You are what you eat. How many of us are human garbage cans? Much could be written on the necessity of eating a proper diet, and We will dwell on this more than some other things.
In Tanquerey's The Spiritual Life: Gluttony is a grievous fault: a) when it ... incapacitates us for the fulfillment of our duties of state or ... for example, when it injures our health. This confirms what Saint Alphonsus said: It is also a sin willfully to produce any serious illness, by eating to excess, or by eating food injurious to health; for we are bound to preserve our life, and to avoid all danger of death. Consuming sugar is a form of gluttony, because the body doesn't need it and in fact works better without it.
Now what food is injurious to health and how much is too much? Some have tried to set limits to people's diets, when this is impossible. A 200 pound ditch digger is obviously going to need more food than a 100 pound old lady. So a limit cannot be set. We can observe, however, that many Americans eat too much, since the average American eats over 3,000 calories a day. (Remember this group includes everyone from newborn infants to 300 pound football players.) Obesity is indeed a great problem in the United States. Some of this is called by an improper combination of food; some by an excess of food. We can refer people to several books that will help people design a proper diet. There is one common problem, the consumption of refined sugar. Overconsmption of sugar causes hypoglycemia.
Hypoglycemia is like height, everyone has some., Dr. Atkins. Although some of his theories may be considered radical, much of his basic premise is quite sound. From what We have studied, and We have studied nutrition extensively, it certainly would not be sinful to follow the Atkins diet. But let us get to the main problem in the modern diet. The modern diet causes hypoglycemia, which in turn causes everything from headaches to heart trouble, to depression to suicide. (Yes, we know of a man, whose suicide was probably caused by hypoglycemia.) In fact the list of things directly connected to hypoglycemia would fill pages. Since this disorder causes so many illnesses, to encourage it is certainly sinful. Hypoglycemia is encouraged by sugar and alcohol. One doctor states that he has never found an alcoholic who wasn't also hypoglycemic. Since the average American eats a sugar bowl full of sugar every day, the average American is hypoglycemic.
We can consider eating too much sugar as sinful in two ways. First is gluttony. Gluttony is a disordered love of eating and drinking., Saint John Vianney. Sweets are supposed to be an occasional thing, reserved for feast days. For instance, we fix sweets for Christmas, Easter and our Baptismal day. When finished we leave them until the next feast day. Gluttony is normally considered a venial sin, unless it causes serious health problems. And this is the second way eating too much sugar is sinful. We are morally certain that eating too much sugar causes serious illnesses, and that continued consumption of too much sugar is certainly a mortal sin. It make take years to become mortally sinful, but eventually overconsumption of sugar becomes seriously sinful. The simplest recommendation to make is to eliminate sugar completely from your diet. If it is causing serious illnesses, this advice binds under pain of serious sin. We can provide more information for those who wish to restore their health.
The second problem with the modern diet is refined carbohydrates. Sugar is the worst refined carbohydrate, containing absolutely nothing of value to our system. In fact, some call it an anti-nutrient, because it saps our system of much needed nutrients. (No, it is not possible to supply these nutrients and still maintain a high level of sugar consumption and maintain health. In fact sugar could be considered a poison.) White flour is almost as worthless, although it is enriched with a few nutrients added back. The same may be said of white rice. Basically the best thing to do is to eliminate white rice, white flour and everything made from it and white sugar from our diet. However, this is normally difficult. True the sugar must go, but white flour and white rice maybe a different question. The simplest and least expensive thing to do is to purchase a wheat grinder, grind your own wheat and make your own bread and pasta. (Whole wheat bread in the store is not that much better than white bread.) Wheat can be purchased for $10 for fifty pounds and a grinder for a hundred dollars. How much do you spend in a year on bread, which is no more bread than this piece of paper is? At most this change would bind under pain of venial sin, and even this may be questionable. However, hypoglycemics should avoid all carbohydrates, since any carbohydrate can set off the hypoglycemic reaction.
What We strongly recommend is that all learn how to improve their diet, in order to improve their health. Although We should long for heaven, it is a sin to dig our graves with our teeth and get to heaven early by a bad diet. True some saints accelerated their death by abstinence, but this was inspired by God and no one got to heaven by gluttony or neglecting their health by eating the wrong things