Carbohydrate
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Carbohydrates are not strictly essential in the same way protein and fat are — the body can produce glucose through gluconeogenesis from protein and fat. However, carbohydrates are the most efficient fuel for the brain, high-intensity exercise, and many cellular processes. For most active adults, eliminating carbohydrates unnecessarily limits performance and food variety without clear benefit.

Going below 50g/day triggers ketosis — the liver converts fat to ketone bodies for fuel. Initial effects include water weight loss, fatigue, and mental fog (keto flu), which typically resolve in 2–4 weeks. Long-term low-carb diets may reduce high-intensity exercise performance since fast-twitch muscles rely on glycolysis. For sedentary people, very low carb can work well; for athletes, some carbohydrate is generally beneficial.

Both have value. Pre-workout carbs (1–4 hours before) top up muscle glycogen for sustained performance. For sessions under 60 minutes, carbs aren't strictly necessary pre-workout if you've eaten adequately that day. Post-workout, consuming 1–1.5g/kg of carbs within 30–60 minutes maximizes glycogen resynthesis — critical for athletes training twice daily or on consecutive days. For most gym-goers training once daily, total daily carbs matter more than timing.

Carb cycling involves alternating high-carb days (training days) and low-carb days (rest days). The theory is to have carbohydrate available when needed (training) while spending more time in a state of higher fat oxidation (rest days). Research is mixed on whether carb cycling provides meaningful advantages over consistent moderate-carb intake. It works well for some people psychologically and can improve dietary adherence by allowing higher-carb meals on training days.