I have spent much of the last fifteen years reporting and writing about issues
of public health, nutrition, and diet. I have spent five years on the research
for and writing of this book alone. To a great extent, the conclusions I've
reached are as much a product of the age we live in as they are my own skeptical
inquiry. Just ten years ago, the research for this book would have taken the
better part of a lifetime. It was only with the development of the Internet, of
search engines and the comprehensive databases of the Library of Medicine, the
Institute for Scientific Information, research libraries, and secondhand-book
stores worldwide now accessible online that I was able, with reasonable
facility, to locate and procure virtually any written source, whether published
a century ago or last week, and to track down and contact clinical investigators
and public-health officials, even those long retired. 
Throughout this research, I tried to follow the facts wherever they led. In
writing the book, I have tried to let the science and the evidence speak for
themselves. When I began my research, I had no idea that I would come to believe
that obesity is not caused by eating too much, or that exercise is not a means
of prevention. Nor did I believe that diseases such as cancer and Alzheimer's
could possibly be caused by the consumption of refined carbohydrates and sugars.
I had no idea that I would find the quality of the research on nutrition,
obesity, and chronic disease to be so inadequate; that so much of the
conventional wisdom would be founded on so little substantial evidence; and
that, once it was, the researchers and the public-health authorities who funded
the research would no longer see any reason to challenge this conventional
wisdom and so to test its validity.  
As I emerge from this research, though, certain conclusions seem inescapable to
me, based on the existing knowledge: 

1. Dietary fat, whether saturated or not, is not a cause of obesity, heart
disease, or any other chronic disease of civilization. 

2. The problem is the carbohydrates in the diet, their effect on insulin
secretion, and thus the hormonal regulation of homeostasisthe entire harmonic
ensemble of the human body. The more easily digestible and refined the
carbohydrates, the greater the effect on our health, weight, and well-being. 

3. Sugarssucrose and high-fructose corn syrup specificallyare particularly
harmful, probably because the combination of fructose and glucose simultaneously
elevates insulin levels while overloading the liver with carbohydrates. 

4. Through their direct effect on insulin and blood sugar, refined
carbohydrates, starches, and sugars are the dietary cause of coronary heart
disease and diabetes. They are the most likely dietary causes of cancer,
Alzheimer's disease, and the other chronic diseases of civilization. 

5. Obesity is a disorder of excess fat accumulation, not overeating, and not
sedentary behavior. 

6. Consuming excess calories does not cause us to grow fatter, any more than it
causes a child to grow taller. Expending more energy than we consume does not
lead to long-term weight loss; it leads to hunger. 

7. Fattening and obesity are caused by an imbalancea disequilibriumin the
hormonal regulation of adipose tissue and fat metabolism. Fat synthesis and
storage exceed the mobilization of fat from the adipose tissue and its
subsequent oxidation. We become leaner when the hormonal regulation of the fat
tissue reverses this balance. 

8. Insulin is the primary regulator of fat storage. When insulin levels are
elevatedeither chronically or after a mealwe accumulate fat in our fat tissue.
When insulin levels fall, we release fat from our fat tissue and use it for
fuel. 

9. By stimulating insulin secretion, carbohydrates make us fat and ultimately
cause obesity. The fewer carbohydrates we consume, the leaner we will be. 

10. By driving fat accumulation, carbohydrates also increase hunger and decrease
the amount of energy we expend in metabolism and physical activity. 

