Summer is fattening. Don’t do it in winter.

Seasonal eating proper

More from T.S. Wiley and Dr. Kruse on seasonal eating in what appears to be the primary model for its justification for use in humans – hibernating mammals.

How it goes, or so they say: in summer, hibernators massively overeat, including carb-rich foods, in order to generate muscle and liver insulin resistance, so as to promote body fat growth.  The long light cycle reduces evening melatonin, which pushes back the usual nighttime peak in prolactin, which causes an abnormal resistance to leptin, which induces hypothalamic NPY and subsequent carbohydrate craving.  Ergo, summer is fattening.  In today’s day, increased artificial lights guarantee year-round pseudo-summer; and we no longer experience the benefits of the short light cycle: longer sleep times (akin to hibernation) and fasting – either complete fasting as in hibernation, or pseudo-fasting, ie, a ketogenic diet.

Graphically, from Florant and Healy (2012):Circadian 1

andCircadian 2

There are a lot of steps in this line of reasoning: light exposure –> melatonin –> prolactin –> leptin –> NPY –> carbs –> obesity.

divide and conquer

There appears to be a relationship between melatonin and prolactin, which is one of the more tricky ones.  Eg, Jimenez-Ortega (2012) showed melatonin suppressed prolactin secretion… this could be interpreted to mean that with less melatonin around (ie, long light cycle = summer), there would be more prolactin (later in the day?); circadian prolactin timing is a bit foggy:

Melatonin prolactin

However, in a rat study (Claustrat et al., 2008), chronic light exposure appeared to increase daytime prolactin levels, which fills that potential gap in the logic:

prolactin and light

Support of prolactin’s role in food intake: Birketvedt (2012) showed that people with “Night Eating Syndrome” had significantly higher levels of prolactin during the day, and overweight NES exhibited a delayed peak in prolactin (although total levels were lower than in normal weight NES):circadian prolactin

Park (2012) reported that prolactin improves insulin sensitivity – lower overall prolactin during long light cycles would support this, ie, when fat mass is growing (prolactin is supposed to be a winter hormone, but it’s only supposed to be elevated at night, after the melatonin surge… shorter sleep times cause prolactin to ‘spill over’ into the daytime = timing matters).  Also, dopamine antagonists (winter brain) increase prolactin whereas dopamine agonists decrease it.  Bromocriptine (summer brain), a classical prolactin antagonist, also reduces leptin (low leptin = hunger, sometimes, which isn’t exactly the same thing as leptin resistance, but may be seen to have a similar effect in some contexts).

It’s a bit of a stretch – trying to extrapolate the abovementioned day-to-day effects to a 10-day icv infusion of prolactin in rats – but Naef (2007) showed this treatment caused a pretty robust resistance to icv leptin injections:

prolactin leptin resistance

Furthermore, Park (2011) showed that ketones, which would be elevated during winter (hibernation or on a ketogenic diet), improve leptin sensitivity; thus, it might be concluded that lower ketones, as promoted by a high carb summer diet, reduces leptin sensitivity (also supported by the effect of fructose, found in summer fare, on leptin sensitivity).

The leptin-NPY connection doesn’t require fact-checking.  It is known.

I am skeptical about food selection or diet choice paradigms in animal studies, but here it goes anyway…

Glass (1997) gave rats two bowls of food, one high fat & another high sugar, and measured food intake before and after NPY injections.  Rats ate waaay more sugar after NPY:NPY

Wang (1998) showed that rats who naturally prefer carbs over other macronutrients had higher NPY levels.  This alone wouldn’t suggest causation, but in combination with Glass’ findings above and Welch’s (1994), I’m comfortable saying that NPY causes animals to “crave” carbs.

So there you have it.  It’s stringing together many different lines of logic, but it makes sense (prolactin is the tricky one, imo).  Lots of indirect, independently sourced pieces of evidence to suggest that summer is, indeed, fattening.  Don’t do it in winter = turn out the lights and go keto.

calories proper

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  • Alicia

    Hey Bill,

    Just wanted to say kudos to you for even attempting to scientifically support Wiely’s hypothesis! I read it earlier this summer and found it very compelling, but also very difficult to keep it all straight. You’re doing a good job with it, keep it coming!


    • William Lagakos

      Hi Alicia, thanks! I had the exact same experience: compelling but difficult to mentally organize so many details.

  • Kindke

    All of my personal experience goes against this, in the summer I drop weight effortlessly and in the winter ( particularly january ) I struggle to keep the fat off.

    ive lost it now but I remember reading about how d2 receptors on the pancreas modify insulin secretion independently in response to carbs such that low dopamine = increased insulin secretion. This effect was completely independent of anything else like insulin resistance etc. Which seems to suggest a pre-program to store more fat in the absence of dopamine.

