= Solution
\b[Exhaled carbonic acid here means <carbon dioxide>; its amount per unit time must be distinguished from its percentage in expired air.] A person can breathe a larger volume of more dilute expired gas and still exhale more total <carbon dioxide>.
At metabolic steady state, pulmonary <carbon dioxide> output approximately reflects its production by <cellular respiration>. Increased muscular activity is therefore a major physiological cause of increased output: walking, climbing and strenuous exercise increase substrate oxidation and usually increase both ventilation and total <carbon dioxide> elimination. Rest lowers the requirement; during ordinary sleep, reduced activity and metabolic demand generally lower output relative to the waking active state. A comparison needs the same time interval and should not confuse a short transient with a sustained resting measurement.
Food affects output in two ways. Digestion, absorption and processing increase metabolic expenditure after a meal, with a time course depending on the meal. The material oxidized also matters. The <respiratory quotient> is the ratio of metabolic <carbon dioxide> production to <oxygen> use. For complete oxidation of <glucose>,
$$
\mathrm{C_6H_{12}O_6}+6\mathrm{O_2}\longrightarrow6\mathrm{CO_2}+6\mathrm{H_2O},
\qquad \boxed{\mathrm{RQ}=1}.
$$
A typical <fatty acid> gives less <carbon dioxide> per unit <oxygen> consumed; for <palmitic acid>,
$$
\mathrm{C_{16}H_{32}O_2}+23\mathrm{O_2}\longrightarrow16\mathrm{CO_2}+16\mathrm{H_2O},
\qquad \boxed{\mathrm{RQ}=\frac{16}{23}\simeq0.70}.
$$
These are substrate comparisons, not a rule that any high-fat meal immediately reduces a person's total daily output. Fasting changes fuel selection as well as metabolic rate, so both processes must be considered.<Anatomy and Physiology 2e by J. Gordon Betts and others/24.5 Metabolic States of the Body>[Absorptive and Postabsorptive States: storage, mobilization and changing fuel use]{ref}
Cold exposure increases output when maintaining body <temperature> requires extra <thermogenesis>, especially <shivering>. Moving from cold to a thermally comfortable environment can reduce that extra production. Heating does not simply increase or decrease output in proportion to air <temperature>: passive warming, heat stress, activity and the thermoregulatory response differ. Growth, body size, age, pregnancy and lactation can change whole-body metabolic requirements. Comparisons between people therefore need to specify whether output is absolute or normalized to <body mass> or another appropriate size measure. Biological sex alone is not an explanation independent of these factors. Daily variation also mixes circadian physiology with sleep, meals and activity.
Ventilatory changes require a separate account. Voluntary <hyperventilation> can initially wash <carbon dioxide> out of body stores faster than it is being produced; breath holding temporarily prevents its pulmonary escape while production continues. Subsequent recovery changes the output again. The ratio measured at the mouth, the <respiratory exchange ratio>, need not equal the tissue <respiratory quotient> during those transitions or during intense exercise with acid buffering. Reduced expired output for a few seconds is therefore not evidence that tissue <metabolism> has stopped.<Physiology, Carbon Dioxide Transport (StatPearls)>[Carbon-dioxide production, transport in dissolved and bicarbonate forms, and pulmonary elimination]{ref}
A measurement should collect total expired gas over a known time and determine its dry <carbon dioxide> fraction, allowing for inspired <carbon dioxide>. With inspired and expired dry gas volumes expressed under the same reference conditions,
$$
\boxed{\dot V_{\mathrm{CO_2}}=\dot V_EF_{E,\mathrm{CO_2}}-\dot V_IF_{I,\mathrm{CO_2}}}.
$$
Here $\dot V_E$ and $\dot V_I$ are expired and inspired volume rates and the $F$ terms their respective fractions. Recording food intake, physical activity, ambient <temperature> and time since the preceding intervention makes the comparison interpretable. <Carbon dioxide> output is a measured flux, not merely an observation that an animal makes a gas collection vessel cloudy.
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