By: Hassan S Dashti, PhD
The most popular New Years resolution by far is weight loss. People kick-start their new year on new ‘detox’ or fad diets with hopes to lose some weight or, less commonly, to adopt a healthy lifestyle, only to quit a few months later. Traffic to websites like caloriecounter.com and weightwaterchers.com hits an all time high in January! (1) People often envision January 1 of every year as an empowering and motivating moment that enables them to consider making these daring lifestyle changes. People might be less inclined to make these commitments on arbitrary dates like March 1 or October 19. With emerging evidence suggesting seasonal changes in the environment and human physiology, driven primarily by seasonal changes in sunlight and temperature, is it possible that certain start dates or seasons are more conducive to successful weight loss?
Seasonal variations have been observed for numerous communicable and non-communicable diseases (2) and both biological and behavioral traits. One of the earliest observations of seasonal variation in a disease was that of rickets, a disease resulting from vitamin D deficiency (3). Clinical observations indicated that rickets was common in spring, but rare in fall. The subsequent finding of seasonal variation in plasma 25(OH)D levels suggested that summer sunlight exposure was indeed an important determinant of vitamin D status. For more complex traits, like obesity, the seasonal etiology, if present, is likely to be multifactorial!
Successful weight loss is largely determined by the ability to reduce overall caloric intake, which depends on food availability and internal hunger cues. Living at a time where food is essentially abundant year-round in the Western world, people are typically not dealing with food shortages. For most processed foods, seasonal price variability is also absent, particularly in metropolitan areas, so people’s intakes are likely to be homogenous year-round (4,5). However, seasonal price variability of nutrient dense fruits and vegetables may limit a person’s likelihood to adhere to diets higher in fruits and vegetables. For example, strawberry prices tend to decrease through the first four months of the year and rise again from September to December. Fresh apples, on the other hand, have a fairly weak seasonal price pattern as a result of new apple varieties with later harvest dates and sophisticated storage technology. But it seems that despite the constant supply of most foods at steady prices, seasonal variation in dietary intake may exist. In the Framingham Heart Study, for example, self-reported total energy intake was 86 kcal/day higher during the fall than in the spring (6). Also, percentage of calories from carbohydrate, fat and saturated fat showed slight seasonal variation, with a peak in the spring for carbohydrate and in the fall for total fat and saturated fat intake. Of course these differences may be due to seasonal differences in self-reporting and recall, but if it’s true, is weight loss in the spring more successful than the fall?
Another important aspect of weight loss to consider is seasonal variability in energy expenditure.
The investigation from the Framingham Heart Study (6) also observed seasonal variation in physical activity, including common activities such as gardening, carpentry, lawn mowing, golf and running for men, and gardening, swimming, health club exercise, dancing and bicycling for women. Not surprisingly, people residing in the Northeast are less inclined to engage in outdoor physical activity. This sedentary lifestyle in the winter may partly explain the reason why people tend to be the heavier in the winter! (7)
Newer studies are investigating more complex physiologic changes that might interfere with energy balance. Recent observations in humans suggest that cold exposure may induce the conversion of white adipose tissue to more metabolically active brown-like adipose tissue (8). This ‘beiging’ effect of cold exposure could potentially have clinical implications for diabetes and obesity. Other studies have observed seasonal variability in metabolism and epigenetics as well (9,10). Whether these physiologic differences can override energy imbalance resulting from seasonal lifestyle differences is currently unknown.
To test whether there are seasonal differences in weight loss success we’d ideally test this in a randomized and controlled weight loss trial whereby people are prescribed hypocaloric diets and assigned random start dates. This can also be investigated analytically in previously conducted weight loss cohorts. Various methodologies are available for the assessment of seasonality and those range from simple comparisons across seasons, to simple models such as fitting monthly counts to a sine curve, or more complex statistical models (2).
Despite the little evidence we have so far relating seasonality and energy balance, healthcare providers, including nutritionists, should account for seasonality in their practice, and tailor their dietary (food and fluids) and physical activity recommendations accordingly – it’d be senseless to recommend berries when they are unavailable at stores or outdoor exercise when it’s uncomfortably warm! But perhaps reaching that point of enthusiasm for weight loss is the most important factor predicting weight loss success, so if January 1 is that date when motivation hits in, then so be it!