Am I underweight? How to check BMI in one minute
BMI is a simple number from your height and weight. You need a scale and a wall to measure your height.
The sum: weight in kilos ÷ (height in metres × height in metres).
Example: 50 kg, 1.68 m tall. 1.68 × 1.68 = 2.82. 50 ÷ 2.82 = 17.7.
Read it:
- Under 18.5: underweight (WHO scale)
- 18.5 to 22.9: normal for Indians
- 23 and above: overweight for Indians (Indian guidelines, 2009)
- 25 and above: very overweight (obese) for Indians
So the person in the example, at 17.7, is underweight. At 54 kg, the same person would be 19.1 and in the normal range. That is the gap this series is about: a few kilos.
BMI is not perfect. A thin person with lots of muscle, or an old person who has lost height, can get a misleading number. But for most adults it is a good first check.
Is being underweight dangerous? 5 things low weight does
Infographic: five effects of low weight: weak immunity, weak bones, missed periods, low stamina, feeling cold
Where the body cuts corners when it is underfed.
1. You fall sick more often
The UK health service, on underweight adults: "Being underweight could mean you're missing out on vitamins and minerals. This could lead to health problems such as a weak immune system and bone fractures."
A weak immune system means the cold that others shake off in three days keeps you in bed for a week. Cuts heal slowly. Every change of season brings something.
2. Your bones get weak
The same line names bone fractures. Bones need calcium, protein and a body that carries some load. An underweight person often eats too little of all three. The break comes years later, from a small fall.
3. Periods go missing
The UK health service lists "sudden weight loss", "doing too much exercise" and "stress" among the causes of missed or late periods.
A woman who has lost weight fast, or who exercises hard on little food, may find her periods late, light, or gone. The body is saying it does not have enough to spare. This matters for health now and for having children later. A doctor should know about it.
4. You have no stamina
Not enough stored energy means the body tires early. Climbing stairs, a long day, a hot afternoon: the thin person gives up first. The old books describe this exactly, as you will see below.
5. You feel cold, and you look older
Many underweight people say they feel cold when others are comfortable, and notice thin hair, dry skin and sunken cheeks. These are not in a guideline, but they are what the "missing vitamins and minerals" line looks like in the mirror: the body spends what little it has on the organs and skips the rest.
What the old book says the thin person suffers
Charaka Samhita, Sutra Sthana, chapter 21, verses 13 to 15. The very thin person cannot bear exercise, large meals, hunger, thirst, cold, heat, illness or medicines. He easily gets diseases of the spleen, cough, wasting, breathlessness, lumps in the belly and piles. His buttocks, belly and neck are dried up.
Verse 16: such a person is often ill, and needs ongoing care to build the body up.
Verse 17: "Of the two, lean is less harmful than obese." That is, the too-fat person suffers more than the too-thin one.
Read the first list again next to the modern one. Cannot bear illness: weak immunity. Cough and breathlessness: the chest infections that keep coming back. Cannot bear exercise, hunger or heat: no stamina. Two thousand years apart, the same patient.
And read verse 17. The old book does not want you to panic. Thin is a problem to fix, not a disaster. The too-fat person is in more trouble. Fix it, do not fear it.
Underweight symptoms: a checklist
Tick what applies to you.
Sign | What it points to |
|---|---|
BMI under 18.5 | Underweight by the number |
Fall sick with every season change | Weak immunity |
Cuts and colds take long to heal | Weak immunity, low protein |
Tired by afternoon, give up on stairs | No stored energy; Charaka's "cannot bear exercise" |
Feel cold when others are fine | Too little fat under the skin |
Periods late, light or stopped (women) | Body short of energy; see a doctor |
Hair thinning, skin dry, cheeks sunken | Body skipping the "extras" |
Cough that keeps coming back | Charaka's "cough, breathlessness"; rule out TB |
Weight going down without trying | Not thinness. Illness. See a doctor now. |
None or one tick, with a steady weight: you are slim, not sick. Three or more: take it seriously. The last row, on its own, is enough.
When thin means "see a doctor"
Being thin since childhood, with a steady weight, is one thing. These are different, and need a doctor soon:
- Losing weight without changing your food or exercise. The UK health service's rule: see a doctor if "you keep losing weight without changing your diet or exercise routine".
- A cough of more than two weeks, fever or night sweats with weight loss. In India, that means a TB test.
- Great thirst and passing urine often. That can be diabetes.
- Loose motions, stomach pain or blood in the stool for weeks.
- Fast heartbeat, shaking hands, sweating for no reason. That can be an overactive thyroid.
- Periods stopped for three months or more, and you are not pregnant.
Our guide to why am I not gaining weight goes through each of these causes.
What to do: the first goal is 18.5
If the doctor finds nothing, or you have simply always been thin, the plan is the one in this series. The first goal is not a six-pack. It is to get your BMI above 18.5, where the body stops cutting corners.
- Eat five times a day. Breakfast, mid-morning, lunch, evening, dinner. Our Indian diet chart for weight gain lays it out.
- Put the fat back. Ghee on dal and rice, full-cream milk, peanuts. Charaka's first cause of thinness is dry food.
- Protein every meal. Dal, milk, dahi, paneer, peanuts, soya. The numbers are in protein for weight gain.
- Sleep 7 to 8 hours. And a short nap after lunch, which the old book allows for the thin.
- 20 minutes of strength exercise, three days a week. Not long running. The body needs to be asked to build, gently.
- Weigh once a week. Half a kilo to one kilo a month is the right speed. For the person in our example, 18.5 is about 2 kilos away. That is two to four months.
The old book's cure, for the record (verses 20, 29 to 34): nourishing food: milk, ghee, dahi, rice, urad, wheat, sugarcane, jaggery dishes; plus sleep, a comfortable bed, a calm mind, no worry, oil massage and pleasant surroundings. That is the plan above, written earlier.
Is being underweight dangerous? Under 18.5, yes. The good news: it is a few kilos away from not being.
If low appetite is what keeps you from five meals a day, Ayuvya's i-Gain is made from Ayurvedic herbs for appetite and nourishment, with a diet chart that follows the old book's list. See i-Gain. If you have any of the warning signs above, see a doctor first.
This article is for general information. It is not medical advice. BMI is a screening number, not a diagnosis. Losing weight without trying, missed periods, a long cough or great thirst need a doctor, not a diet chart. The food advice here is for adults who are underweight and otherwise healthy.
Sources
- NHS (UK). Advice for underweight adults ("missing out on vitamins and minerals"; "weak immune system and bone fractures"). nhs.uk
- NHS (UK). Causes of missed or late periods (sudden weight loss, too much exercise, stress). nhs.uk
- NHS (UK). Unintentional weight loss: when to see a GP. nhs.uk
- Data for India. Measuring obesity in India: WHO BMI cut-offs (underweight below 18.5) and the 2009 Indian cut-offs (overweight 23 to 24.9, obese 25 and above). dataforindia.com
- World Health Organization. Tuberculosis: common symptoms include prolonged cough, weight loss, fever and night sweats. who.int
- Charaka Samhita, Sutra Sthana, chapter 21: verses 11 to 17 on the too-thin person; 20 and 29 to 34 on the nourishing cure. carakasamhitaonline.com
