These are the words I hear from patients regularly. Is it possible to eat "too little" thereby leading to weight gain? NO.
Energy balance determines your body weight. "Calories in, calories out". If you use more energy than you consume, it will lead to weight loss. If you use less energy than you consume, it will lead to weight gain. This is basic science.
In truth, weight loss or weight gain is a bit more complicated than this. You body is dynamic and metabolic needs are constantly changing based on your activity, hormones, sleep deprivation, and genetic differences. Don't forget, as you lose weight, your metabolic needs go down. Therefore if you are consuming the same amount when you are smaller, you will not lose weight at the same rate.
What are the main reasons people don't lose weight?
1) Slow metabolism - There are some medical conditions that can lead to this, including hypothyroidism, polycystic ovarian syndrome, leptin resistance. But having these conditions still doesn't mean weight loss is impossible, just more difficult. The number of people in this category is very small.
2) Eating more than you think you are - We are all poor estimators of what we eat. Oftentimes we think we hardly ate a thing, but forgot about the handful of nuts, the snack in the car, and the leftover chicken strip from your child's plate. Logging and measuring your food, don't estimate. Don't forget to log your snacks. This is the main category that people fall into.
3) Expecting faster weight loss than is realistic - As you age, your metabolic rate declines. You don't see the dramatic weight losses that you may have when you were in your 20's.
Be patient. Stay on track. Even with obstacles, a healthy weight can be in your future!