How to avoid the Thin-Fat body type
People with "thin-fat" (also known as "skinny-fat") body types may not appear obese from the outside, and may be at a healthy weight, as defined by BMI. However, because this is because they have so little muscle that their fat simply offsets the weight and keeps them from being defined as "underweight", they may be unaware that their body composition is less than ideal.
There are a few different things that can contribute to thin-fat, but a primary cause tends to be poor diet, and more specifically poorly-executed diet cycles. Most people who want to change their bodies focus on weight loss, and tend to favor unsustainable forms of dieting. If they combine diet with exercise, it's oftentimes too cardio-heavy, with little to no strength training.
In many people, this causes them to lose weight and muscle mass. Muscle requires a significant amount of calories to maintain, and decrease of muscle mass makes your diet even harder, as it also decreases metabolism.
Once the diet stops (it's difficult and generally not advised to stay on a significant caloric deficit for extended periods of time), they end up smaller overall, with reduced daily energy expenditure due to less muscle. So if they return to eating the same number of calories before they began their diet - or more, as people have a tendency to binge eat if they've been dieting too hard - they'll find themselves in a caloric surplus, which the body can then happily store as fat.
Ironically, they may now find themselves with less muscle and more fat than before they began their diet! And if they try the same method again, with an even more extreme diet (ex: water only for two weeks), an even worse result is likely, and their body composition continuously deteriorates.
So how could one avoid falling into the thin-fat "trap"?
1) Strength Training
Since increased muscle mass can increase your metabolism, and thus the number of calories it takes to maintain your body, building muscle is an important factor in long-term body change and weight loss. A generally accepted method is to train using progressive overload, gradually increasing the level of resistance/weight as the body grows stronger and accustomed to the previous level of intensity. This is arguably a more effective method than "boot-camp" style short intensive workshops where it's harder to maintain as a long-term habit. And while cardio-based exercise such as running or High Intensity Interval Training are not bad in of themselves, they may not be quite suitable at this point in time, as it may make it harder to increase mass.
2) Increase Protein Consumption
Protein isn't a magical food that will give you muscles, but it is the building block for muscle growth. By itself, it's not particularly useful (in terms of muscle building) if you don't stimulate muscle growth, allowing your body to actually make use of protein. If you've never paid attention to how much protein you consume each day, this would be a good opportunity to do so; otherwise, you may not be giving your body enough resources to build muscle mass. It also helps that proteins tend to be more filling, which make a caloric deficient easier to handle.
3) Count your Calories
Maintaining a slight caloric deficit is part of a thin-fat transformation. By calculating your Basal Metabolic Rate using a Body Composition Analyzer and multiplying that with the appropriate Physical Activity Level, you can get a reasonable accurate estimate of how many calories you actually use each day, and therefore how many calories you need to consume each day.
Making sure you're getting enough (not too much, but not too little) is because if your deficit is too deep, your body tends to have difficulty building muscle. It doesn't have to be exact, but it'll take some work upfront to determine how many calories the food items you eat each day will give you. In general, if you're losing a small amount of weight each week, this is a good sign. If you're losing too much fairly quickly, then you need to increase your calories because it's likely you'll also be losing muscle at the same time otherwise. If you're not losing any weight, then you should try reducing your calories slightly, by roughly 200-300.
4) Sufficient Rest
Many people with thin-fat bodies didn't previously have a habit of training, so the first few weeks in particular can take some getting used to. Overtraining is certainly a common issue when beginning, as you have not established what a "normal" workout should be for your body yet. At the other end of the spectrum, it is easy to underestimate the importance of consistent, non-interrupted sleep, which is when the real building of muscle occurs, through repair after being broken down in exercise.
You didn't develop a thin-fat physique in one day, so you won't change your body type away from it overnight either. Like any fitness goal, body change is a long-term process that you will ideally be doing for the rest of your life. But by eating and training correctly, you can save a lot of wasted effort! Change may seem slow, but one day you may look at a mirror and realize how much progress you've made!