    • FrankG

      I agree Kindke and struggle with the idea that ALL of Summer is “weight-gain season”. Although as I mention elsewhere I think that the Fall (Autumn) harvest is a good candidate for additional fat mass gong into the (traditionally) lean Winter months.

      If anything I find I am heavier going into Winter (and probably during it now, with better year round access to food) and lighter during the Summer.

      I also recall reading (not sure where now) that even our distribution of fat can change during the year — hence the advice to not focus so much on “weight” but more about how your clothes fit.

    • William Lagakos

      I don’t really experience seasonal shifts in body weight or composition, but that’s probably because I’m “aware” of my diet. In reality, though, I can see it going either way; ie, some people fatten in the summer, whereas others actively pursue summer 6-pack abs and rebound in winter.

      From these 2 papers, the dopamine-insulin connection seems to be a local effect… might not be related to the high dopamine phenotype of “summer brain.” (not really sure though.)

      DA-mediated autocrine inhibitory circuit regulating human insulinsecretion in vitro.

      DA d2 KO impairs insulin secretion and causes glucose intolerance.

      • Kindke

        Bill yes i think your right in winter I do tend to lose all self control over diet and just mindlessly eat processed carbs. I admit to that. I have to stop it tbh because every year I gain 10 lbs in December/January then have to lose it again in the summer.

        Also sorry for the ramble it was this study I was actually thinking of, I remember briefly skimming over it, but now re-reading it properly it is the alpha2-adrenergic receptor that inhibits glucose stimulated insulin secretion. The dopamine d2 receptor part in the title is what got stuck in my brain though :)

        I like their conclusion btw, “We suggest that treatment with bromocriptine promotes beta cells rest, thereby preventing long-lasting hypersecretion of insulin and subsequent beta cell failure”

        implying that it is excess insulin secretion and not insulin resistance as the primary lesion in t2d. Because if insulin resistance was the primary lesion then bromocriptine should worsen their diabetes.

        • FrankG

          “implying that it is excess insulin secretion and not insulin resistance as the primary lesion in t2d.”

          That is the way that makes most sense to me, as someone with Type 2 D.

          I think we are starting to recognise “normal” or physiological reasons for IR as an healthy response but in Type 2 D it is clearly disordered.

          I see IR as a “down-regulation” mechanism to combat the over-secretion of Insulin — just like how folks who live alongside busy roads will, after a time, no longer hear the noise and may even have trouble sleeping elsewhere, if it is too quiet.

          Of course once IR has kicked in and so long as the dietary drive is still keeping the Blood Glucose (BG) and Insulin high, then you enter into a vicious cycle where one feeds into the other… greater Insulin secretion leads to higher IR., leads to greater insulin secretion… until finally the Beta cells can no longer keep up and overt Type 2 is finally diagnosed; by which time it may be too late to recover.

          Of course there are those who have already decided that it can’t possibly be the Western industrial diet, high in sugars and refined starches, which drives up the BG and Insulin secretion etc… No No! For them the IR starts first as a result of “excessive energy intake” from any source, or some such rationale. It can’t possibly be the Insulin! :-)

          • Kindke

            damn Frank that was spot on couldnt of said it better myself. I also think thats exactly how t2d works.

          • Jane Plain (Woo)

            in t2dm, ir is reactive to mitochondrial damage imparing glucose and fat oxidation. reduced insulin receptors occur secondary to maximal , oxidation of fat and glucose ….which is pathological as mitochondria and therefore energy production poor and inefficient.

            there is a trend to increasing energy with prolonged ketosis specifically because wt losd – low cal intake – promotes mitochondrial genesis.

            there are of course nutritous, infectious, and chemical causes of impared insulin action but mitochondrial issue most specific.

          • Franziska Spritzler

            Fantastic explanation of the progression from hyper-secretion of insulin to IR to DM T2. If only all endocrinologists would recognize that the first step in reversing hyperinsulinemia is drastically reducing carb intake.

        • William Lagakos

          Barbara Corkey gave a great talk about how environmental “obesogens” can ever-so-slightly increase the insulin response to carbs, and this causes insulin resistance:… mostly stuff that would be found in junk food, but she had a ton of in vitro studies to support it.

          • Chris

            will check this one out

      • Jane Plain (Woo)

        I think issue with modern people and summer wt gain is that a lot of people drink sugar heavy alcoholic drinks in summer , and also in summer they stop trying to diet and go on holidays.

        When people follow natural body cues almost everyone loses weight in summer and gains in fall and winter… but national dieting season which starts after early winter to prepare for summer may confound things.

    • Jane Plain (Woo)

      Kindke this (your experience) is actually the reality of how seasonality affects weight. Kruse’s ideas don’t line up with what we know of hibernating animals, or the effects of light & temp patterns on physiology and energy/metabolic/weight results.

