summarize: I may be starting to sound like a broken record, but it should be obvious by
now that many of the problems we want to address or avoid stem from
consuming calories in excess of what we can use or safely store. If we take in
more energy than we require, the surplus ends up in our adipose tissue, one
way or another. If this imbalance continues, we exceed the capacity of our
“safe” subcutaneous fat tissue, and excess fat spills over into our liver, our
viscera, and our muscles, as we discussed in chapter 6.
How many calories you consume has a huge impact on everything else
we’re talking about in this book. If you’re ingesting one thousand extra
calories a day, of anything, you’re going to have problems sooner or later. In
prior chapters, we’ve seen how excess calories contribute to many chronic
diseases, not only metabolic disorders but also heart disease, cancer, and
Alzheimer’s disease. We also know from decades of experimental data
(chapter 5) that eating fewer calories tends to lengthen lifespan, at least in lab
animals such as rats and mice—although there is debate over whether this
represents true lifespan extension or an elimination of the known hazards of
overfeeding, the default state of the control animals in most of these
experiments. (And also, of many modern humans.)
In human beings, as opposed to laboratory animals, caloric restriction
typically goes by a different name: calorie counting. There is plenty of
research showing that people who count their calories and limit them can and
do lose weight, the primary end point of such studies. This is how Weight
Watchers works. The biggest obstacles to doing this successfully are first of
all hunger and second the requirement that you track what you eat in
meticulous detail. The apps that help you do this today are better than ten
years ago, but it’s still not easy. For the right person, this approach works
incredibly well—it’s a favorite of bodybuilders and athletes—but for many,
the requirement of constant tracking makes it unfeasible.
One slight advantage is that calorie counting is agnostic to food choices;
you can eat whatever you want so long as you stay within your daily
allowance. But if you make too many poor decisions, you will be very hungry,
so buyer beware. You can lose weight on a restricted-calorie diet consisting
only of Snickers bars, but you will feel much better if you opt for steamed
broccoli and chicken breasts instead.
There has been a long-running controversy over whether caloric
restriction could or should be applied to humans as a tool to enhance
longevity. It did seem to work for Luigi Cornaro, the dieting Italian
gentleman from the sixteenth century—he claimed to have lived to one
hundred, although he was probably actually in his eighties when he died. This
supposed longevity benefit is, obviously, a difficult proposition to study in
human beings over the long term, for some of the reasons I’ve just outlined.
So the hypothesis was tested in monkeys, in two long-running primate
studies. The results were so surprising that they are still being debated.
In July of 2009, a study published in Science found that rhesus monkeys
that had been fed a reduced-calorie diet for more than two decades had lived
markedly longer than those who were allowed to eat freely. “Dieting Monkeys
Offer Hope for Living Longer,” declared the headline on the front page of
The New York Times. The accompanying photos told the story: on the left was
a monkey named Canto, looking trim and spry at the relatively advanced age
of twenty-seven, while on the right sat Owen, just two years older but looking
like Canto’s flabby, dissipated uncle. Canto had been on a calorically
restricted diet for most of his life, while Owen had eaten just about as much
food as he wanted.
Owen and Canto were two of seventy-six monkeys in this study, begun
two decades earlier at the University of Wisconsin–Madison. Half of the
monkeys (the control group) were fed ad libitum, meaning they could eat as
much food as they wanted, while the other half were placed on a “diet”
allowing them about 25 percent fewer calories than the controls. They then
lived out their lives as the researchers observed them growing older.
Aging studies tend to be about as exciting as watching paint dry, but the
end results were pretty dramatic. In the end, the calorically restricted
monkeys lived significantly longer and proved far less likely to die of agerelated diseases than the ad libitum–fed control monkeys. They were healthier
by many other measures, such as insulin sensitivity. Even their brains were in
better shape than those of the controls, retaining more gray matter as they
aged. “These data demonstrate that caloric restriction slows aging in a primate
species,” the study authors concluded.
Case closed, or so it seemed.
Three years later, in August 2012, another monkey study made the front
page of the Times, but with a markedly different headline: “Severe Diet
Doesn’t Prolong Life,” the paper declared grimly, adding, “At Least in
Monkeys.” This study, also begun in the 1980s, was conducted under the
auspices of the National Institute on Aging, one of the National Institutes of
Health, and the study design was nearly identical to the Wisconsin study, with
one group of monkeys being fed about 25 to 30 percent less than the other.
Yet the NIH researchers found that their calorically restricted monkeys had
not lived longer than the controls. There was no statistically significant
difference in the lifespans of the two groups. From a headline writer’s point
of view, caloric restriction had not “worked.”
Journalists love it when a study contradicts whatever the last wellpublicized study said. In the small world of people who study aging, the NIH
results caused consternation. Everyone had expected that the NIH monkey
study would confirm the results seen in Wisconsin. Now it appeared as if the
two research teams had spent tens of millions of dollars in federal grant
money to demonstrate that caloric restriction lengthens monkey lifespans in
Wisconsin but not in Maryland, where the NIH monkeys were kept.
