Thinness and obesity: why body composition matters more than weight
Nazira Docmac
Integrative, Orthomolecular and Bioregulatory Nutritionist
It is well known that weight has always occupied a central place in how we assess health. We step on a scale at the doctor's office, the nutritionist's, or even the one in our own bathroom, and just by looking at the numbers, we almost automatically decide whether we are "too thin," "normal," or "overweight/obese."
The problem with this reductionist view is that the human body is much more complex than a figure expressed in total kilos.
For example, two people of the same sex and height can weigh exactly the same and yet have completely different bodies, body fat percentages (adipose tissue), muscle percentages, and metabolic profiles.
One person’s weight may be largely accounted for by high muscle mass, while the other may have low muscle mass and a weight that reflects a significantly higher proportion of fat. The scale will show the same weight for both people, yet their body composition—and therefore their nutritional diagnosis—is very different.
That is why, when we talk about thinness or obesity, it is fundamental to change our perspective, and instead of focusing solely on how much we weigh, we should ask ourselves:
"What is the weight I see on the scale actually made of?"
What is body composition?
Our organism is made up of different components: water, muscle, bones, organs, tissues, and fat, among others. All these compartments contribute to body weight, but they do not hold the same meaning from a health perspective.
This does not mean that body weight is irrelevant; on the contrary, it can be important data in the context of a nutritional assessment, especially when its evolution over time is analyzed. The problem arises when it becomes the sole indicator used to define a person's nutritional status.
What is thinness?
The word "thinness" is commonly used to describe a person who is visibly underweight, but it is necessary to clarify that being thin does not necessarily mean being healthy. A person may have a low body weight and "look good" visually, but that low weight may be associated with insufficient food intake, loss of muscle mass, a disease or nutrient absorption problem, or even specific eating disorders.
That is why it is very important to distinguish between aesthetic thinness and a healthy nutritional state, as while they are related concepts, they are not synonymous.
Consequences of extreme thinness
Extreme thinness should not be confused with a naturally slim constitution, as a person can have a small body frame and be perfectly healthy. The concern arises when there is an excessively low weight or body composition as a result of dietary restriction or other factors.
When the body does not receive enough energy and nutrients over a prolonged period, it may begin to use its own reserves. This can affect both fat mass and muscle mass.
Possible consequences of extreme thinness or insufficient energy availability may include:
Significant loss of muscle mass and strength.
Increased weakness and fatigue.
Impaired immune function.
Increased risk of vitamin and mineral deficiencies.
Hormonal imbalances.
Menstrual cycle and reproductive function disturbances during low energy availability.
Decreased bone mineral density and higher risk of bone problems in certain situations.
Lower tolerance to cold.
Impaired wound healing.
Poorer recovery after exercise.
Increased vulnerability to certain diseases or infections.
Cardiovascular complications in severe cases of malnutrition.
"Skinny fat" or "false thin"
Imagine two people who weigh 55 kg and are the same height. The first has an adequate amount of muscle mass and a fat percentage within the normal range for their age.
On the contrary, the second person has very little muscle mass and a very high proportion of fat. Although both weigh exactly the same, their body composition is different, and quite possibly their metabolism is too.
This phenomenon relates to what is popularly called "skinny fat": a person can have an apparently low or normal weight and still have a high amount of fat compared to their muscle mass.
A "light or thin" body is not necessarily a healthy body.
For this reason, from an integrative nutrition perspective, the goal should not be for a person to weigh less, but rather to promote an adequate body composition and dietary habits consistent with good health.
What is obesity?
It is a chronic, multifactorial disease characterized by excessive accumulation of adipose tissue that can negatively affect health. This definition is important because it helps us understand that obesity and body weight are not the same thing.
Weight can be used as an initial approximation to identify potential problems related to excess adipose tissue, but it does not tell us directly how much fat a person has or where it is distributed.
Body fat impacts health differently depending on where it is most accumulated. Adipose tissue that forms around the waist and trunk indicates a person's visceral fat. Since this is fat located around the organs, it is the most dangerous, as it is linked to a higher cardiovascular risk.
Health consequences of obesity
Among the health consequences and problems that may be related to obesity are:
Increased risk of insulin resistance and type 2 diabetes.
High blood pressure.
Lipid profile alterations.
Increased risk of cardiovascular disease.
Metabolic liver disease associated with fat accumulation in the liver.
Increased risk of obstructive sleep apnea.
Joint and osteoarticular problems due to, among other factors, mechanical overload.
Limitations in mobility and functional capacity.
Higher risk of certain types of cancer.
Health issues related to reproductive health in some individuals.
Higher probability of developing low-grade chronic inflammation associated with adipose tissue dysfunction.
