While obesity is a serious public health issue, being underweight is also a challenge that many people deal with. Since weight loss is an issue in the limelight, we may not be aware that there are those who also struggle to gain weight. 

On a population level, the Body Mass Index (BMI) is used to categorize weight into three groups. An ideal BMI is between 20 to 22.9 kg/m2. Below this range, one can be considered as underweight and a BMI of 18.5 to 17.5 kg/ m2 is extremely underweight. People who are underweight are often told that being underweight is better (or healthier) than being overweight or obese. However, the reality is that being too thin comes with its own set of risks and health challenges.

Nutrient Malabsorption:
Being underweight leads to nutrient deficiencies due to nutrient malabsorption. In particular, the bones get affected leading to an increased risk of brittle bones. This makes one susceptible to bone fractures. Nutrients like calcium and potassium get seriously compromised. They are important for healthy bones and a regular heartbeat. A lack of Vitamin C could lead to soft and bleeding gums. Low levels of iron could lead to anemia.

Lowered Immunity:
Lack of adequate nutrients and/ or malabsorption could lead to lowered immunity. Antioxidants like Vitamins A, C and E all prevent various illness. Being underweight negatively affects the body’s ability to fight against illness, viruses and bacteria.

Lack of energy:
When the body is unable to retain fat, nutrient and energy stores are also affected leading to weakness. Fat soluble vitamins- A, D, E and K require fat as a medium for their effective absorption and utilization. Nutrients help release energy from food that can be used by the body. When iron levels drop, this affects the body’s ability to transport oxygen effectively from the blood stream to the various organs. This can lead to a feeling of tiredness and weakness.

Affects Fertility:
A low BMI may also contribute to infertility or make conceiving harder for some women. However, this varies with each individual. Some women are naturally thin in which case their lower-than-average weight will not affect fertility. On the other hand, women who diet excessively/ follow crash diets to lose weight, may find it hard to conceive. The female body physiology requires fat, since this plays an important role in how oestrogen is used. Oestrogen is an important female hormone that helps regulate the menstrual cycle and ovulation.

How can I eat healthy?

• Prepare meals with foods from each food group. Include carbohydrates, protein and healthy fat. Make it a point to also include portions of fruit and vegetables to benefit with the antioxidants, fiber and nutrients they contain.
• Have 3 meals and 2-3 nutrient dense snacks in between. Avoid keeping long gaps between meals.
• Eat healthy and make wise food choices. Do not snack on junk foods, sweets and sugar laden beverages. They contain empty calories and can cause leaching of essential nutrients from the body for their digestion.
• Have a good amount of protein, since this helps you build muscle. Include proteins like lean cuts of chicken, fish and eggs. Vegetarians can opt for soy and its products, pulses and legumes. Have one protein source at each meal. Try and also include strength training to build muscle mass for healthy weight gain.
• Add healthy fat to your meal pattern. Make use of nuts and nut butters (peanut/ almond), olive oil dressings over salads and avocado as a snack or dessert. These fats help the fat-soluble nutrients to be better absorbed and utilized.
• Add dry milk powder or a scoop of protein powder to soups and beverages like milk. This will help increase nutrient intake. Prepare drinks like smoothies which give you nutrients from 2 food groups- milk/ yogurt and fruit.

You need to be consistent with these healthy changes and include calorie dense portions at each meal. Gradually increase the portion size of food, especially protein. In case you are deficient in any nutrients, you can ask your doctor to prescribe a multivitamin supplement. A qualified Dietitian can help you plan a customized diet to be followed for a healthy increase in weight. 
Once your eating habits have improved, you can start with strength training exercises to build healthy muscle. However, do this only under the recommendation of your doctor and a qualified fitness instructor.

Reference:
http://www.todaysdietitian.com/newarchives/tdjan2008pg56.shtml
https://healthyliving.azcentral.com/dangers-being-underweight-8137.html
https://www.foodsforbetterhealth.com/how-can-i-gain-weight-and-stay-healthy-1866
http://drhoffman.com/article/underweight-and-malabsorption-2/
https://www.babycentre.co.uk/x554802/will-being-underweight-affect-my-ability-to-get-pregnant
https://www.everydayfamily.com/why-being-underweight-affects-conception/
https://www.betterbones.com/bone-nutrition/weight-gaining-for-under-weight/