Detoxification symptoms
After a time of having eaten foods, a fast will result in a natural cleansing which the body makes to remove accumulated waste and toxins that originated from food, water, air, and also begin a cleanse of the emotional body (depression, anger etc). Various symptoms are usually felt, their intensity depending on how you have been eating previously, for how long, how gradually you begun the fast, and on other details as well such as how you proceed with the fasting. Most of these symptoms are not caused by the absence of food, but by the presence of food waste and other toxin that are now about to surface to be removed. You are thus likely to feel many accumulated problems that you did not know were there. Some symptoms include:

Nausea - toxins released into blood stream and other areas of the body for elimination
Shortness of breath - more oxygen is needed for removal of various toxins
Headaches - more water may be needed for detoxification, dehydration causes headaches
Muscle pains or cramps - various nerve toxins released from tissues
Gall bladder cleanse - gall bladder empties all of its contents into the stomach; bile, foam, stones, waste, bacteria
Liver cleanse - liver 'flips over' and empties all waste accumulated into it, waste may include crystalline stones
Kidney cleanse - kidneys may hurt due to dehydration (greater need of fluids for cleanse) and elimination of stones and other waste
Diarrhoea or vomiting - digestive system cleansing
Weakness - usually due to a temporary imbalance in the body with all the toxins felt as they are released for removal, creates needs of vitamins, fluids, oils or other materials that are easily found in foods
Palpitations - another symptom of weakness, mineral imbalances, blood imbalance

I do not think it wise to make a full cease of eating and drinking if coming from a recent past of 'average eating'. Especially for persons who have eaten average foods for several years, meaning anyone born and raised an 'average eating person' and has reached adult age. The body will usually need some time to finish the cleansing. Many ailments experienced during this time are likely to inhibit one's ability to sense or work with the aspects that are to activate breatharianism. In most cases, the body will still require some food or drink for cleansing purposes. Whether you want to reach a life of non-eating or not, it is wise to pay attention to the body's needs of additional water, oils, foods or other things it might need during the time of cleanse.

Always pay attention to the body's needs of food or drink, or any other of its needs such as rest. During a cleanse the body may need additional materials from external source (vitamins, water, foods..) to neutralise the toxins and waste that are also materials originating from external source. Not allowing the needed materials for cleanse may stress the body and can in some cases lead to an imbalanced state, in which breatharianism is at that time impossible and health may be at risk. I advise to make the cleansing and detoxifications period one which will include any food, drink or other, that the body may ask for. This allows the body to reach a healthy balanced state, in which breatharianism is more easily reached.

Symptoms of body tuning in to living on light
Some symptoms may appear that are caused when the body is adjusting itself to living on light.

Palpitations - the heart may feel different, however in this case definitely not caused by deficiency of minerals, proteins or other (as it can appear even while eating and not caused by any disease)
Shortness of breath - appears if your blood and body has adjusted itself to living on light, and you eat, the body quickly needs more oxygen for removing food materials and you feel the shortness of oxygen, until the blood re-adapts itself to a state of eating
Dizziness - may be felt for shorter periods of time, possibly due to blood-sugar levels adjusting

Symptoms of not doing well on non-eating
Various symptoms will appear if you are eating very little, yet not adjusting to breatharianism. In such a case it is not much different from starvation which would appear for the 'eating person' who is without eating for some time. There is a distinct difference between breatharian and eating person, and that difference is not "to be without eating". There are various initiations in the physical body and other aspects of the human. If these are not initiated, the body is unable to reach the alternative sources of nourishment. Some of the following symptoms may also in some cases be temporary symptoms in cases where breatharianism is initiating, but are also found in cases of starvation. Distinction must be made between the two.

Becoming too thin
Paleness
Tiredness
Emotional numbness
Difficulty of listening
Suffering
Heart weakness where pulse becomes dangerously high
Problems with teeth
Pinkish rash on skin
Feeling tight pressure from inside throat, chest or stomach (are caused by chakra problems, dehydration or other)
Problems with bad body odor (a strong smell, caused by body braking down its fats in order to get the water contained in fats, the odor being from the waste product of fats)
Thirst that is not cared for
Hunger that is not cared for
Collapsing, fainting
Difficulty standing, uncomfortable feelings in spine (due to dehydration in spinal fluids and other)
Fevers (can become extremely dangerous; breatharians would never have a fever!, fevers are caused by infections)
Sadness, depression

In such cases where breatharianism is not initiated, one must re-evaluate one's relations toward food. Never forget, in any case, to pay attention to the physical body's true physical needs of food or drink. Denial of hungers, or thinking that food is not needed, while the body is in fact in great need, is very unwise. Breatharianism is not reached by denials, it is reached by a conscious approach toward who you are. This consciousness will activate the body to live on light.