However, it is fundamental to make a distinction: having obesity does not mean that a person necessarily has all these complications. The risk depends on, among other factors, the amount and distribution of fat, the duration of obesity, the presence of muscle mass, genetics, physical activity, and other health-related factors.
Chronic inflammation and adipose tissue
Adipose tissue is not merely a passive energy storage site. It is a metabolically active tissue that participates in the production of different substances and signals that can influence processes related to metabolism and the inflammatory response.
When there is an excess of adipose tissue, especially visceral fat, the normal functioning of adipose tissue can be altered, promoting a state of low-grade chronic inflammation. This inflammation does not have to manifest as obvious inflammation with pain or swelling; instead, it can develop silently and persist for long periods.
It is very important to know that inflammation associated with excess body fat is not exclusive to people with visible obesity—let’s remember the case of "skinny fat" individuals.
A person can be thin to the naked eye and yet have a relatively high fat percentage in relation to their muscle mass. In certain cases, this situation may be accompanied by an unfavorable distribution of fat, especially at the abdominal level, and metabolic alterations.
This is what partly explains why "being thin" does not automatically equate to "being metabolically healthy."
Important considerations
A person can be thin and have a high fat percentage.
A person can have obesity and present a much higher amount of fat than a thin person.
And yet, none of these situations should be evaluated solely by looking at weight. What is important is not only how much a person weighs, but what proportion of that weight corresponds to fat, where that fat is located, and what impact it is having on their metabolic health. This is why speaking of body composition allows for a much more complete assessment than speaking exclusively of kilograms.
Moving beyond "good or bad" thinking
One of the greatest difficulties surrounding body weight is that we have learned to interpret the body through very simple categories:
"Thin = healthy, successful"
"Fat = unhealthy, failure"
When we know the reality is much more complex than that. In terms of nutrition, it has been observed that a person with obesity can significantly improve their health through relatively modest changes in their body composition and habits, even before reaching a specific "ideal weight."
Likewise, a thin person may need to improve their diet, increase their muscle mass, or review certain habits; health cannot be observed exclusively from the exterior, as physical appearance allows us to know a person's body composition, metabolic condition, or habits with precision.
Therefore, a responsible nutritional assessment must move away from both the stigma associated with excess weight and the idealization of thinness.
I eat healthy and work out, but I'm not losing weight: what is happening
When a person begins going to the gym or performing other types of exercise, they may not see their effort reflected on the scale, as strength training can cause them to gain muscle and lose fat simultaneously. In this case, the change is not reflected in weight but in the shift in body composition, a process called "body recomposition."
Clothes may fit looser, the waist may shrink, strength may increase, and the fat percentage may decrease even if the scale barely moves.
If a person only looks at the scale, they might erroneously conclude that they "have had no changes or progress," when in reality they have achieved a great deal of progress.
This does not mean that we should never step on a scale again; it means that while weight is a useful indicator, it must be integrated with other data in a nutritional assessment.
How to know if the nutritional treatment is working
What should you look for to know if the nutritional treatment is yielding results?
- Weight evolution.
- Body fat percentage (nutritional assessment via skinfolds or bioelectrical impedance).
- Waist circumference.
- Percentage of muscle mass or fat-free mass.
- Strength and physical performance.
- Eating habits.
- Level of physical activity.
- Sleep quality.
- Metabolic parameters and blood test results when applicable.
- Sense of energy and well-being.
- Functional capacity.
- Others.
Analyzed together, these indicators allow for a much more complete view of a person's nutritional and health status.
Diets for rapid weight loss: what to keep in mind
When a person seeks to reduce their weight quickly through very restrictive diets, the scale may show a decrease of several kilograms. However, before getting happy, it must be analyzed that this value does not mean all the loss corresponds to fat; certainly, a large part of the weight lost can be generated by losses of water, glycogen, and muscle mass.
From a body composition standpoint, this is a very poor strategy for losing weight. The ideal is to lose fat while preserving as much muscle mass as possible.
The question in weight loss treatments should not focus on how many kilos have been lost, but on how much fat has been lost and how much muscle mass has been preserved or increased.
Conclusion
Thinness and obesity should not be analyzed solely from the number that appears on a scale; body weight is a piece of data, but it does not explain the composition of the organism by itself. Body fat percentage, along with muscle mass, waist circumference, physical activity, eating habits, and other health indicators, can help us obtain a much more complete view.
But we should not turn fat percentage into a new obsession either. There is no universal perfect number, and chasing excessively low levels of fat is not synonymous with health.
The true goal should be to achieve a healthy, functional, and sustainable body composition, accompanied by good habits and a balanced relationship with food and one's own body.
Nazira Docmac
Integrative, Orthomolecular and Bioregulatory Nutritionist
