In order to lose weight, you must eat at a caloric deficit (consume less calories than you burn). In other words, over a period of time, say, 3 months to reach a weight goal, most days should be focused on eating less calories than your body burns for energy. This puts you at a negative energy balance.
A negative energy balance promotes the catabolism of your own tissues. In other words, your body begins to break down its own tissues to be used for energy. Sounds great right? Well, what most people don't realize is that although you are losing adipose tissue (body fat) during times of catabolism, you will lose some muscle mass as well (fat-free mass).
A study in Advances in Nutrition looked at how catabolism works and how our dietary choices affect what tissues are being catabolized:
Sustained periods of negative energy balance decrease body mass due to losses of both fat and skeletal muscle mass. Decreases in skeletal muscle mass are associated with a myriad of negative consequences, including suppressed basal metabolic rate, decreased protein turnover, decreased physical performance, and increased risk of injury. Decreases in skeletal muscle mass in response to negative energy balance are due to imbalanced rates of muscle protein synthesis and degradation. However, the underlying physiological mechanisms contributing to the loss of skeletal muscle during energy deprivation are not well described. Recent studies have demonstrated that consuming dietary protein at levels above the current recommended dietary allowance (0.8 g·kg−1·d−1) may attenuate the loss of skeletal muscle mass by affecting the intracellular regulation of muscle anabolism and proteolysis. However, the specific mechanism by which increased dietary protein spares skeletal muscle through enhanced molecular control of muscle protein metabolism has not been elucidated. This article reviews the available literature related to the effects of negative energy balance on skeletal muscle mass, highlighting investigations that assessed the influence of varying levels of dietary protein on skeletal muscle protein metabolism. Further, the molecular mechanisms that may contribute to the regulation of skeletal muscle mass in response to negative energy balance and alterations in dietary protein level are described.
Sustained periods of negative energy balance decrease body mass due to losses of both fat and skeletal muscle mass
Mentioned above. If you eat less than you burn, your body will begin to look inward for energy. This is not a perfect system. Ideally, your body would burn only adipose tissue and leave your muscles alone, but sadly, that is not the case.
In general, the proportion of body mass loss at the recommended dietary allowance of protein (0.8 g·kg−1·d−1) is: ~75% adipose tissue and ~25% fat-free mass
So, if you were to consume protein at a "standard" dietary recommendation of 0.8g/kgxday (so that would be 65g protein for a 180lb person), then you will lose approximately 75% adipose to 25% muscle. In other words, every 10lbs you lose, 7.5lbs of that would be body fat and 2.5lbs would be muscle.
Consuming a high-protein diet may contribute to the regulation of muscle mass by maintaining whole-body protein turnover in response to either acute or prolonged periods of negative energy balance
Researchers have determined that consuming a diet higher in protein than the recommended 0.8g/kgxday can actually help to tip those numbers towards preserving even more muscle mass when at an overall caloric deficit.
A high protein diet during most studies was 1.5 g·kg−1·d−1, nearly twice the recommended dietary allowance of 0.8 g·kg−1·d−1
We're still not talking about crazy-high amounts of protein here, that is still only 123g of protein for a 180lb person.
Leucine-containing food products during exercise stimulate the mTORC1 pathway, increasing muscle protein synthesis and decreased whole-body proteolysis
So, foods containing the branch chain amino acid (BCAA) leucine have shown positive effects of further maintaining muscle mass, so make sure if you are eating at a caloric deficit, to make sure every one of those calories that you do ingest count, by loading up on leucine sources.
Recommended leucine intake is currently 14 mg·kg−1·d−1, but the amount required to maximize the stimulation of muscle anabolic intracellular signaling may be at least 40–65 mg·kg−1·d−1, and even up to 7–12 g·d−1 to contribute to the preservation of muscle mass during stressors such as energy restriction
You can use dietary means to try and reverse some of the effects of muscle breakdown during weight loss. I would also recommend adding resistance training to target all areas of the body to "signal" the body, letting it know that your muscles are being used and are needed. This will also help not only preserve muscle, but to preserve strength as well, as your goal of losing weight is pursued.