Dehydration and the Calculation and Standards of Body Mass Index (BMI)

In medicine, the term "dehydration" is frequently heard. A small percentage of dieters also experience dehydration during their weight loss process. What is dehydration? Dehydration refers to a condition where the body loses a large amount of water due to illness, and the body cannot replenish it promptly, leading to metabolic disorders. In severe cases, it can cause collapse and even be life-threatening, requiring intravenous fluid replacement. Significant reduction in body fluids often occurs in cases of severe vomiting, diarrhea, excessive sweating, or bleeding. Dehydration is further divided into hypotonic dehydration, which is a decrease in extracellular fluid combined with low blood sodium levels. This is common in hypertonic or isotonic dehydration where only water is replenished without timely salt replenishment, such as in cases of significant loss of digestive fluids. Hypertonic dehydration is a decrease in extracellular fluid combined with high blood sodium levels, such as in cases of insufficient water intake, excessive water loss, or increased water demand in high-temperature environments without adequate replenishment. Isotonic dehydration is a decrease in extracellular fluid with normal blood sodium levels, primarily caused by medical factors such as acute massive blood loss or traumatic effusion.

Regardless of the type of dehydration, there is a reduction in extracellular fluid volume. Extracellular fluid accounts for approximately 20% of the body weight of a normal adult, while intracellular fluid accounts for 40%. Extracellular fluid is further divided into plasma (5% of body weight) and interstitial fluid (15% of body weight). Under normal circumstances, the amount of body fluid varies considerably between individuals, mainly depending on age, sex, and degree of obesity.

Based on the amount of weight loss (water loss) and clinical manifestations, dehydration is classified into three degrees: First, mild dehydration, where water loss leads to a weight loss of 2% to 3%, or 5%, with only general neurological symptoms such as headache, dizziness, weakness, and slightly reduced skin elasticity; second, moderate dehydration, where water loss leads to a weight loss of 3% to 6%, or 5% to 10%, with obvious signs of dehydration and the onset of signs of circulatory insufficiency; and third, severe dehydration, where water loss leads to a weight loss of more than 6%, or more than 10%, with the aforementioned symptoms worsening, and even shock and coma.

Water is the most likely component of the human body to cause changes in weight. There will be significant changes in weight before and after drinking water or before and after sweating.

Measure your height and weight to find your Body Mass Index (BMI).

Body Mass Index (BMI), also known as body weight, is a numerical value calculated by dividing weight in kilograms by height in meters squared. It was first proposed by the Belgian polymath Quetelet in the mid-19th century. Now primarily used for statistical purposes, BMI is a neutral and relatively reliable indicator when comparing and analyzing the health impact of weight on people of different heights. BMI is a globally recognized method for assessing obesity levels, and the World Health Organization (WHO) also uses BMI to define obesity or overweight. It was a popular standard in the 20th century for measuring body fat and overall health.

The concept of Body Mass Index (BMI) is defined as follows: BMI = weight (kg) ÷ height² (m).

For example: A person is 1.75 meters tall and weighs 68 kilograms. His BMI = 68 / (1.75²) = 22.2 (kg/m²).

BMI is an indicator closely related to total body fat, taking into account both weight and height. BMI is simple, practical, and reflects overall overweight and obesity. It is more accurate than simply relying on weight when assessing the risk of heart disease, high blood pressure, and other health problems caused by being overweight. It has evolved from a purely medical indicator to a general fitness benchmark for the general public.

According to the World Health Organization (WHO) standards, an Asian BMI above 22.9 is considered overweight. Asians and Europeans belong to different races, and the WHO standards are not entirely suitable for the Chinese population; therefore, a Chinese reference standard has been developed.

Method: ① Measure your height and weight; ② Calculate your body mass index (BMI); ③ Refer to the Chinese reference standards in the table below to evaluate whether you need to control your weight.

**Body Mass Index (BMI) Table**

| WHO Standards | Asian Standards | Chinese Standards | Risk of Disease Onset |

| :--- | :--- | :--- | :--- | :--- |

| Underweight | <18.5 | | | Low (but increased risk of other diseases) |

| Normal | 18.5~24.9 | 18.5~22.9 | 18.5~23.9 | Average Level |

| Overweight | ≥25 | ≥23 | ≥24 | |

| Slightly overweight | 25.0~29.9 | 23~24.9 | 24~27.9 | Increased |

| Obesity | 30.0~34.9 | 25~29.9 | ≥28 | Moderately increased |

| Severe obesity | 35.0~39.9 | ≥30 | ------ | Significantly increased |

| Extremely severe obesity | ≥40.0 | | | Very serious increase |

As can be seen from the table above, the ideal body mass index (BMI) is 22.

The World Health Organization's obesity standards are based on research data from Western populations, and some of this data may not be suitable for Asian populations. The same BMI may represent different levels of obesity across different ethnic groups. Overall, BMI levels in Asia are lower than in European countries. Multiple studies have shown that Asians already have a higher risk of cardiovascular disease at lower BMI levels. In other words, the risk of hypertension begins to increase in Chinese people when their BMI exceeds 25. This meta-analysis report from the China Obesity Working Group indicates that an increase in body mass index (BMI) is associated with a rise in the incidence of coronary heart disease and stroke. Overweight and obesity are independent risk factors for coronary heart disease and stroke. For every 2-point increase in BMI, the relative risk of coronary heart disease, stroke, and ischemic stroke increases by 15.4%, 6.1%, and 18.8%, respectively. Once a BMI reaches or exceeds 24, the probability of developing serious health-threatening diseases such as hypertension, diabetes, coronary heart disease, and dyslipidemia increases significantly.

Based on a compilation and analysis of data from the Chinese population in the 1990s, the China Obesity Working Group proposed obesity standards suitable for Chinese adults: a body mass index (BMI) of 24 or higher is considered overweight, and 28 or higher is considered obese. Waist circumference is an important indicator for measuring abdominal obesity, reflecting the degree of abdominal fat accumulation, which is related to a series of metabolic abnormalities. We will discuss waist circumference in more detail later.