'Graduating with a Doctoral degree (PhD) is easier than gaining weight,' I confessed to my Research Supervisor at twenty-seven. Despite being lively and healthy, my slender stature was a concern for everyone except my doctors, parents and me.
Encounters made for the first time (with total strangers!) would be 'humble' unsolicited tips (free of charge) on how to gain weight. Through this ordeal, my 'well-wishers' failed to understand how I could feast without worrying about adding a few points to the scale.
At thirty, this body naturally gained 10 kilograms (22.05 lbs). Yet, according to the Body Mass Index (BMI) for Indian populations, I am still underweight! Undeterred by this finding, I remain confident. Read more on why BMI has limitations.
Still getting used to fitting perfectly into my previously baggy clothes, I noticed one day that my left leg felt more tightly squeezed into my 'skinny' jeans than my right. My partner reassured me that my left leg wasn't swollen (as presumed by me) and looked exactly like my right.
Have you ever wondered how you are so symmetrical? How does all that extra weight gained as fat accumulate proportionally on both our left and right sides? While several articles online tackle the first question, almost none address the second.
Don't you think there must be some factor controlling 'symmetrical' fat distribution? If not, we would have blobs of healthy fat accumulating in all the wrong places, making us look like the 'Bacteria Monster' from the cartoon SwatKats (nostalgia for millennials?).
In this article, I address How body fat is distributed symmetrically.
Getting Started
1) What Does Symmetry Mean in Human Beings?
Humans are bilaterally symmetrical. Imagine you have a clay moulded into a human figure. You draw a line down the middle of its body, from the top of its head to the tips of its toes. You cut the human mould along the line drawn into two halves. The left half mirrors the right half, hence bilaterally symmetrical.
2) Where is Body Fat Stored in the Human Body?
Say your caretaker packed you a box of sweets in case you get hungry or need an energy boost. When you want to re-energise, what would you do? You would go for the food reserve, those sweets in the box.
Similarly, the human body has energy reserves. Fat is stored within box-like, specialised cells called adipose cells or adipocytes. These fat cells are present in various body parts: Under the skin (subcutaneous), between muscles, surrounding organs (including the heart), in the abdomen, thighs, upper arms and buttocks.
When the body urgently needs energy, the fat reserves are metabolised (broken down), releasing energy. Read more to understand how energy reserves (adipocytes) are metabolised during starvation.
Did You Know? Interesting Facts on Energy Reserves
The largest energy reserves are the subcutaneous adipose tissue (a collection of cells).
The amount and distribution of the subcutaneous adipose tissue determines the body's shape.
Factors that Determine the Location and Pattern for Fat Distribution
Do you agree that the overall body fat distribution or accumulation pattern is visually different in a man from a woman? What causes this difference in fat distribution?
Did You Know? Surprising Facts About Fat Accumulation
A woman accumulates more fat subcutaneously (under the skin) in the upper arms, hips, thighs and buttocks. This pattern in fat accumulation is termed the peripheral fat distribution.
A man accumulates more fat viscerally or in the abdomen.
Age and Sex Hormones: Their Role in Fat Distribution
Estrogen and testosterone are sex hormones present in both males and females. Yes! Both these sex hormones are in you. However, the proportion shall vary. If you are a female, you have more estrogen (primary sex hormone) than testosterone. If you are a male, you have more testosterone (primary sex hormone) than estrogen.
How Does Estrogen Affect Body Fat Distribution?
Imagine you have a box full of sweets. However, the box is locked. You cannot open the box without the key. With the box closed, you cannot access the stored sweets inside to re-energise yourself.
Let us relate to the above analogy. The 'box' is the adipocyte, the 'sweets in the box' is the accumulated fat in the adipocyte, the 'key' is the estrogen and the 'key-hole' is the estrogen receptor (estrogen binds to it).
Now, a tricky situation: You are low on energy, have the key and want to savour the sweets. However, there are multiple boxes, and not all boxes open with the key you have. You can eat sweets only from those boxes that open with the key. The more the boxes opened, the lesser the overall sweets retained.
With the analogy explained, here's how estrogen manages body fat distribution. The number of estrogen receptors (ERα) influences estrogen's access to adipocytes (fat cells) with accumulated fat and its breakdown. In women, fatty tissues in the abdomen region have higher ERα receptors. On the other hand, lower in the thigh and buttock regions.
With lower ERα receptors, estrogen cannot access the fat for metabolism, leaving the fat accumulated in the fatty tissues.
Did You Know? Affected Estrogen Levels Alter Fat Distribution Pattern
In women with polycystic ovarian syndrome (PCOS) and postmenopausal women (period has stopped permanently), due to affected estrogen levels (lower levels), the fat accumulation or distribution patterns are similar to that in men.
