Fat Storage

Cells: The Fundamental Units of Life

  • Approximately 37 trillion cells in the human body.
  • Over 200 different types of cells in humans, each with specific functions.
  • Human cells are animal cells containing specific structures (organelles).

Important Cell Structures

  • Nucleus: Contains genetic material for protein production and cell functions.
  • Cell Membrane: Made of phospholipids (a type of fat), protecting the cell by regulating the entry and exit of substances.
  • Cytoplasm: The area where many chemical reactions occur, including the initial steps of glucose breakdown.
  • Mitochondrion/Mitochondria: The energy-forming site in the cell (ATP production from glucose and fats).
    • Cells have multiple mitochondria, varying in number depending on cell function.

Adipocytes: Fat Storage Cells

  • Adipocytes are cells specialized for storing fat (triglycerides).
  • Adipose tissue is formed when many adipocytes come together.
  • Adipocytes have a unique, almost perfectly spherical shape, unlike most cells.
  • They are large in size, even in lean individuals.
  • A large lipid droplet in the middle stores triglycerides, pushing other organelles to the side.

Adipose Tissue Structure

  • Adipocytes are held together by a mesh of collagen fibers, forming bag-like structures.
  • Collagen is a protein that provides structural support.

Fat Storage Mechanisms

  • Two ways to store excess fat:
    • Adding triglycerides into existing adipocytes, causing them to grow larger.
    • Synthesizing new adipocyte cells to store triglycerides.
  • The number of adipocytes in an average adult is about 50 billion, but varies among individuals.
  • Once new adipocytes are formed, they persist for a long time and do not disappear quickly.
  • The method of fat storage depends on where the triglyceride is stored in the body.

Study on Overeating (2010)

  • A study with 28 healthy adults in their late 20s examined the effects of overeating.
  • Participants gained an average of 4 kg over eight weeks.
  • Nearly 3 billion new fat cells formed in the legs, but not significantly in the abdomen.
  • Weight gain in legs and buttocks involves new fat cell formation, while the abdomen uses existing cells.
  • This difference may explain why it's harder to lose weight from the legs and buttocks.

Adipocyte Lifespan and Turnover

  • Adipocytes do not disappear when you lose weight; they shrink as triglycerides are used for energy (released as carbon dioxide).
  • Adipocytes have a finite lifespan and die after about ten years.
  • During this period, they grow and shrink repeatedly.
  • Weight loss during this time reduces the need for new fat storage, leading to fewer new adipocytes formed when old ones die.
  • All fat cells in the body are renewed within a ten-year period.
  • In obesity, the death rate of fat cells can be 30 times higher due to inflammation of adipose tissue.
  • This inflammation is caused by the inelastic collagen mesh surrounding enlarged adipocytes.
  • Prevention of weight gain is better than trying to cure it due to potential complications.

Fat Deposits in the Body

  • Two main fat deposits:
    • Subcutaneous Fat: Located under the skin, covering the entire body.
    • Visceral Fat: Surrounds internal organs, providing protection against shock.
  • Smaller fat deposits are also found in other areas.

Apple vs. Pear Shaped Bodies

  • Apple Shaped Bodies: Excessive fat stored in the abdominal region (mostly visceral fat).
  • Pear Shaped Bodies: Fat stored around the buttocks and legs (mostly subcutaneous fat).
  • Apple shaped bodies are less healthy due to the hormonally active visceral fat, which releases hormones that can cause inflammation, heart disease, and diabetes.
  • Visceral fat protects internal organs, but too much is harmful.
  • Subcutaneous fat releases hormones, but to a lesser extent than visceral fat.

Other Fat Deposits

  • Fat pads under the balls of feet (heels) protect against impact when walking.
  • Fat deposits in knee joints aid in walking.
  • Fat in baby cheeks fills space between jaw muscles, aiding the sucking mechanism for milk.

Types of Adipose Tissue

  • White Adipose Tissue (WAT): For energy storage (triglycerides).
    • Releases stored fat during starvation, fasting, or exercise for energy.
  • Brown Adipose Tissue (BAT): Primarily for producing heat (thermogenesis).
    • Shivering heat production (muscle contraction).
    • Non-shivering heat production (BAT).
  • BAT is important for maintaining body temperature at 37 degrees Celsius.

