Fats

Fats are the most calorie-dense macronutrient — 9 kcal/g by Atwater factor, more than twice the density of protein or carbohydrate. Dietary fats include triglycerides (the bulk of food fat), phospholipids, sterols (cholesterol), and fat-soluble vitamins A, D, E, and K.

What dietary fat does in the body

• Energy substrate. Fat-derived acetyl-CoA enters the citric acid cycle; a gram of fat yields more ATP than a gram of carbohydrate or protein.
• Structural. Phospholipid bilayers, myelin sheaths, and steroid hormones (testosterone, estrogen, cortisol) all derive from dietary and endogenous lipid metabolism.
• Absorption of fat-soluble vitamins. A near-zero-fat meal limits absorption of vitamins A, D, E, K.
• Signalling. Fatty acids act as signalling molecules via PPAR receptors and other pathways.

Fat sub-categories

• Saturated fat (SFA): no double bonds. Abundant in animal products, tropical oils.
• Monounsaturated fat (MUFA): one double bond. Olive oil, avocado, nuts.
• Polyunsaturated fat (PUFA): multiple double bonds. Omega-3 (fish, flax, chia) and omega-6 (most seed oils) fall here.
• Trans fat: synthetic or naturally-occurring partially-hydrogenated fats. Artificial trans fats are banned or restricted in many jurisdictions on cardiovascular-risk grounds.
• Medium-chain triglycerides (MCT): shorter carbon chains, metabolised differently; some niche use in ketogenic and clinical contexts.

Intake recommendations

• Floor: roughly 0.5 g/kg bodyweight per day to support hormonal health and fat-soluble vitamin absorption.
• Typical range: 20–35% of total calories. The US DRI sets 20–35% as the "acceptable macronutrient distribution range" for adults.
• Ketogenic protocols: 65–75% of calories from fat, by design.

Fat and thermic effect

Dietary fat has the lowest thermic effect of any macronutrient — 0–3% of calories spent on processing. This is sometimes cited as "fat is easier to store" content, and there is a grain of real physiology in it: with everything else equal, calories from fat are more efficiently stored as body fat than calories from protein. In practice, the effect at realistic intakes is small compared to total caloric balance, and fat intake supports satiety and palatability in ways that make long-term adherence easier.

Tracking considerations

Fat grams in consumer databases tend to be reasonably accurate for whole foods. The main error source is hidden fat in restaurant and takeout meals — cooking oils, finishing butter, emulsified sauces — which routinely add 150–400 kcal not captured by naïve portion estimation. Scale-logging a dish after preparation captures the weight but not the macro split; the split must come from a database entry or a thoughtful estimate.

The "low-fat is dead" framing, nuanced

The low-fat orthodoxy of the 1980s and 1990s is no longer supported by major evidence-based guidelines. But the replacement is not "eat as much fat as you want" — it is "fat quality and source matter more than total fat percentage, and total calorie balance still drives weight outcomes." A Mediterranean-style diet at 40% of calories from fat is well-supported for cardiovascular outcomes; an industrial-ultra-processed diet at the same fat percentage is not. Source and food matrix matter.