcal per kg approximation, work out roughly how many weeks your current deficit would take to lose 1 kg. No need for precision, just a rough sense of the pace you are actually on.
Check in with yourself
- Have you ever tried a much larger deficit than recommended? How long did it actually last?
- If you are several weeks into a cut, does a planned week at maintenance sound useful right now, or too soon?
- Any new takeaway from today’s lesson you want to remember?
Recalculate your target if it has been a while since your calorie needs were last worked out.