Given the above point you might ask yourself why you should lift weights while dieting. For starters, the health benefits associated with lifting weights go well beyond fat loss and muscle gain. The biggest thing is a strong body tends to be a healthy body. Along those same lines, I buy into the old saying of, “you don’t use it you lose it.” Resistance training helps build and maintain strength and mobility and carrying these things with you as you age can never hurt. Strength training has also been shown to increase bone mineral density. I don’t know about you but I’m not a fan of getting old and breaking a hip. Resistance training has also been shown to prompt cardiovascular improvements, believe it or not. Increased insulin sensitivity, improved blood pressure, improved blood lipid profiles… the list goes on and on. Resistance training is good for you.
In relation to my point though I want to restate the original question: Why should I lift weights while dieting? For starters, complete novices who have never touched a weight before as well as people who are carrying around a lot of extra fat benefit from the ability to gain muscle and lose fat simultaneously. This depends a good bit on genetics too, but in general, simultaneous muscle gain and fat loss (hereafter referred to as recomposition) is next to impossible to achieve outside the realm of novices, overweight individuals and drug users. The reason being, keeping this simplistic, is that muscle building (hypertrophy) is a very intensive process, energetically speaking. Put differently, you need a surplus of calories (more energy in than out) to facilitate recovery and growth of new muscle. While dieting for fat loss, you don’t have adequate calories to maintain your current tissue (hence the loss in weight accompanying a caloric deficit). So adding something as metabolically active as muscle tissue probably isn’t going to happen in the face of an energy deficit. As I’m typing this it seems a bit wordy but suffice it to say, unless you're fat and/or untrained… don’t expect to gain an appreciable amount of muscle while dieting.
This probably makes you more eager to have the question answered. If I’m not going to be adding muscle why the heck should I waste my time resistance training when I can spend my time doing cardio? For starters, a weight lifting routine can be cardiovascular, but that’s a different subject for a different time. To my point, a major reason people aren’t content when they actually reach their goal weight is due to the simple fact that they didn’t lift weights while dieting or they lifted weights using screwy programming/parameters.
Including an appropriately structured weight lifting routine in your diet program gives your body a reason to hold on to as much muscle as possible. Without it, chances are good as you get closer to your goal weight that a higher proportion of weight lost will be from muscle rather than fat. Remember, there is a huge difference between losing fat and losing weight. Unless you’re interested in being a lighter, still soft version of your former self, I suggest you listen to this point.
On the same token:
Good nutrition = A
Resistance training = B
Cardio = C
Total Health and Good Physique = D
A+B+C=D
Take A, B, or C out of the equation and you are operating sub-optimally and giving up unique benefits associated only with the dropped variable.
Losing weight is as simple as creating a deficit. This deficit can come solely from diet, it can come solely from cardio exercise, it can come solely from weight training.
But that's assuming weight loss is the only thing that matters to you. In most, if not all cases things like appearance, health and performance matter more than the simplistic number on the scale. If that's the case, it's going to take a balanced approach utilizing all of the variables. Often times resistance training is the first variable dropped or worst, the variable that’s not even considered.