In considering these conclusions, one must address the obvious question: can a
diet mostly or entirely lacking in carbohydrates possibly be a healthy pattern
of eating? For the past half century, our conceptions of the interaction between
diet and chronic disease have inevitably focused on the fat content. Any
deviation from some ideal low-fat or low-saturated-fat diet has been considered
dangerous until long-term, randomized control trials might demonstrate
otherwise. Because a diet restricted in carbohydrates is by definition
relatively fat-rich, it has therefore been presumed to be unhealthy until proved
otherwise. This is why the American Diabetes Association even recommends against
the use of carbohydrate-restricted diets for the management of Type 2 diabetes.
How do we know they're safe for long-term consumption? 
The argument in their defense is the same one that Peter Cleave made forty years
ago, when he proposed what he called the saccharine-disease hypothesis.
Evolution should be our best guide for what constitutes a healthy diet. It takes
time for a population or a species to adapt to any new factor in its
environment; the longer we've been eating a particular food as a species, and
the closer that food is to its natural state, the less harm it is likely to do.
This is an underlying assumption of all public-health recommendations about the
nature of a healthy diet. It's what the British epidemiologist Geoffrey Rose
meant when he wrote his seminal 1985 essay, "Sick Individuals and Sick
Populations," and described the acceptable measures of prevention that could be
recommended to the public as those that remove "unnatural factors" and restore "
'biological normality'that is . . . the conditions to which presumably we are
genetically adapted." "Such normalizing measures," Rose said, "may be presumed
to be safe, and therefore we should be prepared to advocate them on the basis of
a reasonable presumption of benefit." 
The fat content of the diets to which we presumably evolved, however, will
always remain questionable. If nothing else, whatever constituted the typical
Paleolithic hunter-gatherer diet, the type and quantity of fat consumed
assuredly changed with season, latitude, and the coming and going of ice ages.
This is the problem with recommending that we consume oils in any quantity. Did
we evolve to eat olive oil, for example, or linseed oil? And maybe a few
thousand years is sufficient time to adapt to a new food but a few hundred is
not. If so, then olive oil could conceivably be harmless or even beneficial when
consumed in comparatively large quantities by the descendants of Mediterranean
populations, who have been consuming it for millennia, but not to Scandinavians
or Asians, for whom such an oil is new to the diet. This makes the science even
more complicated than it already is, but these are serious considerations that
should be taken into account when discussing a healthy diet. 
There is no such ambiguity, however, on the subject of carbohydrates. The most
dramatic alterations in human diets in the past two million years,
unequivocally, are (1) the transition from carbohydrate-poor to
carbohydrate-rich diets that came with the invention of agriculturethe addition
of grains and easily digestible starches to the diets of hunterepilogue
gatherers; (2) the increasing refinement of those carbohydrates over the past
few hundred years; and (3) the dramatic increases in fructose consumption that
came as the per-capita consumption of sugarssucrose and now high-fructose corn
syrupincreased from less than ten or twenty pounds a year in the mid-eighteenth
century to the nearly 150 pounds it is today. Why would a diet that excludes
these foods specifically be expected to do anything other than return us to
"biological normality"? 
It is not the case, despite public-health recommendations to the contrary, that
carbohydrates are required in a healthy human diet. Most nutritionists still
insist that a diet requires 120 to 130 grams of carbohydrates, because this is
the amount of glucose that the brain and central nervous system will metabolize
when the diet is carbohydrate-rich. But what the brain uses and what it requires
are two different things. Without carbohydrates in the diet, as we discussed
earlier (see page 319), the brain and central nervous system will run on ketone
bodies, converted from dietary fat and from the fatty acids released by the
adipose tissue; on glycerol, also released from the fat tissue with the
breakdown of triglycerides into free fatty acids; and on glucose, converted from
the protein in the diet. Since a carbohydrate-restricted diet, unrestricted in
calories, will, by definition, include considerable fat and protein, there will
be no shortage of fuel for the brain. Indeed, this is likely to be the fuel
mixture that our brains evolved to use, and our brains seem to run more
efficiently on this fuel mixture than they do on glucose alone. (A good
discussion of the rationale for a minimal amount of carbohydrates in the diet
can be found in the 2002 Institute of Medicine [IOM] report, Dietary Reference
Intakes. The IOM sets an "estimated average requirement" of a hundred grams of
carbohydrates a day for adults, so that the brain can run exclusively on
glucose, "without having to rely on a partial replacement of glucose by [ketone
bodies]." It then sets the "recommended dietary allowance" at 130 grams to allow
margin for error. But the IOM report also acknowledges that the brain will be
fine without these carbohydrates, because it runs perfectly well on ketone
bodies, glycerol, and the protein-derived glucose.) 
Whether a carbohydrate-restricted diet is deficient in essential vitamins and
minerals is another issue. As we also discussed (see page 32026), animal
products contain all the amino acids, minerals, and vitamins essential for
health, with the only point of controversy being vitamin C. And the evidence
suggests that the vitamin C content of meat products is more than sufficient for
health, as long as the diet is indeed carbohydraterestricted, with none of the
refined and easily digestible carbohydrates and sugars that would raise blood
sugar and insulin levels and so increase our need to obtain vitamin C from the
diet. Moreover, though it may indeed be uniquely beneficial to live on meat and
only meat, as Vilhjalmur Stefannson argued in the 1920s, carbohydrate-restricted
diets, as they have been prescribed ever since, do not restrict leafy green
vegetables (what nutritionists in the first half of the twentieth century called
5 percent vegetables) but only starchy vegetables (e.g., potatoes), refined
grains and sugars, and thus only those foods that are virtually without any
essential nutrients unless they're added back in the processing and so
fortified, as is the case with white bread. A calorie-restricted diet that cuts
all calories by a third, as John Yudkin noted, will also cut essential nutrients
by a third. A diet that prohibits sugar, flour, potatoes, and beer, but allows
eating to satiety meat, cheese, eggs, and green vegetables will still include
the essential nutrients, whether or not it leads to a decrease in calories
consumed.