During a fast, you may notice that your urine develops a stronger or simply different smell.
It can seem sharper than usual, and changes in colour or frequency may occur at the same time.
That can lead to the assumption that the smell is evidence of “toxins pouring out”. The reality is more useful — and more interesting — than that simple explanation.
Urine Is a Concentrated Waste Fluid
Your kidneys continuously filter the blood and regulate water, salts and many dissolved substances. Urine carries a range of metabolic waste products out of the body.
The concentration of those substances changes depending on how much fluid is available and what is happening metabolically.
Concentrated Urine Usually Smells Stronger
If you are passing less water, the compounds in your urine become more concentrated.
More concentrated urine is often darker and can have a stronger smell. This is one reason odour may become more obvious if fluid intake falls during a fast or if you are losing more water through sweating.
Fasting Changes Your Metabolism
When food intake stops, the body changes how it supplies energy.
As stored carbohydrate becomes less available, fat use increases and ketone production rises. Some ketones can leave through urine as well as through the breath.
These metabolic changes can contribute to differences in urine odour during fasting.
Why Does Urine Colour Change During a Fast?
Colour is strongly affected by concentration.
Pale urine generally contains more water, while darker yellow urine is more concentrated. However, colour can also be influenced by medicines, supplements, foods and some medical conditions.
That is why urine colour should be interpreted in context rather than used as a stand-alone measure of health.
Does Strong-Smelling Urine Prove You Are Detoxing?
Your kidneys are always involved in removing metabolic waste from the blood, whether you are fasting or eating.
Fasting changes metabolism and therefore changes some of the substances being processed and excreted. But smell alone cannot identify what is present in the urine or prove that a particular stored toxin has been released.
A strong smell is an observation, not a laboratory analysis.
What About Sediment or Cloudiness?
People fasting sometimes report cloudiness or visible sediment, particularly when urine is left standing.
Urine contains dissolved salts and other substances that can crystallise or become more visible as conditions change. Cloudiness can also have other causes, including urinary problems, so appearance alone should not be used to diagnose what the body is eliminating.
The Natural Hygiene Perspective
Natural Hygiene recognises elimination as a continuous part of normal physiology. The kidneys, bowel, lungs and skin are all involved in handling substances the body no longer needs.
During fasting, digestive demands fall and metabolism changes considerably. Urine may therefore change in concentration, frequency, colour and smell.
Those changes are worth observing, but they do not need exaggerated explanations. The body is doing real physiological work without us having to assign every colour or smell to a particular toxin.
When Should Strong-Smelling Urine Be Checked?
Seek appropriate medical assessment if a change in urine is persistent or accompanied by burning or pain when urinating, blood in the urine, fever, significant lower abdominal or back pain, inability to urinate, severe dehydration or feeling seriously unwell.
Fruity or unusual-smelling urine alongside severe thirst, frequent urination, vomiting, abdominal pain, deep or rapid breathing, confusion or marked weakness can require urgent medical attention, particularly in someone with diabetes.