      High temp + high light = better blood sugar, energy, weight loss.

      As stated previously the problem with artificial light is not that it “mimics summer” because it doesn’t; if it mimiced summer people would be eternally energized and dropping weight with a high mood. More accurate to say artificial light disrupts circadian patterns and promotes chronic insomnia/decreased sleeping times, which worsen any seasonal weight trend.

      Winter is a lot less fattening if you can sleep 12 hrs a day like many of us are supposed to. Winter is extremely fattening if you are on your smartphone in the middle of the night and have to wake up in 4 hours to go to work. 1 way ticket to diabetes and fat gain. The very low light exposure and low temperatures in the winter promote significant insulin resistance, lethargy, hyperphagia and fat gain.

      Decreased sleeping and high energy is normal in summer, weight gain only occurs *after* the summer, when the brain and nervous system is sensitized / expectant of very high levels of NTs, only to have them withdrawn abruptly when the sun diminishes. This promotes that ‘cozy” hibernating low stimulation withdrawal, lie in bed and sleep feeling we all associate with fall time. Wt gain and eating lots is normal, which is why fall is associated with national pig out holidays (candy at holloween, thanksgiving, christmas “season”).

      Contrast to the summer holidays, which have very little to do with food, and much more to do with activity and being outdoors like going to the beach or watching fireworks… food is just an afterthought.

      • William Lagakos

        “*after* summer”
        I should’ve picked up on that – it’s even in the first two figures. Autumn, light cycle still long relative to winter, but getting shorter.

  • FrankG

    This whole idea makes a great deal of sense to me: right from the earliest life on this planet (our distant ancestor) our lives have been guided (or at some level affected) by the tidal pull of the Moon, the daily cycle of the Sun (the Earth’s rotation really), the monthly Lunar cycle, the annual progression of the season as we orbit the Sun on our tilted (and slightly wobbly) axis, and even the longer progression of the constellations around us.

    How could we not be adapted to these patterns?

    I’m attracted to the idea of my stone-age kin in ice-age Europe coming across a field of wild-blueberries in the Fall and eating their fill (helped on by hormonal changes) and laying down extra fat (also an adaptive bio-chemical change) before the long, lean Winter months.

    This may not work quite so well for more distant ancestors (what 70,000+ years ago) living near Equatorial Africa but even there is some seasonality to the food supply — but perhaps more a case of “dry season” vs “wet season”.

    • William Lagakos

      I remember reading somewhere that seasonality of coconut availability meant that equatorial peoples could get a ketogenic boost, similar to what people got in colder regions when they were eating a ketogenic diet in winter.

  • anon

    anyone who cites Jack Kruse puts their credibility in to question.
    A Cenegenics hormone injection doc and advocate

  • Jane Plain (Woo)

    This is all very interesting Bill but a lot of the underlying physiology might need to be reexamined.

    For example, that light in summer leads to leptin resistance. Most evidence suggests metabolic parameters are augmented in bright light and warm environments, like the well known seasonal shift variation of people’s body weights (down spring/summer up fall/winter) and blood sugar trends as well.

    Melatonin actually causes insulin resistance and helps fat gain, it does not promote fat loss. Lower melatonin will result in more activity of NTs like 5ht ne and dopamine, more activity of the sympathetic nervous system, and this is the same mechanisms exploited by weight loss drugs like stimulants nicotine, caffeine, cocaine, virtually *any* drug known to cause weight loss.

    Sleep promotes healthy weight and metabolism but drugs (or seasons 😉 ) that cause weight loss will cause insomnia by decreasing melatonin secondary to excesses of 5ht/ne/dopamine and the sympathetic nervous system and CRH.

    Prolacin and melatonin, tend to promote insulin resistance, body fat growth and hyperphagia. The seasonal variation with lower trends in warm months secondary to higher “active” nts is correlated with weight loss and energy wastefulness, it does not promote weight gain.

    In hibernating mammals, the primary mechanism for inducting hibernation is a loss of sensitivity to dopamine, which is directly caused by end of summer maximal dopamine levels.

    The time to become fat and resistant to insulin is not the summer, but THE END OF SUMMER, early fall, when dopamine is maximal and sensitivity becomes low as a direct result of the high levels of the summertime.

    When the sunlight patterns naturally wane with late summer, this reduces 5ht/dopamine/ne, and increases melatonin. This promotes an adaptive energy conservation program which leads to hibernation in hibernating animals, although in obese humans tends to be called “atypical depression” or “SAD” if it is named at all.

    Prolactin is primarily dependent upon dopamine and follows it accordingly – low dopamine(sensitivity) more prolactin. First line for prolactinoma is bromocriptine for this reason; melatonin less direct/important.

