Bulking and cutting are two vital parts of bodybuilding, whilst many fitness experts doubt whether the average man/woman on the street needs to bother with this cycle, the use of both for bodybuilding is undeniable.
So What is Bulking?
Basically bulking is the process of gaining muscle through exercise and an increase in calories, to put you into a calorie surplus.
To build muscle you require the necessary calories, and when bulking it is important that this is done by increasing calories rather than through lowering activity.
Other benefits of increasing calories (particularly from fat and protein) is that you will get an increase in Testosterone and Growth Hormones which are responsible for muscle protein synthesis.
High levels of protein are also required for muscle protein synthesis, which is why protein is so often the number one macronutrient for bulking.
There are two ways that you can bulk, there's the traditional ‘Bro Bodybuilder‘ way which involves absolutely no thought process other than see food, eat food, enjoy the strength and hypertrophy gains, whilst worrying about the gain in body fat at a later date.
Then there is the non-traditional, scientific method (guess which one works best?). This method concentrates on adding calories in a way that leads to maximal muscle gains with minimal fat gains.
This method does take a long time to succeed though, so if you're in a hurry the bro method would suit you.
So the scientific way to bulk is to treat it like a cut, in other words measure your calories, concentrate on hitting your macro targets (the desired percentage of protein, fat, and carbohydrates).
The main difference is that add calories slowly on a weekly basis, whilst keeping an eye on how your body is adapting to the bulk.
Did you gain weight too quickly over the last 3 weeks? Then consider lowering the calories slightly, or changing the protein to carb ratio. This will leave you with more control over fat gain, and will make cutting a lot easier.
So What is Cutting?
Cutting is lowering your calories to the point where you are now in a calorie deficit (less calories consumed per day than burned).
Putting yourself in a deficit forces the body to use stored energy (fat) to address the balance – like how you would have to dip into your savings if you spent more money than you earned.
Essentially cutting is just another word for dieting, and as such you have probably heard of hundreds of different approaches (carb-free, fat-free, zone diet, Mediterranean diet). All of them will have one thing in common though, a reduction in calories that leads to fat loss.
As with bulking the most successful way to cut is to lower calories slowly over a long time, rather than dropping your calories by 1,000 within a day.
Again, high protein is the best way to go as it will enable you to maintain your lean body mass (muscle) whilst reducing body fat .
How do they work together?
For non bodybuilders the bulk and the cut might not really have anything to do with each other, most people decide that they want to lose weight and that's all.
So they would go on a cut/diet until they were satisfied and then look to maintain that size. Or if you've been a skinny guy all your life you probably won't need to be cutting any time in the future.
But, if you are trying to make a long lasting change – i.e. you are about 80kg but want to be 100kg with low body fat – then you will almost certainly require both a bulk and a cut. This is what bodybuilders require too.
The idea is that a well controlled bulk will build as much muscle as possible without gaining too much body fat. That means that your cutting phase will be a lot easier to manage and can possibly lead to you getting leaner than you planned.
So when should you cut?
The short answer is when you've reached your target muscle-wise, see a good bulk should be planned out well in advance.
Have a target weight, maybe some strength goals, and some size goals. You can also have a contingency plan for if you do gain too much body fat.
Working out your Lean Body Mass (bodyweight minus body fat) is the best way to do this – though it is only an estimate.
Once you have reached this target you can either maintain for a bit, or start to lower the calories back down again. If you have done this correctly you should be able to hit your new target (low body fat percentage) within a couple months. Obviously this is dependent on starting weight/physique.
 Pasiakos, S., Cao, J., Margolis, L., Sauter, E., Whigham, L., McClung, J., Rood, J., Carbone, J., Combs Jr., G., Young, A. 2013. Effects of high-protein diets on fat-free mass and muscle protein synthesis following weight loss: A randomised controlled trial. The FASEB Journal 27(9): 3837-3847