We've all heard the phrase "abs are made in the kitchen" - which quite honestly is true. It's not about how many crunches you can do, or how long you can hold a plank for (although certain exercises will help to strengthen the abdominals), it's more about the percentage of body fat that you have in order to see the abdominals. Not everyone is fussed about seeing these muscles; we may have other goals we're working towards, but essentially weight loss or gain comes down to calorie deficits or a calorie surplus in order to notice changes. Research suggests that to notice a change within our body and how we look, diet accounts for 80% of the changes whilst the exercise accounts for 20%.
Regular calorie deficits mean that you burn more calories than you consume, therefore meaning that weight gain comes from consuming more calories than you burn, thus creating a calorie surplus. Saying this, the quality of foods that you consume still need to be taken into consideration whether talking about weight loss or gain; consuming only one 800 calorie milkshake in the day and nothing else would mean that, although you might be a in a calorie deficit and therefore logically would lose weight if you did this regularly, the quality of nutrients that you are consuming is very low and there isn't much variety. It would be far more realistic and manageable to eat a varied diet of smaller portions throughout the day with a wide variety of ingredients that provide much better nutritious content.
For those looking to gain weight - usually in muscle form - you would need to look again at the quality of food that you are eating. A typical example of this would be to look at a boxer, since weight categories are very specific when competing. During their training schedule it wouldn't be enough to just add a milkshake into their diet each day when looking to gain weight, because the nutrient content of the milkshake is far more likely to be fats as opposed to protein. Proteins help to build and repair muscle, and since muscle weighs more than fat, it makes far more sense for a boxer to consume healthy proteins in order to gain a quality level of muscle to make the weight category for competition, than it would be for them to consume high amounts of saturated fats to quickly gain weight. This reason for this being is that high levels of fat on the body can have drastic effects on the organs and muscles, meaning the boxer may not perform as well as he could, as well as risking the chance of heart disease, and other health-related problems.