Expert Advice

Are There Any Proven Benefits to Fasting?

By Joe Sugarman

Mark Mattson is a professor of neuroscience in the Johns
Hopkins School of Medicine and also serves as chief of the Laboratory of
Neurosciences at the National Institute on Aging.

You probably know that too many calories aren’t good for
your waistline, but as it turns out, they aren’t good for your brain either.

According to research conducted by neuroscientist Mark
Mattson and others, cutting your energy intake by fasting several days a week
might help your brain ward off neurodegenerative diseases like Alzheimer’s and
Parkinson’s while at the same time improving memory and mood.

Mattson’s studies have built on decades-old research
establishing a connection between caloric intake and brain function. In
laboratory experiments, Mattson and his colleagues have found that intermittent
fasting–limiting caloric intake at least two days a week–can help improve
neural connections in the hippocampus while protecting neurons against the
accumulation of amyloid plaques, a protein prevalent in people with Alzheimer’s
disease. “Fasting is a challenge to your brain, and we think that your brain
reacts by activating adaptive stress responses that help it cope with disease,”
says Mattson. “From an evolutionary perspective, it makes sense your brain
should be functioning well when you haven’t been able to obtain food for a
while.”

But why fasting? Wouldn’t just eating fewer potato chips
a day have the same effect? Apparently not, says Mattson. He explains that
every time you eat, glucose is stored in your liver as glycogen, which takes
about 10 to 12 hours to be depleted. After the glycogen is used up, your body
starts burning fats, which are converted to ketone bodies, acidic chemicals
used by neurons as energy. Ketones promote positive changes in the structure of
synapses important for learning, memory, and overall brain health. But if you
eat three meals a day with snacks between, your body doesn’t have the chance to
deplete the glycogen stores in your liver, and the ketones aren’t produced.
Mattson says exercise can also get your body to lower its glycogen levels, and
not coincidentally, exercise has been shown to have the same positive effects
on brain health as fasting.

Mattson recommends people try one of two strategies for
incorporating calorie restriction. The first is called the 5:2 diet, which has
gained popularity in recent years, particularly in England after the BBC aired
a 2012 documentary called Eat Fast and Live Longer in which Mattson
was featured. That diet calls for limiting your caloric intake to 500 calories
two nonconsecutive days per week while eating a healthy diet in the normal
caloric range (2,000 for women; 2,500 for men) the rest of the week. Five hundred
calories Fasting is a challenge to your brain, and we think that your brain
reacts by activating adaptive stress responses that help it cope with disease.

means maybe a fried egg for breakfast and a small serving
of lean protein with vegetables for lunch or dinner.

Another strategy is a time-restricted diet in which you
pack all your meals into one eight-hour period a day so your body has time to
exhaust its supply of glycogen, start burning fat, and produce ketones. Mattson
says animal studies have shown that the time-restricted diet has effects
similar to those of intermittent fasting.

If you do decide to try fasting, don’t dive in too
quickly, Mattson advises. “The analogy with exercise applies here as well. If
you’ve been sedentary and then all of a sudden you try to run five miles, it’s
not very pleasant and you’ll likely get discouraged. It’s the same thing as if
you’ve been eating three meals a day plus snacks, and then you’re not eating
anything at all for two days; you’re not going to like it.”

Mattson suggests easing into the routine by starting with
one day of moderate fasting per week and then building up to two. There will
likely be a week or two of headaches, lightheadedness, and/or grouchiness,
which are common side effects, but after the initial phase, experiments show
that your mood should pick up.

Mattson collaborated on one six-month study of people
practicing the 5:2 diet that demonstrated people’s well-being improved over
time. Neurochemically, he says, when the brain is challenged by physical
exertion, cognitive tasks, or caloric restriction, the body produces a
protein called BDNF (brain-derived neurotrophic factor), which not only
strengthens neural connections and increases the production of new neurons but
can also have an anti-depressive effect. In his experiments with mice, he’s
found that those with exercise wheels in their cages have higher levels of BDNF
and show fewer signs of depression. “Probably during evolution, BDNF evolved to
play an important role in increasing neuroplasticity in the brain and forming
new synapses crucial to learning and memory as well as mood and motivation.”

But like so many others, will the 5:2 diet and its ilk
become just another diet du jour?

“I hope it’s not a fad,” says Mattson, who is
currently working on a study involving obese subjects at risk for cognitive
impairment and the effects of intermittent fasting. “There’s a lot of science
behind it, and the science is only increasing.”