    • William Lagakos

      Back to the drawing board!
      Melatonin & prolactin seem antagonistic (but only sometimes):
      1) melatonin infusion lowered prolactin levels; and
      2) chronic light exposure increased prolactin.
      But both are supposed to be winter-dominant… perhaps the missing link is in their circadian relationship (?).
      It’s difficult to find evidence to support (or refute) the steps around and including: “summer prolactin spill over into daytime”… which is why I prefer to say it’s “tricky,” as opposed to it’s “bollixed.”
      (to say it’s bollixed would imply that I have a greater understanding of it than I actually do.)

  • La Frite

    I don’t want to discuss the line of logic presented here as I think it makes sense to some extent. However, the implicit background is that people experience a real summer and winter. I would just point out that this is not the case everywhere, and certainly not in the part of the world where modern humans came from. Of course, humans have explored and settled in many areas where there is a strong contrast between seasons but there are still many densely populated areas where the contrast does not really exist.

    • Jane Plain (Woo)

      In my experience, humans who relocate from an environment with stark seasonality, to an environment with a constant season of bright light and warmth will state that any signs of fall and winter energy conservation stop happening to them.

      I remember one woman told me she would gain and lose 5 pounds every year – gain in fall/winter lose in spring/summer, when she relocated to a constant bright/warm environment this cycle stopped happening.

      Another woman, who had manic depression (bipolar disorder I) experienced fall/winter depression episode, and spring/summer manic episode every year. When she moved to south , the depression episode stopped, but the manic episode continued to happen.

      Of course not all humans exhibit seasonality sensitivity, it tends to be more common in women and people perhaps with health conditions like mood disorders with circadian clock origins like BPAD.

      • William Lagakos

        I believe it! (individual differences in sensitivity to light cycles.) I read about this woman with a really bad case of SAD (i think it was SAD) that oddly started when she moved from Maine to Arizona. She couldn’t find a livable treatment and eventually had to move back (and was “cured”).
        On the other hand, I didn’t experience any of these symptoms moving from NJ to San Diego.

  • William Lagakos

    This study showed bona fide seasonal changes in body weight in agreement with the circannual patterns discussed here, but they chalked it all up to activity levels:

    The impact of seasonality on changes in body weight and physical activity in Mexican-American women.

  • donny

    It sort of makes sense that even though in the natural order of things, animals lose fat in the winter, when food becomes scarce–a tendency to take advantages of windfalls, and fatten when the opportunity arises, would increase chances of survival to spring.

    • William Lagakos

      agreed. A lot of it makes sense, and I’m pretty sure there are some hibernating animals that live like this today. The biochemical & neurohormonal explanations might need some fine tuning, but the seasonal changes in body weight most definitely occur, for example, in bears that hibernate.

  • gretchenbronson

    I’ve always found that I loose weight btwn Nov- Mar. like clockwork and In the summer, especially btwn Aug-Oct gain weight. Interesting thing, my LABS actuall get better in winter, and worse in summer. My IGF-1 increase -cold, low light levels, definitely influence this and as I move into summer my IGF-1 drops like a stone. My ASI isn’t as good either in the summer. My hsCRP is awesome in the winter (.02) and hasn’t been that steller in the summer (ranging from 1.0 -4.0) It was enough for my PCP to ask me what was different. When I told her cold – she told me to invest in a commercial grade Ice machine and get cold year round! I’m definitely light sensitive. We’ve been actively wearing blue light blocking glasses at night after sundown, and limiting all artificial light with the exception of red, at night. My sleep is amazing, across most of my cycle as well. I’ve always loved winter, and winter sports, it took Dr. Kruse shining the flashlight for me to put it all together. Now that its cooling off, I’m back in the 100 gallon Stock Tank as often as possible. last week I ran my quarterly labs for the fall I’m looking for huge improvements with the change in weather.

    you can check out how I put all this together at

    • William Lagakos

      You’re paleo! a lot of those circannual changes make sense, and are nearly opposite of those seen in Westernized SAD lifestyles (eg,
      I’ve yet to embrace the cold, but I’m pretty sure blue blockers have improved my sleep quality.
      Thanks for the link, I look forward to checking it out.

      • gretchenbronson

        Bill What’s interesting is that even before I went paleo, This was my pattern – I always lost weight in winter w/o trying and summer it was always gaining weight, or battling to stay where I was at. The only exception, would be when I lived in Guam for 18 months. 12 hour day/night cycles, and constant supply of fresh fish and lots of activity outdoors. The only time I’d be inside regularly was during typhoons! Winter was the season I missed most when there though…

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  • Chris

    turn off the lights and turn on the reproduction mechanism as spring approaches :)

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