All of these questions can largely be answered by fat cells: specifically, fat cell quantity and size. Allow me to explain.
Growth and Reproduction of Fat Cells
From one to about 18 years old, we body gain fat mainly through the increase in the number of fat cells . Losing fat during those years comes through the decrease in fat cell size. During adulthood, we basically become thinner and fatter through the change in our fat cell size  (we can increase fat cell quantity during adulthood, but that only seems to occur in one specific scenario which I'll get to later). Considering what you just read, you may ask yourself: if we gain fat cells during childhood and adolescents, and if we increase and decrease fat cell size after that, then when do we lose fat by getting rid of fat cells? The answer is that we don't: fat cells, once added, are part of a marriage to our body where divorce isn't an option (at least through natural methods).
A study measuring the growth of fat cell size and number from four months to 19 years old in 288 subjects, both male and female, produced the data charted below .
The dark line is for lean individuals and the dotted line is for obese people. The chart on the left shows changes in fat cell volume over those 18-plus years. As you can see, after year one, it barely climbs until 18, when it increases more significantly in the obese subjects. On the right side is the chart for fat cell quantity. Both groups gain quite a bit, but the obese people are way ahead and increase at a much greater rate.
The fat cells we acquire until age 18 basically determines the leanest that we can possibly become as adults. As mentioned, we can decrease our fat cell size, but only to a certain extent. There seems to be a range that fat cells can shrink to, which is about 3-6 microns [no online link for the study: Fat Cells and Body Weight, Sjostrom]. Obese and lean people can all shrink into that size range, but the difference in fatness at that point is determined by fat cell quantity.
Fat cells also have maximum limits for growth, too. This brings me to the exception mentioned before: the only time adults seem to increase the amount of fat cells that they have is during a state of extreme obesity . The logical conclusion is that people in this state cannot expand their fat cells any further, so new ones are created to meet the demand for more fat storage.
Applying the Data to Our Lives
Here are my suggestions based on how I interpreted the studies discussed:
- Clearly something is going on during the childhood and adolescents of obese people which causes that horrifying fat cell replication rate shown in the chart. What is the cause? There are no definite answers, but there is a study that could help with prevention. Researchers studied fat cell development in rats from three groups: one that ate ad libitum, one that ate a calorie-restricted diet, and a third that ate ad libitum but took part in a lot of regular exercise . Fat cells replicated the least in the exercise group. Those rats predictably gained the least amount of fat in the months following the study. Granted the study looked at rats, I think it could lead to an important point: kids and adolescents need to be physically active. Perhaps we can prevent the likely irreversible damage of gaining too many fat cells. While calorie restriction in rats wasn't as successful in preventing fat cell growth, I think focusing on diet quality in youth is another hugely important factor. The health habits of the youth are setting the table for the quality of life that they will have for the rest of their lives! We need to take every precaution possible when it comes to exercise/physical activity and diet.
- For those of us who are already adults, it's unlikely that we will find out how many fat cells we have and what the sizes of those fat cells are. What I can tell you is if you are obese, no matter how many pounds you need to lose, you need to act responsibly with your lifestyle habits. As mentioned, you could be at or close to a point where your body will produce more fat cells, therefore creating irreversible damage and further decreasing the chances that you can lose weight in the future. The sooner you act, the better your potential outcome will be.
- Finally, for those of you who are overweight/obese but can't seem to lose more weight despite honestly eating well and exercising consistently, I think you should continue what you're doing. You might be in the unfortunate situation where your fat cells are too abundant but fall into the normal size range. However, as I have stated before, health and fatness are separate matters, and the right diet and exercise habits will improve your health, regardless of weight changes. Even if your lack of weight loss frustrates you, I bet you still care about your longevity and quality of life. Diet and exercise can give you both, so keep going.