Alternate-Day Fasting Gets a Safe Bill of Health

Heilbronn, Leonie K.; Panda, Satchidananda

Sep 3, 2019

Cell Metabolism. 2019; 30(3):411-413.

Various forms of fasting improve health and longevity in preclinical models. However, safety, outcomes, and the molecular changes underpinning human fasting are unclear. Stekovic et al. (2019) report improved markers of health for up to 6 months and associated metabolic changes among healthy adults who followed alternate-day fasting.

Reducing daily caloric intake by ∼20%–40% (a.k.a. caloric restriction or CR) in animals increases total and healthy lifespan—a feat rarely achieved by any drug. However, counting calories for CR in humans can be challenging, and chronic CR in rodents can dampen immune function and bone metabolism. Intermittent fasting (IF) is an alternative strategy involving 2–4 days per week when food intake is severely reduced or completely withheld, interspersed with ad libitum eating the remainder of the week.

Mice subjected to alternate-day fasting (ADF)—a form of IF—double their caloric intake on feast days, maintaining body weight close to their ad lib cohorts. Yet ADF mice enjoy pleiotropic health benefits found among CR mice who weigh much less (Mattson et al., 2014). This observation raised the possibility that prolonged fasting elicits a pro-longevity metabolic state. Thus, testing the feasibility of IF in humans and its health impacts, and assessing accompanying molecular changes are crucial to devise optimum nutrition to support long healthspan.

Few human trials on IF have been conducted: they are mostly in obese individuals and are modified to allow consumption of ∼500 kcals during the fasting day (Harvie et al., 2011, Harvie et al., 2013, Trepanowski et al., 2017). Whether ADF imparts any benefit to healthy people and unbiased omics-based molecular changes in ADF have not been addressed. As ADF can involve fasting for up to 36 h, there was also concern that it might trigger excessive muscle loss among healthy cohorts.

In a study published in this issue of Cell Metabolism (Stekovic et al., 2019), healthy individuals who had been under a self-imposed complete ADF for >6 months (long-term ADF) were matched to a comparator group who were naive to IF. This comparator group was then randomized to 4 weeks of ADF (short-term ADF) or ad lib control. The ADF group was in marked CR (∼37% in short-term ADF and ∼28% in long-term ADF) versus control (∼8%); i.e., the ADF group did not double their caloric intake, but modestly increased calorie intake to >125% on feast days. This inability to consume enough calories to maintain weight is observed in other studies of ADF/IF (Hutchison et al., 2019).

Significant weight loss was observed in ADF, which was due to reductions in both lean (∼2.7%) and fat mass (∼8.9%). The reduction in fat mass occurred from the trunk area and particularly the android area, which is considered lipotoxic (Figure 1). The expected fasting-induced surge in ketone bodies (specifically beta hydroxybutyrate or BHBA), which are products of fatty acid oxidation, was observed. Keeping in mind that the ADF group ate >125% of caloric intake, the increase in ketones following feast days was unexpected. There was also no reduction in resting metabolic rate, despite weight loss. These findings raise important questions about whether postprandial metabolic rate, relative fuel utilization, or substrate interconversion is altered by ADF in healthy adults. In mice that undergo 16 h of fast every day, post-prandial energy expenditure was increased without any change in energy expenditure during the rest or fast period (Hatori et al., 2012).

Figure 1. Alternate-Day Fasting Is Safe for Normal Weight Healthy Adults

Ad libitum eating every other day leads to both a prolonged period of fasting and an overall reduction in caloric intake. This results in several metabolic changes that correlate with healthy lifespan. ADF does not change the immune cell composition or bone mineral density.

Among hormonal changes, the chronic ADF group showed relatively less circulating fT3, although the TSH and free thyroxine did not change. This reflects normal thyroid function even after 6 months of ADF. However, there was a marked increase in parathyroid hormone, which may partly explain the preservation of bone mineral density (BMD) following ADF. This is an important finding as a slight reduction in BMD was reported in a relatively younger cohort following chronic CR, in which participants reduced calories by ∼12% for more than 12 months (Villareal et al., 2016).

Did ADF improve metabolic health? Since the participants were healthy to begin with, the authors assessed metabolic changes that are predictive of cardiometabolic health. Long-term ADF induced multiple beneficial health effects including reduced cholesterol and inflammatory markers and reduced risk for cardiovascular disease versus the comparator group.

How comparable are these metabolic benefits with those observed in other ADF/IF studies? While there is general agreement that ADF/IF imparts metabolic health benefits, protocol differences often hinder direct comparison. For example, many IF studies prescribe a reduced caloric intake on “fast” day, so participants don’t experience ∼24 h of true fast. Other groups prescribed 24 h fasts initiated immediately after breakfast (Hutchison et al., 2019) or dinner (Halberg et al., 2005), or advocate for “breaking” the fast in the middle of the day (Trepanowski et al., 2017). There is accumulating evidence that timing of nutrition impacts peripheral clocks, which modulate several aspects of metabolism and physiology (Skene et al., 2018).

Uniquely, the authors used omics approaches to assess molecular changes after 12 or 36 h fast among the ADF group. Approximately 20% of the plasma metabolome was transiently elevated after 36 h fast, with lipids contributing a major portion. Another ∼20% of the metabolites were lowered by the extended fast, and amino acids were the major contributor. These plasma metabolite changes may reflect changes in adipose tissue and liver metabolism. The liver is a key player in extended fasting, converting fatty acids released from adipose tissue to ketones, and amino acids as substrates for gluconeogenesis. ADF also reduced the pro-aging metabolite methionine, which may have prolongevity effects (Richie et al., 1994).

The proteome, measured from peripheral blood mononuclear cells (PBMCs) in fasted state, showed inactivation of pathways involved in lipid metabolism, mitochondrial biogenesis, and stress response. Although there was no significant compositional change in blood cells, the proteomic changes suggest a distinct molecular state of PBMCs. So it remains to be tested whether people under ADF mount a different immune response upon challenge.

The similarity of the metabolome changes with those seen in chronic CR (Mitchell et al., 2016) either implies a common mechanism or was due to unintended CR in the current study. Whether the reduced caloric intake accompanied disproportionate changes in the source of macro- and micro-nutrient (e.g., processed or raw) cannot be ruled out. Further, a disproportionate reduction in carbohydrate relative to fat intake might also explain the observed increase in BHBA after an overnight fast. Similarly, there was some indication of slight, albeit clinically insignificant, reduction in iron, RBC, and hematocrit. Altogether, this study gives a safe bill of health to relatively long-term ADF in healthy adults and highlights that a combination of fasting with optimum nutrition may ultimately increase healthy human lifespan.


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Alternate day fasting improves physiological and molecular markers of aging in healthy, non-obese humans. Cell Metab., 30 (2019), pp. 462-476.

Trepanowski, J.F.; Kroeger, C.M.; Barnosky, A.; Klempel, M.C.; Bhutani, S.; Hoddy, K.K.; Gabel, K.; Freels, S.; Rigdon, J.; Rood, J.;et al. Effect of alternate-day fasting on weight loss, weight maintenance, and cardioprotection among metabolically healthy obese adults: a randomized clinical trial. JAMA Intern. Med., 177 (2017), pp. 930-938.

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