People don’t just want to lose weight, but rather lose fat. In most cases because if you lose weight quickly, it’s mostly water weight. When you step on a scale, it measures not just body fat and muscle, but also bones, organs (like your lungs, heart, and liver), bodily fluids, and waste inside your digestive system you haven't yet eliminated. Don’t rely on the scale as your only measure of success. A leaner (less body fat), more toned, well-nourished and hydrated body will weigh more than one that's thinner but flabby, undernourished, and dehydrated.
Losing fat requires eating fewer calories than you burn. When you're at a calorie deficit, it's hard to fuel your muscle, that’s why strength training is the best way to boost your metabolism, as it helps to build and maintain muscle. A beginner in strength training will gain muscle faster in what’s usually known as ‘Newbie Gains’.
Lifting weights stresses and breaks down your muscles, which responds by building more muscle cells to compensate for the new loads being placed upon them. These new cells require energy, and burn three times more calories than fat cells, even at rest.
Fat loss takes time, and muscle gain typically takes even longer. Exercise is important, but fat loss happens primarily in the kitchen. When you're talking metabolism, the key is protein. You need to eat enough protein because they work to spike your metabolism in several ways.