As a mother of a child with celiac disease, I was confronted with the sight of my daughter losing weight at the age of 8, even though she was eating all day long. Apart from that, she had very worrying physical and mental health conditions: recurrent respiratory and gastroenterological diseases, physical and mental exhaustion, depression, etc. Having a clear diagnosis with medical tests to prove it and a well-defined diet, we had ​​no doubt about implementing it. But although most health problems were disappearing quite quickly, her weight still decreased for a few months and then it was stable without any increase during two long years.
Nowadays, as a nutrition therapist, I am often confronted with cases where I see weight loss without being able to find the underlying cause with conventional medical tests. In these cases, people have come to situations of such extreme thinness, that they are terrified of the fact of losing a single more gram. The concern about the situation of being underweight is such that it gains priority over having diseases that cause strong pain such as fibromyalgia or arthritis. 
Often people are so underweight that the body uses the muscle to feed themselves; it’s the cachexia or wasting syndrome. 
Leaving aside cases where a poor amount of food is eaten, either due to anorexia, extreme poverty or hunger strike, etc., or where the cause is emotional, such as depression, marginalization, etc., or it’s the side effect of a medicine or therapy, such as chemotherapy, we are confronted with cases in which there is no apparent cause of the weight loss. In such cases, mainly the proposed solution is to go on a nutritional therapy. 
 

Eating a lot makes us gain weight, eating little makes us lose weight

 
The myth of when someone eats a large amount, this person will surely gain weight is not true at all. We've all seen people eating lots of food being still very thin. So the solution to the fact of losing weight is not to eat more food, but on the contrary; in many cases, overeating is one of the causes of thinness.
As discussed in this another article, overeating is one of the causes of intestinal dysbiosis and as a consequence, of malabsorption of nutrients as well.

Eating a lot in order to grow very tall

 
Since early childhood we have been told that it is necessary to eat everything on our plate to "grow up ". It is true that a certain amount of food is needed for children to grow and stay healthy, but how much food is that? Is it really necessary to have three full meals and two snacks a day?
Finding the right amount of food and the proper frequency of eating it is not easy. There is no universal rule about the amount of food each person should eat. But usually, three meals plus two snacks is really too much for an adult who is basically leading a sedentary life. A person in good health should be able to live without any problem eating a moderate amount of food.
For a child, the amount depends on the age, and as they grow, they must be capable of withstanding grabbing snacks without any problem. They would ideally decide themselves how much to eat, so we should not force them to finish everything they have on their plate, instead, they would stop eating when they feel that they are full. Can we trust them to eat the necessary amount if we give them the freedom to decide? Yes, we can, provided that they do not eat products that cause addiction, which are usually sugars, gluten, dairy products (from about 3 years old), stimulants such as chocolate and coke, etc. If they are given healthy meals, we can trust that they will eat what is necessary, and they won’t reject food because they prefer to eat other foods that give them more pleasure, but because their body is already full.
On the other hand, there are certain foods that stimulate the growth of children, such as dairy products. We must be aware of the fact that milk naturally contains a growth hormone, the so called IGF-I., which serves as a quick developer of small calves in large cows. Is this really what we want for our children? We must think about the effect of a hormone of this kind of features can cause on humans. Of course, it boosts growth, but what might happen if excessive cell growth is stimulated? The IGF-I is an insulin-like hormone, but with growth functions; bovine and human are identical. Numerous studies show a link between IGF-I and the development of various tumors (thyroid, bone, kidney, breast tumors, etc.) (1) (2)
Consequently, it is not true that for the proper growth of a child it is necessary to eat a lot of food. The important thing is to meet their needs in all aspects: healthy food, clean air and water, sunshine, exercise, rest, parental love, friendly social environment, academic and spiritual education, etc.
 

Causes of weight loss
 

Weight loss is mainly due to poor absorption of nutrients in the body. You can find the causes of malabsorption detailed in this article about the relationship between overeating and nutritional deficiencies as well. 

Treatment
 

The proposal is to go on a diet that takes into account the intolerances and nutritional lacks. But obviously, the person who loses weight is not willing to accept a diet, as a diet is traditionally associated with losing weight, which is just what these people should avoid. However, diets do not only work for weight loss, but for the person to find his/her proper weight.
So the treatment consists of remedying the deficiencies of nutrient absorption, repairing intestinal villi as well as the intestinal flora and avoiding foods such as nutrient “thieves”.
When someone starts a nutritional treatment, it is normal to lose a little more weight at first, which really scares those affected. Here is what Dr. Campbell-McBride says about the weight loss one experiences when starting the GAPS diet:
“Regular consumption of carbohydrates and cereals causes water retention in the body. When we stop eating these foods, we lose the excess water, and therefore, we lose a little weight, which usually happens during the first weeks. Without the water retention, it is possible to see someone’s real weight and measure, which show the real extent of malnutrition of a person. Following the GAPS nutritional protocol, the digestive system begins to absorb foods properly and feed the body; bones start to grow denser, muscles, other tissues and organs become healthier, and therefore the result is weight gain. A person can continue being thin the rest of his/her life, if that is what his/her physique is like, however, they will be stronger and more energetic.”
 Moreover, it is also common to experience an die-off reaction when intestinal pathogenic bacteria die, releasing a large amount of toxins at once, causing a worsening in the symptoms of the disease that is being treated. As a consequence, nutritional therapy should be started very slowly, so that the body is able to bear the process of detoxification. Therefore, this treatment usually lasts several years.
In some cases, the nutritional program must even incorporate fasting in case an in-depth detoxification is required. Paradoxically, the thinner and more malnourished a person is, the higher  the toxicity in his/her body is, and therefore, the diet must be stricter, and maybe even a certain type of fast might be necessary.
 

Bibliography:

(1) « Growth hormone, IGF-I and cancer. Less intervention to avoid cancer? More intervention to prevent cancer? »,

(2) http://www.notmilk.com/