More abdominal fat accumulation. The access (open) to the fat-accumulated adipose tissues is lower due to low estrogen levels (though the ERα receptors are higher).
How Does Testosterone Affect Body Fat Distribution?
Think of testosterone as the caretaker who packs your lunch (in a box) in the right proportion. However, in the absence of the caretaker, the lunch packed is always in excess (is it not?).
Similarly, testosterone controls the fat packing (fat accumulation) in the adipocytes (box-like cells) and breaks down fat through testosterone receptors. Due to lower testosterone receptors in the abdominal adipocytes, the influence of testosterone is lower in the abdominal region than in the thighs (higher testosterone receptors).
Did You Know? Altered Testosterone Levels Affect Fat Accumulation Pattern
Women with PCOS have higher testosterone. They exhibit a male fat distribution pattern.
Men in their lowest testosterone levels (after 70 years) have excess abdominal fat accumulation.
So far, the active roles of estrogen and testosterone in regulating fat accumulation in specific body regions in women and men through receptor-induced metabolism have been explained.
How is Fat Distributed Symmetrically on Both Sides of the Body?
To answer this question, one must understand how humans are bilaterally symmetrical.
Just like a building layout is planned into a blueprint, the body layout of an organism is determined by a genetic blueprint. Also, just like an engineer oversees the execution of the plan, specific genes (HOX genes) direct the body plan.
During embryo development, HOX gene proteins (like a controller) regulate the expression of other genes that form specific structures (body parts) when expressed.
By switching genes on (genes expressed) and off in a developing embryo, HOX gene proteins control the development of tissues and organs in the correct location and orientation, contributing to bilateral symmetry. Learn more about 'Homeotic Genes and Body Patterns.'
With the role of HOX genes understood, let's explore why your left leg is an exact mirror image of your right. Why is one leg or an arm not bulkier than the other?
For fat to be distributed symmetrically, the storehouse of fat, the adipose tissue or adipocytes have to be distributed and positioned symmetrically.
Imagine a person assigned the task of arranging boxes within the outline of a human figure. The person places each box in a pattern that perfectly mirrors both sides. These boxes would later be filled with sweets.
Let us relate to the above analogy. HOX gene is the 'person', the fat tissue is the 'box,' and fat is the 'sweet.'
As mentioned earlier, the HOX gene is a developmental gene that controls the development of tissues and organs in a developing embryo while maintaining a bilaterally symmetric body plan. The same gene (HOX gene) meticulously orchestrates the developmental pattern of 'box-like' adipocytes (adipose tissue) for a symmetrical fat distribution on both sides of the body.
Recap: Bilateral Symmetry in Fat Accumulation
As a developing embryo, my HOX genes meticulously orchestrated the expression of genes to create a bilaterally symmetrical body, including my legs. The same genes also regulated the development of fat tissues in specific locations, ensuring that fat would be deposited symmetrically on both the left and right sides of my body.
The mystery is solved if you are still wondering why my left leg felt more tightly squeezed into my skinny jeans than my right. Two words and that is manufacturing defect.
Have you ever wondered how your body maintains its symmetry? How much did you know about fat distribution patterns? What did you think of this article? Leave a comment below to let me know!
If you enjoyed this article, please share it on your social media. The more readers, the better the dissemination of information and the greater the motivation for me to keep writing!
References
Ahn, J., Wu, H., & Lee, K. (2019). Integrative analysis revealing human adipose-specific genes and consolidating obesity loci. Scientific Reports, 9(1), 3087.
Frank, A. P., de Souza Santos, R., Palmer, B. F., & Clegg, D. J. (2019). Determinants of body fat distribution in humans may provide insight about obesity-related health risks. Journal of lipid research, 60(10), 1710–1719.
Yuhasz, M. S. (1982). Body fat patterning of the subcutaneous adipose tissue. Anthropologiai Közlemények- Anthropological Communications, 26(1-2), 163-173.
Very interesting facts. Waiting for such blogs...
Fascinating! First of all, good for you for mentioning gaining weight. That's hardly mentioned, yet people do have problems with it. And I never thought about fat distribution, or much about body symmetry at all. Very interesting.
Wow! This was such a great and informative read. I've honestly never thought about this too much before, but now that I've read this, I find it SO interesting! Our bodies are amazing. Thank you for taking such a complex topic and breaking it down in a way to easily understand.
I love the way you explain complex things in a simple way, especially for someone who has virtually no knowledge of medicine or the human body. I particularly liked the picture of the sweets in the box. I now know how fat is symmetrically distributed in our bodies.
Thank you for sharing this informative article.
Smruthi has a talent to make complex concepts in short stories. I found the article engaging and crisp. The flow was smooth and the illustrations cute and clear. Waiting for the next one.
Best wishes
Smitha Hegde