Over weight is ranges of weight that are greater than what is generally considered healthy for a given height. It is generally associated with several relatively common diseases such as diabetes, hypertension, heart disease, and some cancers. [1]  Generally for white Caucasians,  when the body mass index (BMI) is within 18.5 to 24.99 is considered normal weight [2]. For Asians, because the body fat/muscle ratio is greater than the Caucasians, the BMI should not be over 21 which is considered as over weight. . When the BMI is greater than 22 the risk of getting diabetes is great. [3]

Genetics: although genes play an important role in obesity but it needs other behavior factors like over eating and lack of physical activities to cause obesity.

Eating habit: over eating habit of children can be acquired from parents or from people around them.

Using steroids: Using steroid type of drugs and increase eating appetite and cause weight gain. Clinically applied glucocorticoid type of drugs include: cortisone, hydrocortisone, prednisone, betamethasone (Celestone), clobetasol propionate, dexamethasone, fludrocortisone (Florinef), etc.

Stress: constantly under stress increases the steroid hormone, glucocorticoid ( cortisol). After the stress situation subsided, the excess cortisol is toxic for the brain. Estrogen helps clear the left over cortisol. If the cortisol remain high, it may  increase the eating appetite and cause weight gain.

Sex hormone: lack of sex hormone causes weight gain. Women gain weight during pregnancies and menopause because of the lowering of sex hormone. As people get older the sex hormone production decreases and most people become heavier. This is why castration is being used to increase weight of farm animals for meat production. When sex hormone is diminishing, the body is less able to handle stress and vice versa.

Serotonin effect: low level of serotonin may create depression and also creates the urge to eat sweet or carbohydrates because the body tries to use sugar to produce more serotonin.

Long term use of certain group of mood medication: To address low level of serotonin, usually the group of drugs called selective serotonin re-uptake inhibitors (SSRIs) are used. But, long term use of  SSRIs (selective serotonin re-uptake inhibitors)  causes weight gain. Among them Paxil is the worst of all. Study shows it increases breast sizes and increases the production of prolactin. This leads to low estrogen level and amenorrhea (lack of menses). Low estrogen level may cause weight gain. Tricyclic Antidepressants besides being used as antidepressants are also used to treat sleep disorders and to help manage pain. TCAs can cause significant weight gain, worse than SSRIs. The nonselective irreversible MAOIs can cause weight gain similar to TCAs.

Food high in fat and carbohydrates: like potatoes, sweet potatoes, ice cream, cakes, cookies, soft drink with sugar, sweet dessert, cheese, large amount of rice, or pasta.

SSRIs commonly prescribed are Citalopram (Celexa),  Fluoxetine (Prozac),  Fluvoxamine (Luvox),  Paroxetine (Paxil),  Sertraline (Zoloft)

The Tricyclic Antidepressants (TCAs)  inhibit the re-absorption (re-uptake) of serotonin and norepinephrine and to lesser extent, dopamine. They are Amitriptyline (Elavil),  Amoxapine (Asendin),  Clomipramine (Anafranil),  Desipramine (Norpramin, Pertofrane),  Doxepin (Adapin, Sinequan),  Imipramine (Janimine, Tofranil), Nortriptyline (Aventyl, Pamelor),  Protriptyline (Vivactil),  Trimipramine (Rhotrimine, Surmontil),.

Nonselective, irreversible MAOIs include:Isocarboxazid (Marplan),  Phenelzine (Nardil),  Tranylcypromine (Parnate). MAOIs (monoamine oxidase inhibitors) inhibit the enzyme (monoamine oxidase) the cleans up the neurotransmitters serotonin, norepinephrine and dopamine.

[1] http://www.cdc.gov/genomics/training/perspectives/obesity.htm

[2] http://www.who.int/bmi/index.jsp?introPage=intro_3.html

[3] http://www.halls.md/bmi/singapore.htm

BMI = body weight in kg / (height in meter)²