How do salt, fat, fructose - make water?
What can we learn from camels, desert sand rats and cosmonaut experiments?
Fat provides water
Fat has two major functions. The first one, is to store calories for a later time when food is unavailable. The other major but lesser-known function of fat is to provide water. Take camel humps, for example. They are all fat-filled, not water. So how does one get water out of a lump of lard?
Metabolic Water
Water is also generated during fat and glycogen metabolism. Fat is anhydrous and contains only 10% water by weight, but when fat is oxidized, water and carbon dioxide are released. For every gram of fat metabolized, 1.12 ml water is generated. Liver or muscle glycogen also generates 0.6 ml water per gram of glycogen metabolized. Because glycogen is water soluble, it also releases potassium and water during metabolism, accounting for an additional 3 ml water per gram of glycogen metabolized. The marked diuresis after initiation of a low-carbohydrate diet is partially because of water released during glycogen metabolism. Although glycogen metabolism produces metabolic water, most organisms store more fat than glycogen. Thus, fat is the major source of metabolic water for most animals. …
Some obligatory water loss by the lung occurs during fat metabolism because of the need to excrete carbon dioxide that may counter the gain of water provided during fat metabolism. However, animals like camels have developed techniques to reduce water loss from their airways and skin. - R Johnson et al
Lessons learned from a desert sand rat
The desert sand rat Psammomys obesus, inhabits Northern Africa’s salty marshes and deserts. It mainly feeds on the stems of Salicornia - glasswort - its sap being filled with salty briny water.
Fructose production can be stimulated by dehydration, which then drives fat production and storage. A high-salt diet helps the sand rat convert the low amount of ingested carbohydrates, into fructose. This pathway helps the species survive when food and fresh water resources are low.
In contrast, when the rat is brought into captivity and given the standard lab rat chow consisting of 50% carbohydrates, obesity and diabetes rapidly develop. But if instead it is fed fresh vegetables, the rodent remains lean. Also, rodents with access to more water slowed their weight gain and prediabetes, even with sugar/ fat rich diets.
French Fries plus Soda Pop = Obesity
Salty crisps and high osmolarity fructose filled soda [no ‘free water’] stimulate glycogen and fat storage. These mechanisms seem to be linked, both are stimulated by dehydration and hyperosmolarity. Lack of hydration and excessive salt intake contribute to obesity.
The transport and metabolism of fructose occurs in only a few tissues - the liver, intestine, kidney, adipose tissue, and muscle - and insulin is not required. Most of the dietary fructose converts into glucose. It metabolizes predominantly in the liver, replenishing liver glycogen and synthesizing triglycerides. When sodium is high, these enzymes get rev’d up, driving the reactions towards more storage.
Russian Space Station Simulation Experiments - MARS-500 and SIRIUS19
SIRIUS (Scientific International Research in Unique Terrestrial Station) are studies of Russian cosmonauts, held in isolation to simulate space travel.
I lived for 120 days without sun, fresh air, friends, fresh vegetables or fruit, phone or the internet. 120 days amongst colleagues, that have since become family and friends, in a hermetically-sealed station that resembles a large metal barrel. 120 days performing over 80 scientific experiments in physiology and psychology of the mind under extreme conditions. - RBTH
Their data revealed that eating more salt made them less thirsty but somehow hungrier. And when this scenario was tested in mice, they found that they burned more calories when they ate more salt, eating 25% more just to maintain their base weight.
From a Russian space program of 135-day simulation of life on the Mir space station:
A striking finding emerged: a 28-day rhythm in the amount of sodium the cosmonauts’ bodies retained that was not linked to the amount of urine they produced. And the sodium rhythms were much more pronounced than the urine patterns.
The sodium levels should have been rising and falling with the volume of urine. Although the study wasn’t perfect — the crew members’ sodium intake was not precisely calibrated — Dr. Titze was convinced something other than fluid intake was influencing sodium stores in the crew’s bodies. - G Kolata. NYTimes
The researchers then varied the salt in their diet. As expected, when the crew ate more salt, they excreted more salt; the amount of sodium in their blood remained constant, and their urine volume increased. The surprising finding was fluid intake. Instead of drinking more when dosed with high salt, the crew drank less. So the body had produced the extra excreted water when salt intake was high. As for the hunger, urine tests showed an increasing levels of glucocorticoid hormones, such as cortisol.
And back to that study of lab mice: the more salt in their diet, the less water they consumed. They, too, increased their levels of glucocorticoid hormones, which can break down fat and muscle in their own bodies, freeing up the water for the body’s use. That catabolic process burns energy, stimulating the mice to eat more food on the high-salt diet. These adrenal hormones, too, may be behind that monthly cycle in body sodium content.
More fascinating and controversial research is provided by the Weston A. Price Foundation website. It definitely deserves a read for those seeking to unravel this Gordian Knot of understanding salt intake and determining what is best for you.
REFERENCES
R Johnson. Two surprising reasons behind the obesity epidemic: Too much salt, not enough water. August 22, 2022 https://theconversation.com/two-surprising-reasons-behind-the-obesity-epidemic-too-much-salt-not-enough-water-184128
R Johnson et al. Metabolic and Kidney Diseases in the Setting of Climate Change, Water Shortage, and Survival Factors. JASN August 2016, 27 (8) 2247-2256; DOI:https://doi.org/10.1681/ASN.2015121314 https://jasn.asnjournals.org/content/27/8/2247
SJ Dholariya; JA. Orrick. Biochemistry, Fructose Metabolism, StatPearls. October 25, 2021. https://www.ncbi.nlm.nih.gov/books/NBK576428/
G Kolata. May 8, 2017. Why Everything We Know About Salt May Be Wrong https://www.nytimes.com/2017/05/08/health/salt-health-effects.html
How to ENJOY isolation: Confessions of a cosmonaut trainee RBTH
12 May 2020.
Weston A. Price Foundation https://www.westonaprice.org/health-topics/abcs-of-nutrition/the-salt-of-the-earth/
Takashi Matsuda et al,
Two parabrachial Cck neurons involved in the feedback control of thirst or salt appetite,
Cell Reports (2023).
DOI: 10.1016/j.celrep.2023.113619