Fats: building blocks, types, and why the old classification is crumbling
For decades a simple rule applied: saturated bad, unsaturated good. Research has since become more differentiated — and in places more open than the brevity of the rule suggested. An overview of structure, types and the current state.
Structure
Dietary fats consist predominantly of triglycerides: a glycerol backbone with three fatty acids attached. The properties of a fat follow almost entirely from those fatty acids.
Fatty acids are carbon chains of differing length. What matters is whether and how many double bonds they contain. Without a double bond a fatty acid is called saturated, with one monounsaturated, with several polyunsaturated. This structure determines whether a fat is solid or liquid at room temperature and how sensitive it is to heat and oxygen.
No natural fat consists of only one kind. Butter also contains unsaturated fatty acids, olive oil also saturated ones. Assigning a food to a category describes an emphasis, not a pure form.
Why fat cannot simply be left out
Fat is the most energy-dense of the three main nutrients at around nine kilocalories per gram — roughly double that of carbohydrate and protein. That makes it a compact energy source and explains why fat-rich foods deliver a lot of energy quickly.
Beyond that, fat performs tasks that other nutrients cannot cover. Cell membranes consist substantially of fat components. Certain signalling molecules are formed from fatty acids. And vitamins A, D, E and K are absorbed only in conjunction with fat — a completely fat-free diet is therefore not a sensible option.
Two polyunsaturated fatty acids cannot be produced by the human body: linoleic acid and alpha-linolenic acid. They have to be taken in through food and are therefore called essential.
Saturated: the rule becomes more differentiated
The long-standing recommendation to limit saturated fatty acids in favour of unsaturated ones rests on observations about blood lipid profiles. Those observations hold up; what is disputed is how far predictions can be derived from them.
More recent reviews point out that the nature of the replacement is decisive: what saturated fatty acids are replaced with changes the outcome considerably. Individual saturated fatty acids also behave differently, and the effect of a food cannot be derived from its fatty acid pattern alone — the overall structure plays a part.
The honest summary is therefore: the blanket division into “good” and “bad” does not do justice to the data, without an all-clear following from that. Professional bodies mostly maintain a limit but phrase it more cautiously than before.
Omega-3 and omega-6
The names describe where on the carbon chain the first double bond sits. The omega-6 group includes linoleic acid, abundant in many plant oils. The omega-3 group includes alpha-linolenic acid from plant sources such as linseed and rapeseed oil, as well as the longer-chain forms EPA and DHA, found mainly in oily sea fish.
The body can convert alpha-linolenic acid into EPA and DHA, but only to a limited extent; the conversion rate in humans is low and varies between individuals. That is why plant and marine omega-3 sources are treated separately in dietary recommendations.
On the much-discussed ratio between omega-6 and omega-3 the evidence is less clear than the figure’s popularity suggests. Whether a particular ratio should be aimed at, or whether absolute amounts matter more, continues to be debated.
Trans fatty acids
Trans fatty acids arise during the industrial hardening of plant oils and in small amounts naturally in the digestive tract of ruminants. They are the case where the evidence is unusually clear — accordingly, numerous countries including the EU have set maximum levels for industrially produced trans fatty acids in food.
Through this regulation the amounts in processed foods have fallen markedly. In practice this means the point demands less attention today than it did twenty years ago.
Heat and storage
The more double bonds a fatty acid has, the more sensitive it is to heat, light and oxygen. Polyunsaturated oils are therefore less suited to strong heating and should be stored dark and cool. Oils with a higher share of monounsaturated or saturated fatty acids are more robust in this respect.
The practical cue is smell: rancid oils smell distinctly different from fresh ones. An oil that smells unpleasant after long storage no longer belongs in the pan.
How much fat?
Professional bodies state ranges as a share of total energy, not fixed amounts. There is a reason: the sensible share depends on how much energy is taken in overall and how the other nutrients are distributed.
In practice, fat follows from food choice for most people rather than from a calculation. Anyone planning a split usually sets total energy and the protein share first, then distributes the rest between carbohydrate and fat — along what fits everyday life and personal preference.
A very low-fat diet over a longer period carries drawbacks, because essential fatty acids and fat-soluble vitamins are affected. Where exactly the lower limit lies cannot be stated in general.
The key points
- The properties of a fat follow from its fatty acids — no natural fat consists of only one kind.
- Fat provides around nine kilocalories per gram and is needed for absorbing vitamins A, D, E and K.
- The body cannot produce linoleic acid and alpha-linolenic acid itself.
- The simple “saturated bad, unsaturated good” division no longer does justice to today’s evidence.
- Conversion of plant omega-3 fatty acids into the longer-chain forms is limited in humans.
- For trans fatty acids the evidence is unusually clear; the EU has set maximum levels.
- The more double bonds, the more sensitive to heat, light and oxygen.
Last reviewed: 14.08.2026
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