Bulking and cutting: Is it safe for your metabolism?

If you’ve ever spent any time online trying to figure out the best way to build muscle or get in shape, you’ve probably come across a celebrity, athlete or fitness influencer somewhere advocating for the “bulking and cutting” method.

While this method certainly isn’t new – indeed, it first became a phenomenon in the 1960s thanks to bodybuilders – it continues to be popular, even among amateurs, because many claim it’s the best dietary method for efficiently building muscle while also losing fat. But this could actually be harmful, especially if followed for a long period of time.

Bulking and cutting involves two distinct phases, which can lead to significant fluctuations in weight – at least for bodybuilders.

During the bulking phase, you consume a calorie surplus, usually with a diet high in both protein and carbohydrates. This is supposed to promote an “anabolic” (building) state, where you can build new muscle tissue and grow in size. For bodybuilders, this phase normally takes place during the off-season, when they aren’t competing. But many people who aren’t bodybuilders may choose to bulk during the colder months.

Bulking also tends to lead to an increase in fat, which is why the cutting phase is needed afterwards. This phase involves eating in a calorie deficit in order to promote a “catabolic” (breakdown) state which will lead to fat loss and enhanced muscle definition.

On average, a bulk is typically 4-6 weeks and a cut is typically longer, around 6-8 weeks.

Increasingly, there are concerns online that bulk and cut cycles can “break” your metabolism. Indeed, this concern does have some element of truth – though it isn’t quite that straightforward.

Research does show that extreme weight loss – losing more than 28kg over ten to 23 weeks – can lower your resting metabolic rate (the number of calories your body burns before doing any sort of physical activity), at least in obese study participants. However, it’s currently uncertain whether or not this change remains permanent. This is why gradual weight loss is better, as research shows it preserves your resting metabolic rate better.

In addition, the more weight gained during a bulk, the more likely there is to be an increase in fat, too. More body fat means less insulin sensitivity. With time, this could lead to type 2 diabetes. And, ironically, this can make it more difficult to build muscle, as insulin needs to work properly in order to do so. But, at least in the short term, research shows that two weeks of overeating does not appear to blunt your ability to build muscle, even in people who are overweight.

Bulking phases can also lead to an increase in the number of fat cells, more so on a severe bulk. While dieting can reduce the fat actually stored in these cells, it doesn’t reduce the number of fat cells we have. This may make it easier to gain weight in the future.

The relentless focus on nutrition with bulking and cutting could also result in disordered eating. Indeed, in a 2019 study of 348 female bodybuilders, 47% were identified as having an eating disorder, while nearly half had used methods such as purging to control their weight. Whether a similar relationship exists in men has not been established.

Ultimately, severe weight fluctuations are associated with greater risk of poor health and metabolic syndrome (the combination of diabetes, obesity and high blood pressure). So if you are planning to try bulking and cutting for whatever reason, it’s important to do it safely and without extremes to avoid risk of harm.

Some people online advocate for the “dirty bulking” method to achieve rapid weight (and muscle) gain. This is done by consuming large amounts of calories, with no foods off limits. Some people may also use high calorie shakes and “mass gainers” (high calorie protein shakes) during this period.

But eating too many processed foods can lead to an increase in cholesterol and blood sugar levels, which may lead to cardiovascular disease and diabetes over time. Even as little as four weeks eating an unhealthy, high-calorie diet can see young, healthy adults begin to show changes in their body that are associated with type 2 diabetes. Dirty bulking can also lead to a potential increase in liver fat, which can ultimately lead to liver damage in the form of non-alcoholic fatty liver disease.

There’s no evidence that a dirty bulk is any better than a clean bulk for building muscle. But given the potential harms that may come from an unhealthy, high-calorie diet – even just for a couple of muscles – it’s probably best avoided.

During the cutting phase, it’s important to avoid using extreme methods such as supplements that promise to help you shed weight. Not only is there no evidence these work, some have even been associated with causing liver toxicity.

While bulking and cutting may be a popular method, there’s little evidence to suggest it’s superior for building muscle or losing fat over other methods. But if you really want to try it, it’s best to do this in a “clean” way. During the bulking phase, avoid processed, high fat and high sugar foods. Focus on eating nutrient-dense foods, and only increase calories moderately (by around 500 calories daily) to avoid excess weight gain.

When cutting, reduce calories moderately with the aim of only losing 0.5% to 1% of your body weight per week – so 0.4kg to 0.8kg per week for a person who is 80kg. Not only is this safer, it will still help you achieve your desired fat loss without muscle loss.

Of course, you may be able to gain muscle without gaining any fat, by only moderately increasing the amount of calories you eat daily. This will make it possible to gain muscle slowly over time, while minimising fat gain.