But sometimes science tells us more when an experiment “fails” than
when it yields the expected results, and so it was with the monkeys. When
examined side by side, the two monkey studies had some seemingly minor
differences that turned out to be hugely significant—and very pertinent to our
strategy as well. Together, the dueling monkey studies constitute one of the
most rigorous experiments ever done about the complex relationship between
nutrition and long-term health. And like many of the best scientific
experiments, this one happened at least partly by accident.
The most profound difference between the two studies was also the most
fundamental, for a diet study: the food that the monkeys ate. The Wisconsin
animals ate an off-the-shelf commercial monkey chow that was
“semipurified,” meaning its ingredients were highly processed and rigorously
titrated. The NIH monkeys were fed a diet that was similar in its basic
macronutrient profile, but their chow was “natural” and less refined, custom
formulated from whole ingredients by an in-house primate nutritionist at
NIH. The most glaring contrast: while the NIH monkey chow contained about
4 percent sugar, the Wisconsin diet comprised an astonishing 28.5 percent
sucrose, by weight. That is a greater proportion of sugar than you’ll find in
vanilla Häagen-Dazs ice cream.
Could that alone have explained the difference in survival outcomes?
Possibly: more than 40 percent of the Wisconsin control monkeys, the ones
not subject to calorie limitations, developed insulin resistance and prediabetes,
while just one in seven of the NIH controls became diabetic.
[*1] And the
Wisconsin control monkeys proved far more likely to die from cardiovascular
causes and cancer than monkeys from any other group. This could suggest
that caloric restriction was eliminating early deaths because of the bad
Wisconsin diet more than it was actually slowing aging—which is still useful
information, as avoiding diabetes and related metabolic disorders is important
to our strategy.
The Wisconsin researchers defended their diet as more similar to what
Americans actually eat, which is a fair point. The comparison is not exact by
any means, but in human terms the Wisconsin monkeys were more or less
living on fast food, while the NIH monkeys were eating at the salad bar. The
Wisconsin control monkeys ate the most calories, of the worst food, and their
health suffered. Makes sense; if your diet consists mostly of cheeseburgers
and milkshakes, then eating fewer cheeseburgers and milkshakes is going to
help you.
The NIH diet was much higher in quality. Instead of ultraprocessed
ingredients like corn oil and cornstarch (another 30 percent of the Wisconsin
diet), the NIH monkey chow contained ground whole wheat and corn, and
thus more phytochemicals and other possibly beneficial micronutrients like
those typically found in fresh food. While not exactly natural, it was at least
closer to what rhesus monkeys would actually eat in the wild. So giving the
NIH monkeys more or less of that feed may have had less of an impact
because the diet was not as harmful to begin with. Upshot: the quality of your
diet may matter as much as the quantity.
Taken together, then, what do these two monkey studies have to tell us
about nutritional biochemistry?
1. Avoiding diabetes and related metabolic dysfunction—especially by
eliminating or reducing junk food—is very important to longevity.
2. There appears to be a strong link between calories and cancer, the
leading cause of death in the control monkeys in both studies. The CR
monkeys had a 50 percent lower incidence of cancer.
3. The quality of the food you eat could be as important as the quantity. If
you’re eating the SAD, then you should eat much less of it.
4. Conversely, if your diet is high quality to begin with, and you are
metabolically healthy, then only a slight degree of caloric restriction—or
simply not eating to excess—can still be beneficial.
I think this last point is key. These two studies suggest that if you are
eating a higher-quality diet—and are metabolically healthy to begin with—
then severe caloric restriction may not even be necessary. The NIH control
monkeys ate as much as they wanted of their better diet and still lived nearly
as long as the CR monkeys in both studies. Interestingly, the post facto
analyses also revealed that the NIH control monkeys naturally consumed
about 10 percent fewer calories per day than the Wisconsin controls, likely
because their higher-quality diet left them feeling less hungry. The
researchers speculated that even this very slight degree of caloric reduction
may have been significant—certainly, it supports our thesis that it is better to
avoid being overnourished.
Note that these study results do not suggest that everyone needs to
undertake a drastic, severe reduction in caloric intake. Limiting calories can
be helpful for people who are metabolically unhealthy and/or overnourished.
But I’m not convinced that whatever longevity boost long-term, deep caloric
restriction may confer is worth some of the trade-offs—including potentially
weakened immunity and greater susceptibility to cachexia and sarcopenia
(muscle loss), not to mention constant hunger. These unwanted side effects
would accelerate some of the negative processes that already go along with
aging, suggesting that in older people especially, caloric restriction might do
more harm than good.
The monkeys teach us that if you are metabolically healthy and not overnourished, like the NIH animals, then avoiding a crap diet may be all you
need. Some of the NIH CR monkeys ended up with some of the longest
lifespans ever recorded in rhesus monkeys. It seems quite clear, then, that
even for monkeys, limiting caloric intake and improving diet quality
“works”—it’s how you pull it off that is tricky. As we’ll see in the next
section, there are many other strategies we can adopt to limit the calories we
consume and to tailor our food consumption to suit our metabolism and way
of life.