Distribution and Structure

  • WAT is present throughout life and is the largest volume of adipose tissue.
  • BAT was initially thought to be mainly in infants but is also found in adults in smaller amounts.
  • WAT is distributed throughout the body, including subcutaneous and visceral regions.
  • BAT is mainly found around the collarbone and chest area.
  • WAT has a large lipid droplet, pushing organelles aside.
  • BAT has multiple small lipid droplets and contains many mitochondria to break down fats for heat production.

Brown Fat and Heat Production

  • High concentration of mitochondria in brown fat gives it a brownish appearance due to iron content.
  • Babies are born with more brown fat to maintain body temperature.
  • Animals that hibernate also have large amounts of brown fat.
  • Exposure to cold weather activates brown fat.

Mechanism of Heat Production

  • Glucose and fatty acids are broken down to make energy in the form of ATP through respiration.
  • Brown fat makes some ATP but also produces heat energy.
  • Brown fat cells need to break down more fatty acids to make the required ATP, resulting in heat production.
  • Brown fat helps use up more stored fat, aiding in weight loss.
  • Brown fat is associated with lower BMI and lower body fat percentage.

Recent Discoveries

  • Beige Adipocytes: Display characteristics of both brown and white adipocytes.
    • Emerge within subcutaneous white adipocytes.
    • Formation can be induced through diet, exercise, pre/post-biotics, pharmaceutical agents, and adipokines.
    • Seem to have a protective effect against obesity.
  • Pink Adipocytes: Found only in rodents so far.
    • Originate from subcutaneous adipocytes.
    • Occur during late pregnancy or lactation in female mice.
    • Have an epithelial cell appearance with villi.

Adipokines: Signaling Molecules

  • Adipokines are signaling molecules (e.g., hormones) produced by adipose tissue.
  • They can have local effects on adipose tissue or travel to other parts of the body.
  • Involved in inflammation, metabolic pathways, insulin sensitivity, and immune function.

Leptin

  • Leptin is a hormone produced by the OB gene.
  • Acts as an appetite suppressant, signaling the brain to indicate fullness.
  • Empty fat cells secrete less leptin; full fat cells secrete more.
  • In obesity, leptin is highly expressed, but the body may become resistant to its effects.
  • Leptin binds to leptin receptors on the cell surface, targeting the brain, central nervous system, liver, and pancreas.
  • Congenital leptin deficiency (CLD) results in constant hunger and morbid obesity.
  • Synthetic versions of leptin can treat CLD, but do not work for obese individuals without the mutated gene due to leptin resistance.

Weight Loss Methods

  • Liposuction: Removes subcutaneous fat, but does not provide the health benefits of natural weight loss (decreased blood fat, insulin levels).
    • Does not remove harmful visceral fat.
  • Weight Loss Pills: Require careful understanding of the mechanism of action and potential side effects.
    • DNP (2,4-Dinitrophenol): An example of a dangerous weight loss drug that was discovered in the early 1900s.
      • It uncouples ATP production, leading to a dangerous increase in body heat.
      • Leads to death, blindness and organ failure.
      • The drug was taken off the market once they found out these detrimental health effects, and it was made illegal to sell.

Body Mass Index (BMI)

  • BMI is a measure of body fatness based on weight and height, invented in the early 1800s.
    BMI=Weight(kg)Height(m)2BMI = \frac{Weight (kg)}{Height (m)^2}
  • Indicates if a person is underweight, at a healthy weight, overweight, or obese.
  • Does not account for gender, age, or muscle mass.
  • In Australia, two-thirds of adults are overweight or obese (67% in 2017-2018).
  • Fat storage location is important for assessing disease risk.
  • BMI is not always accurate, especially for elite athletes, classifying some as overweight.
  • More accurate ways to assess body fat include skin fold thickness, underwater weighing, and dual-energy X-ray absorptiometry (DEXA), but these are time-consuming and expensive.