If you attended secondary school in West Africa, you probably remember the biology laboratory. Shelves were lined with bottles containing frogs, lizards, snakes, and other specimens suspended in a clear liquid. At the time, most schools did not perform dissections, but almost everyone remembers the smell. It was sharp, pungent, and capable of making your eyes sting. A few years later, many of us encountered that same unmistakable smell again, only much stronger. This time, it was in the cadaver room, where preclinical students in medicine, physiotherapy, nursing, and dentistry were learning human anatomy.
Interestingly, long before the biology laboratory and the cadaver room, the chemistry behind that unmistakable smell had been part of everyday life without attracting much attention. Almost everyone has experienced forgetting a pan of cooking oil on the stove until the kitchen fills with acrid smoke that stings the eyes and catches in the throat. Anyone who has lingered behind the exhaust pipe of a running vehicle has encountered another source of similarly reactive chemicals with their own unpleasant smell. These familiar experiences seem completely unrelated, but they all introduce us to members of the same chemical family known as aldehydes.
For most people, aldehydes belong to laboratories, industrial chemistry, cigarette smoke, vehicle exhaust, or burnt cooking oil rather than everyday biology. In reality, they are woven into the very fabric of life, continuously generated by every living cell as nutrients are transformed into energy. Under healthy conditions, these reactive molecules are produced, transformed, detoxified, and removed with remarkable efficiency as part of normal metabolism.
The question is, why does formalin, a diluted solution of an aldehyde, preserve the dead so effectively in biology laboratories and cadaver rooms? The answer is that aldehydes readily react with proteins and other biological molecules to form chemical cross-links that stabilize tissues and slow their breakdown. This property makes them invaluable for preserving biological specimens and cadavers for study. The same chemistry that protects a specimen from decomposition also explains why formalin has such a pungent smell and why it irritates the eyes and airways.
Fortunately, living cells are not jars of formalin. They possess sophisticated defense systems that continuously keep aldehydes under control. Enzymes such as aldehyde dehydrogenases, together with the glyoxalase system and glutathione-dependent pathways, work around the clock to transform and remove these reactive molecules before they accumulate. Life depends not on eliminating aldehydes but on keeping them moving through metabolism.
The story changes when metabolic flow begins to slow. In this case, nutrients continue entering the body, but their processing, oxidation, and disposal cannot always keep pace. As metabolic congestion develops, reactive intermediates linger longer than they should. One example is methylglyoxal, a highly reactive aldehyde produced during glucose metabolism. Similar reactive aldehydes can also arise during fat metabolism. These molecules are not foreign invaders or evidence that one nutrient is inherently worse than another. They are ordinary products of metabolism that become problematic when biochemical traffic becomes congested.
This is where the chemistry of the biology laboratory intersects with the chemistry of chronic disease. Formalin preserves tissue by chemically modifying proteins and making them resistant to breakdown. Chronic metabolic disease does not embalm the body, but persistent exposure to reactive aldehydes gradually modifies long-lived proteins throughout the body. Collagen in blood vessels becomes stiffer, basement membranes in the kidneys slowly thicken, and lens proteins in the eyes gradually lose their clarity. Over years and decades, these long-lived structures accumulate chemical scars that are difficult to erase because they are replaced only very slowly.
Normal physiology depends on two continuous forms of flow. Metabolic flow transforms nutrients into useful work while preventing reactive intermediates from lingering. Mechanical flow, generated through regular physical activity, preserves the flexibility, strength, and continual remodeling of long-lived tissues. Chronic disease often reflects the gradual failure of both.
The consequences become visible across many metabolic diseases. In diabetes, reactive aldehydes contribute to the gradual modification of proteins in nerves, kidneys, and small blood vessels. In fatty liver disease, they accompany oxidative stress and tissue remodeling. In hypertension, they participate in the gradual stiffening of arterial walls. In the aging brain and in chronic kidney disease, aldehyde-derived modifications accumulate alongside many other pathological processes. These conditions have different causes and different clinical features, but they often share one biochemical consequence. Reactive aldehydes and other reactive intermediates persist longer than healthy metabolism was designed to permit.
Therefore, chronic metabolic disease is less a story of one harmful molecule than of impaired metabolic flow. Metabolism continuously produces reactive intermediates because life depends on them. Health depends on transforming them as efficiently as they are formed. Sedentarism does not create aldehydes, but it allows them more opportunity to linger.
Perhaps the greatest surprise comes from plants. One of the first exportable, metabolically useful nutrients produced by life’s primary energy-capturing system is glyceraldehyde-3-phosphate, or G3P, an aldehyde. Long before humans understood metabolism, plants were already using this remarkably reactive chemistry to capture sunlight and build the molecules that sustain nearly all life. Aldehydes have therefore been part of biology’s energy story from the very beginning.
The chemistry that preserves the dead is also part of the chemistry that sustains the living. The body has evolved sophisticated enzyme systems to produce, transform, detoxify, and remove aldehydes because they are indispensable intermediates in metabolism. Chronic metabolic disease is not evidence that aldehydes are inherently harmful. Life’s energy story begins with an aldehyde, and chronic metabolic disease reminds us what happens when aldehydes stop passing through metabolism and begin lingering within it.
Mukaila Kareem is a doctor of physiotherapy and founder of metabolichealthliteracy.com
