The principle: an energy deficit
You lose fat when you take in less energy than you use over a period of weeks. Which diet style you use for that is secondary, as long as it reliably creates the deficit and contains enough protein.
The starting point is your maintenance needs. Formulas like the one by Mifflin and St Jeor (1990) estimate resting energy expenditure from weight, height, age and sex; with an activity factor you get total needs. It remains an estimate that can be clearly off in individual cases. Your real needs only show when you compare intake and weight trend over a few weeks.
The right pace
The bigger the deficit, the faster your weight drops, but the greater the risk of losing muscle and training performance. For contest preparation of natural bodybuilders, Helms, Aragon and Fitschen (2014) recommend a weight loss of 0.5 to 1 percent of body weight per week.
In a study by Garthe and colleagues (2011) with elite athletes, the group that lost weight more slowly (about 0.7 percent per week) did better on fat-free mass and strength than the group with roughly double the pace. The leaner you already are, the more the lower end of the range is worth it.
The bulk and cut calorie calculator converts a target pace in percent per week into a daily calorie number.
Protein and training keep your muscle
In a deficit your body needs a clear reason to keep muscle. Heavy training provides it: keep the weights and exercises you built with. You can lower the volume a little if recovery suffers, but ideally not the intensity.
Protein needs tend to rise during a diet. For lean strength athletes in a deficit, Helms and colleagues (2014) give 2.3 to 3.1 g per kg of fat-free mass per day. Protein also keeps you full, which makes the deficit easier.
Measuring progress
In a cut, water retention often masks fat loss for a few days. Then your weight seems to drop suddenly. If you judge every daily value, you ride a rollercoaster. The weekly rate of your trend weight is more reliable.
- Trend weight and its change per week as a percentage of body weight
- Waist circumference, once a week under the same conditions
- Photos with the same light, the same pose and the same distance
- Strength in the main lifts: if it stays stable, that is a good sign
Hunger, everyday life and cardio
A cut rarely fails because of knowledge, but because of hunger and everyday life. High-protein meals, plenty of vegetables and other bulky foods with little energy per bite help you feel full. Fixed meal times and a rough plan for weekends keep one evening from using up the deficit of the whole week. Lack of sleep also makes the diet noticeably harder.
More movement in everyday life, such as extra steps, widens the deficit without putting much strain on your recovery from strength training. Cardio is a tool, not a must: in moderation it helps, too much of it can lower your performance in the gym. Strength training has priority.
Plateaus, adaptation and diet breaks
As your weight falls, your energy expenditure falls too: a lighter body needs less energy, and many people unconsciously move less in daily life. Trexler, Smith-Ryan and Norton (2014) describe this adaptation in their review. The deficit gets smaller over the weeks, even if you eat the same amount.
Before you cut calories, check whether the trend has really stalled for two to three weeks, and not just the scale. Some people plan diet breaks: one to two weeks at maintenance. In the MATADOR study (Byrne and colleagues 2018) with men with obesity, two-week breaks led to a more favorable outcome. Whether this carries over to lean athletes is open; as an aid to staying the course, a break can still make sense.
After the cut
When you have reached your goal, you raise calories to maintenance. In the first few days your weight usually rises a bit, because glycogen and water are refilling. That is not fat. After two to three weeks the trend shows whether you are holding your new weight.