IANAN (I am not a nutritionist) but I strongly suspect that the physical and mental exhaustion is primarily related to dehydration (as Mahdi Yusuf himself indicates by bolding "drink"). Looking at some of the comments here, the main difference between those who describe positive experiences with fasting and what the author describes is that the former drink water throughout the day. In that light, let me quote from a very good article [1]:

A normal average human body is made of approximately 60% of water. Water is the natural transport fluid involved in all important communication networks of the body. Drinking a lot of water (medium to low mineralization) ensures that the organs responsible for filtering and cleansing have sufficient carrying capacity and can eliminate toxins and wastes. You can see the argument as just plain good sense that we need sufficient water for dilution of wastes. Chemists and physicists know water as a truly exceptional fluid on many accounts. It is not by chance that life emerged and organized itself with water. Water is the best solvent that can dissolve an astonishing number of solid substances. Dehydration is a dominant cause of tiredness, pain and chronic diseases. Drinking about two liters per day plays marvel to keep energy and remove fatigue. Indeed, unknown to or often forgotten by the public is the fact that many chronic diseases may be associated with an insufficient intake of water. Such a simple gesture as drinking water regularly during the day may go a long way towards avoiding fatigue and remaining healthy.

I emphasize drinking pure water and not sodas, cokes, processed juices, coffees and so on. Just plain water. The drinks that are processed by agro-business are in general provided with added sweeteners that are known to promote malignant cell growth [Servan-Schreiber, 2009]. In addition, drinking with sugar (glucose, saccharine) prevents the body from strengthening its metabolic pathway of storing fat in time of surplus and burning fat efficiently in case of need. By feeding our body continuously with sweet drinks and sugar, we saturate our blood with sugar and we weaken considerably the metabolic processes of storing and retrieving sugar, making us more vulnerable to hypoglycemia in the rare cases where external sugar intakes stop. The consequence is to easily feel weak and tired. In contrast, letting the body be just flushed by plain water for hours ensures the build up of its metabolic capacities to burn fat efficiently. This is like muscle build-up by training. Art de Vany [2010] develops convincing supportive arguments for this. He correctly argues that our body is basically inherited from our hunter-gatherer evolution and we are thus adapted to strive in a patchy and varying environment for which our metabolism has derived efficient solutions to the energy flow problem. This backfires with our modern caloric and sugar rich, but nutritionally depleted, foods that are available at little expenditure of energy [de Vany, 2010], in the form of chronic diseases, an on-going so-called epidemic of obesity and many other modern so-called developed country diseases [Campbell and Campbell, 2006].

For the determination of the amount of needed water, a rule of thumb is to keep urine transparent. Do you feel a bit tired? Drink water. The effect is almost instantaneous. I constantly keep one or two liters on my office desk and drink when I feel like it and outside meals. I always carry water with me on trips. A minor nuisance is to drink it all before going through airport security._

_Of upmost importance is to drink our water outside meals. Most people use breakfast, lunch and dinner times to fill their body with both the liquid and solid nutrients that their bodies require. This is logical since meals are the times when we re-fuel our body. However, this behavior constitutes a fundamental mistake. Ponder this question: what other mammals in the whole animal kingdom drink their water together with their solid meals? None! We are the only one among about 5500 known mammal species who do it. The convenience of tools and the development of technology have put bottles on our meal tables to consume at the same time we ingest solid food. This apparent gain of civilization collides against a healthy diet for at least three reasons:

(i) Drinking lubricates and help swallow insufficiently chewed morsels; but digestion in the stomach requires the comminution of our food into particles that should be as minute as possible in order to maximize surface over volume ratio and therefore facilitate the digestive chemistry performed by the gastric secretions. This is just plain and simple good sense chemistry. When digesting unbroken food morsels, the stomach and the whole digestive system has to secrete more, takes more time to process our food, all this cumulatively increasing tiredness and fatigue on the body over the long run. I therefore recommend chewing so that you “drink your food”. Similarly, water and liquids should stay a while in the mouth before swallowing to warm up and mix with saliva so that you “eat your drinks”. A difficult digestion starting in the mouth is probably significant contributor to the feeling of tiredness after a meal.

(ii) Starch and other vegetable substances start their digestion with the help of enzymes found in the saliva; lengthy chewing ensures optimal chemical reactions with these enzymes and saves energy for the rest of the process in the stomach and intestines.

(iii) Ingested fluids dilute the stomach secretions, thus hindering the digestion process. Again, plain and simple good sense chemistry.

The article was originally written by a true scientist for his Ph.D. students. I think the physical dangers of our work is quite similar in that programmers and phd candidates or scientists will tend to get absorbed in front of a screen, sit for too long at a stretch, forget about or get negligent about food and drink, etc etc. I recommend this article.

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[1]	D. Sornette. Optimization of brain and life performance: Striving for playing at the top for the long run. Schweizer Monat, pages 38–49, Dec 2011.

(I believe the article is available on Arxiv.)