What does 'count their calories' mean?

[Hoanghuy]

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #315 "Health and Nutrition Questions Test (1)", question 9

When some people go on a diet, they count their ......... every day.

(a) chopsticks
(b) digestion
(c) calories

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #315 "Health and Nutrition Questions Test (1)", answer 9

When some people go on a diet, they count their calories every day.

Correct answer: (c) calories

Your answer was: incorrect
When some people go on a diet, they count their chopsticks every day.
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I do not understand, please explaines for me: Why and How counting the calories everyday in people who is a diet

Hoanghuy

[Torsten]

When somebody wants to reduce their weight, the "go on a diet" This means they select their food carefully paying special attention to their calorie consumption. In other words, they 'count their calories'.
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