Lipids and the Body

  With autumn approaching, appetites increase. We all know that overeating can lead to obesity, high blood pressure, heart disease, and stroke. In particular, excessive intake of fats (chemically called lipids) is important. However, lipids are also essential for the body. Our body’s surface is covered with skin cells, each of which is surrounded by a cell membrane. The cell membrane’s main component is lipids, particularly phospholipids. This membrane, lined with these lipids, acts as a barrier to water and various substances from the outside world. Nutrients and drugs must pass through the membrane-permeable proteins embedded in the lipids, which selectively allow the passage of substances and ions.

   In addition to membrane lipids, the body also contains lipid-derived substances, such as vitamins A, E, D, and K. However, the most important role of lipids is to store the energy needed for bodily function. Food contains sugars, such as starch, which are energy sources, while meat, fish, and milk contain fats. We survive by ingesting these nutrients, but it is generally difficult to obtain the necessary energy from our diet, so a certain amount of surplus energy is always stored as neutral lipids in fat cells under the skin and in our internal organs. When there is danger or trouble around us, we use our muscles and brains to deal with it. When danger is detected, the sympathetic nervous system becomes excited and adrenaline is secreted from the adrenal glands. This adrenaline acts as a signal to bind to the liver and fat cells, where the lipids stored in lipid globules are broken down by an enzyme called lipase to produce fatty acids, which are then released into the blood. The fatty acids in the blood enter the brain and muscle cells, where they are broken down to synthesize the energy source ATP. ATP is the driving force behind brain and muscle function, allowing us to deal with danger.

    Let’s take a closer look at how it works. Lipids are substances that store energy, but where is it stored? As shown in the diagram, lipid molecules, and in particular the neutral lipids that serve as energy sources, are made up of a group of over 10 carbon atoms called fatty acids bonded to glycerin. The majority of the subcutaneous and visceral fat in the body is made up of neutral lipids. Fatty acids, which are made up of aligned carbon atoms, are poorly soluble in water and gather together to form clumps called micelles. Fat droplets are giant micelles.

Within a fatty acid molecule, more than ten carbon atoms are lined up and bonded to each other. Energy is stored in these carbon atom bonds. When these carbon linkages (chemical bonds) are broken down by the action of enzymes, energy is released. After decomposition, the carbon atoms combine with oxygen atoms and are excreted from the body as carbon dioxide. The released energy is then re-stored in the bonds between phosphorus and oxygen atoms in ATP molecules using machinery within the mitochondria. ATP with stored energy is used to perform tasks in the nerve cells and muscles of the brain.

    Here, we will introduce the results of a study by H. Yang et al. (H. Yang et al., LYVAC/PDZD8 is a lysosomal vacuolator, Science (2025) Aug. 21 issue, pp. 1-16) as an example of what new information is being learned about these lipids at the cutting edge of medicine and biology. Within cells, there are numerous intracellular membrane structures made of lipids, one of which is called a lysosome. These sac-like structures contain numerous enzymes that break down proteins, lipids, and carbohydrates. When cells ingest external invaders, such as viruses, they fuse with lysosomes and destroy them. In this way, lysosomes function as cleaning factories. Therefore, as the amount of foreign substances increases and degradation progresses, the breakdown products accumulate within the lysosomes, causing water to flow in from the outside and expanding. This expansion requires an increase in the lysosomal membrane, which adds new lipids, a membrane component. According to research by H. Yang et al., lipids for the lysosomal membrane are supplied by the endoplasmic reticulum membrane, which is connected to the nuclear membrane. Therefore, contact between the lysosomal membrane and the endoplasmic reticulum membrane is observed. In their research, they discovered a special protein at this contact site and further revealed that within this protein molecule is a structure that acts as a tunnel through which water-insoluble lipids can move from the endoplasmic reticulum to the lysosome. This discovery is extremely interesting, and the protein has been named “Lysosomal vacuolator” based on this finding. Increased membrane size is also found in many membrane structures within cells, including cell membranes, and the discovery that a protein exists that allows lipids to pass through suggests future developments. Detailed research is being conducted at the molecular level into the mechanisms underlying bacterial and viral infections in the body’s cells. Further elucidation of the mechanisms of lipid passage is expected to lead to the discovery of new drugs.