Personally I believe the obesity epidemic can be blamed primarily on two phenomena:
1. The abundance of high calorie foods high in fats & carbs (I'll call them CF) in ever larger portions, and
2. Liquid calories loaded with sugar and/or fat
To prevent obesity my solution is simple: Keep the fats and carbs separate. If you're going to eat carbohydrate, eat it with lean protein and/or in whole form so you get sufficient fiber. Go easy on the fat. If you're going to eat fat, chances are it is attached to protein, forgo the carbs. If you simply must eat CF foods, rely on portion control/calorie counting and not on satiety to determine how much you eat. Keep the total caloric load low, perhaps in the 2-300 cal range, to keep the unnatural assault on your metabolism to a minimum.
My reasons for this are twofold:
1. Our paleolithic ancestors, from whom we differ very little genetically, did not have access to foods that were rich in both lipid and carb content simultaneously. I don't envision Paleo dude saved up his tubers to cook in rendered wild boar fat to plate tuber fries with his boar ribs and a side of some veggie also cooked in boar fat. It seems far more likely that Paleo dude ate the tubers if in abundance perhaps even delaying the need for a hunt, or saved those tubers for a rainy day when a kill was available for the eating. Paleo dude was mostly an opportunistic eater in a scarce world. Obesity was not a problem nor did Paleo chick worry over a little belly roll. We are programmed to store energy in its most efficient form (lipid) in amounts that are seemingly unlimited. There would have been no evolutionary advantage to not being able to store energy, or not wanting to partake in this energy source during times of abundance to save for times of scarcity. There is no physiological reason to limit lipid intake on any given day. I see no reason to doubt Eaton's work indicating that Paleos ate a relatively low fat diet. Further underlining the need for taking it while they could get it -- e.g. overconsumption one day if necessary -- and weak signaling at best. Bottom line, our metabolisms seem designed to switch between fuels depending which was more available, not deal with being bombarded by mixed fuels.
(2) As outlined in Nutrient Fates After Absorption dietary intake of protein and carbohydrate share the following in common:
(a) they invoke an insulin response
(b) they are on the order in terms of quantity with the body's storage capacity (nitrogen "pool" and glycogen)
(c) their intake stimulates their metabolism (protein synthesis, oxidation)
(d) their absorbed form is as metabolic substrate (amino acid, glucose)
As such, our hormonal signaling is tightly attuned to intake of these macronutrients to maintain levels in a relatively narrow window. Dietary fat, OTOH, is
(a) once absorbed, packaged as triglycerides in chylomicrons and transported mostly to the adipose tissue for immediate storage.
(b) as chylos, the absorbed form is not the metabolic substrate for lipids, that being free fatty acids (NEFA/FFA).
(c) in quantity, orders of magnitude less than total stored lipid even in the leanest of humans
(d) can virtually be stored without limit thus eliminating any need to limit intake in one feeding
Circulating NEFA levels are controlled indirectly by release from storage. Dietary fat does not significantly contribute directly to the levels of this energy source. My take-away message from the post/article is that our appetite/satiety signals are finely attuned to intakes of carb and protein out of necessity to maintain structure and storage/circulating levels within a relatively narrow range. Our metabolisms change remarkably within 24-48 hrs of deprivation. Lipid storage, even on a lean person, can last weeks (or more). Even gorging on fat is a drop in the bucket of the amount of lipid we store (again, even in the lean), so there's no need to limit this in the short term. Fat mass regulation seems almost independent of dietary fat when you think about it.
So, whenever I hear the query "Why do we overeat", in many cases it is a passive process. Our bodies were not made to handle the caloric punch of CF foods, so we tend to eat more calories before the stop signals go up were we consuming just carbs (usually with lots of fiber) or fats (usually with lots of protein).
Over on the personal blog ( When to Eat ), Helen wondered about separating carbs and fats on a daily basis -- e.g. alternating high fat day(s) with high carb day(s). This was actually the impetus for this post because my reply became too lengthy for the comments feature here to handle. In any case, here are my thoughts on that:
Let's say during the day I have a fatty breakfast and a carby dinner or a carby breakfast and a fatty dinner. Either way, if I'm consuming basically maintenance-caloric levels of these foods, my metabolism will do a bit of switching up within its normal mode. After carbs, lipid oxidation will be down-regulated and the carbs burnt off or converted to glycogen (see that Nutrient Fate link), but as the glucose is "cleared", lipid oxidation ramps up again. After fats, lipid oxidation remains as it was. If there's no to minimal carb in the meal, insulin is likely low so NEFA are released from the fat cells to replenish the IMCL being "burnt". The metabolism is "normal".
It takes a few days, however, for the body to transition to a "fat burning" (low carb) metabolism. Our bodies are inefficient during this transition -- spilling ketones, etc. This can probably be used to our advantage to get a little more out of weight loss, but is it healthy? I don't have the answer to that and I'll try to put it on my "to do list" to look into. My educated guess is that so long as you're not overdoing it caloriewise this is probably OK as diacylglycerols and ceramides shouldn't build up. It takes a while for IMCL to accumulate anyway on an HF diet. I don't know if it will accumulate if one's "average" diet is HF, even if some days are LF.
From a weight loss perspective, I do feel this switching up was probably responsible for the whooshes I would experience upon returning to LC after carb cheats. Was this healthy? Who knows. I look at it as a trade-off in the end. Whatever I did to get here, I'm way better off for it now.
I worry more, however, over the transition from a HF day to a HC day. The switch gets flipped back almost immediately. I've posted that as little as a single high fat meal can induce IR, and impaired glucose tolerance (IGT) the following day. If one is not efficiently burning lipids in short HF stints, this could potentially turn the HC day into a "diabetic day". I've played with a glucose meter to see about this for myself. Perhaps the fact that I don't eat particularly high fat (as a % or on a gram basis) version of LC, I've not had issues with tolerating carbs. Meters are cheap, and you can get strips relatively inexpensively too. Rather than guess, or look to studies to see, testing one's own response to this is probably better. But I do think alternating days like this has a greater potential for creating issues than mixing it up "separately" throughout the day. If one experiences a degree of IGT, I think a good bout of exercise between the last high fat meal and the first high carb meal might be all that's needed.
Companion post to follow ....